[12335] in Perl-Users-Digest
Perl-Users Digest, Issue: 5935 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Wed Jun 9 16:07:20 1999
Date: Wed, 9 Jun 99 13:02:34 -0700
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Wed, 9 Jun 1999 Volume: 8 Number: 5935
Today's topics:
Re: Rounding excessive trailing decimals <tchrist@mox.perl.com>
Separating array into alphabetical array of arrays (Marc Bissonnette)
Re: Using a hash slice with references? <d-edwards@nospam.uchicago.edu>
Re: writing to a gzip file (Greg Bacon)
Re: writing to a gzip file <sidelko@stripe.Colorado.EDU>
Re: XS: newSVpv() and malloc (Ilya Zakharevich)
Special: Digest Administrivia (Last modified: 12 Dec 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 9 Jun 1999 13:35:39 -0700
From: Tom Christiansen <tchrist@mox.perl.com>
Subject: Re: Rounding excessive trailing decimals
Message-Id: <375ec20b@cs.colorado.edu>
[courtesy cc of this posting mailed to cited author]
In comp.lang.perl.misc,
"Anthony Lalande" <tonyboy@earthling.net> writes:
:I can't seem to find a way to have PERL return a shortened version of a
:number such as (1.94456543 -> 1.94 or 1.9).
:
:Can anyone help me with this?
The perlfaq4 manpage answers this on your own system. Read it.
--tom
NAME
perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date:
1999/05/23 20:37:49 $)
DESCRIPTION
The section of the FAQ answers question related to the
manipulation of data as numbers, dates, strings, arrays, hashes,
and miscellaneous data issues.
Data: Numbers
Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
The infinite set that a mathematician thinks of as the real
numbers can only be approximate on a computer, since the
computer only has a finite number of bits to store an infinite
number of, um, numbers.
Internally, your computer represents floating-point numbers in
binary. Floating-point numbers read in from a file or appearing
as literals in your program are converted from their decimal
floating-point representation (eg, 19.95) to the internal binary
representation.
However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly
represented as a decimal floating-point number. The computer's
binary representation of 19.95, therefore, isn't exactly 19.95.
When a floating-point number gets printed, the binary floating-
point representation is converted back to decimal. These decimal
numbers are displayed in either the format you specify with
printf(), or the current output format for numbers (see the
section on "$#" in the perlvar manpage if you use print. `$#'
has a different default value in Perl5 than it did in Perl4.
Changing `$#' yourself is deprecated.
This affects all computer languages that represent decimal
floating-point numbers in binary, not just Perl. Perl provides
arbitrary-precision decimal numbers with the Math::BigFloat
module (part of the standard Perl distribution), but
mathematical operations are consequently slower.
To get rid of the superfluous digits, just use a format (eg,
`printf("%.2f", 19.95)') to get the required precision. See the
section on "Floating-point Arithmetic" in the perlop manpage.
Why isn't my octal data interpreted correctly?
Perl only understands octal and hex numbers as such when they
occur as literals in your program. If they are read in from
somewhere and assigned, no automatic conversion takes place. You
must explicitly use oct() or hex() if you want the values
converted. oct() interprets both hex ("0x350") numbers and octal
ones ("0350" or even without the leading "0", like "377"), while
hex() only converts hexadecimal ones, with or without a leading
"0x", like "0x255", "3A", "ff", or "deadbeef".
This problem shows up most often when people try using chmod(),
mkdir(), umask(), or sysopen(), which all want permissions in
octal.
chmod(644, $file); # WRONG -- perl -w catches this
chmod(0644, $file); # right
Does Perl have a round() function? What about ceil() and floor()? Trig functions?
Remember that int() merely truncates toward 0. For rounding to a
certain number of digits, sprintf() or printf() is usually the
easiest route.
printf("%.3f", 3.1415926535); # prints 3.142
The POSIX module (part of the standard perl distribution)
implements ceil(), floor(), and a number of other mathematical
and trigonometric functions.
use POSIX;
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3
In 5.000 to 5.003 Perls, trigonometry was done in the
Math::Complex module. With 5.004, the Math::Trig module (part of
the standard perl distribution) implements the trigonometric
functions. Internally it uses the Math::Complex module and some
functions can break out from the real axis into the complex
plane, for example the inverse sine of 2.
Rounding in financial applications can have serious
implications, and the rounding method used should be specified
precisely. In these cases, it probably pays not to trust
whichever system rounding is being used by Perl, but to instead
implement the rounding function you need yourself.
To see why, notice how you'll still have an issue on half-way-
point alternation:
for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
0.8 0.8 0.9 0.9 1.0 1.0
Don't blame Perl. It's the same as in C. IEEE says we have to do
this. Perl numbers whose absolute values are integers under
2**31 (on 32 bit machines) will work pretty much like
mathematical integers. Other numbers are not guaranteed.
How do I convert bits into ints?
To turn a string of 1s and 0s like `10110110' into a scalar
containing its binary value, use the pack() and unpack()
functions (documented in the section on "pack"
L<perlfunc/"unpack" in the perlfunc manpage):
$decimal = unpack('c', pack('B8', '10110110'));
This packs the string `10110110' into an eight bit binary
structure. This is then unpack as a character, which returns its
ordinal value.
This does the same thing:
$decimal = ord(pack('B8', '10110110'));
Here's an example of going the other way:
$binary_string = unpack('B*', "\x29");
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on whether
they're used on numbers or strings. The operators treat a string
as a series of bits and work with that (the string `"3"' is the
bit pattern `00110011'). The operators work with the binary form
of a number (the number `3' is treated as the bit pattern
`00000011').
So, saying `11 & 3' performs the "and" operation on numbers
(yielding `1'). Saying `"11" & "3"' performs the "and" operation
on strings (yielding `"1"').
Most problems with `&' and `|' arise because the programmer
thinks they have a number but really it's a string. The rest
arise because the programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of
`"\020\020" & "\101\101"') is not a false value in Perl. You
need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules (available from
CPAN) or the PDL extension (also available from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect the
results, use:
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the
results:
foreach $iterator (@array) {
some_func($iterator);
}
To call a function on each integer in a (small) range, you can
use:
@results = map { some_func($_) } (5 .. 25);
but you should be aware that the `..' operator creates an array
of all integers in the range. This can take a lot of memory for
large ranges. Instead use:
@results = ();
for ($i=5; $i < 500_005; $i++) {
push(@results, some_func($i));
}
How can I output Roman numerals?
Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
Why aren't my random numbers random?
If you're using a version of Perl before 5.004, you must call
`srand' once at the start of your program to seed the random
number generator. 5.004 and later automatically call `srand' at
the beginning. Don't call `srand' more than once--you make your
numbers less random, rather than more.
Computers are good at being predictable and bad at being random
(despite appearances caused by bugs in your programs :-).
http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
Phoenix, talks more about this.. John von Neumann said, ``Anyone
who attempts to generate random numbers by deterministic means
is, of course, living in a state of sin.''
If you want numbers that are more random than `rand' with
`srand' provides, you should also check out the
Math::TrulyRandom module from CPAN. It uses the imperfections in
your system's timer to generate random numbers, but this takes
quite a while. If you want a better pseudorandom generator than
comes with your operating system, look at ``Numerical Recipes in
C'' at http://www.nr.com/ .
Data: Dates
How do I find the week-of-the-year/day-of-the-year?
The day of the year is in the array returned by localtime() (see
the section on "localtime" in the perlfunc manpage):
$day_of_year = (localtime(time()))[7];
or more legibly (in 5.004 or higher):
use Time::localtime;
$day_of_year = localtime(time())->yday;
You can find the week of the year by dividing this by 7:
$week_of_year = int($day_of_year / 7);
Of course, this believes that weeks start at zero. The
Date::Calc module from CPAN has a lot of date calculation
functions, including day of the year, week of the year, and so
on. Note that not all businesses consider ``week 1'' to be the
same; for example, American businesses often consider the first
week with a Monday in it to be Work Week #1, despite ISO 8601,
which considers WW1 to be the first week with a Thursday in it.
How do I find the current century or millennium?
Use the following simple functions:
sub get_century {
return int((((localtime(shift || time))[5] + 1999))/100);
}
sub get_millennium {
return 1+int((((localtime(shift || time))[5] + 1899))/1000);
}
On some systems, you'll find that the POSIX module's strftime()
function has been extended in a non-standard way to use a `%C'
format, which they sometimes claim is the "century". It isn't,
because on most such systems, this is only the first two digits
of the four-digit year, and thus cannot be used to reliably
determine the current century or millennium.
How can I compare two dates and find the difference?
If you're storing your dates as epoch seconds then simply
subtract one from the other. If you've got a structured date
(distinct year, day, month, hour, minute, seconds values), then
for reasons of accessibility, simplicity, and efficiency, merely
use either timelocal or timegm (from the Time::Local module in
the standard distribution) to reduce structured dates to epoch
seconds. However, if you don't know the precise format of your
dates, then you should probably use either of the Date::Manip
and Date::Calc modules from CPAN before you go hacking up your
own parsing routine to handle arbitrary date formats.
How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same
format, you can split it up and pass the parts to `timelocal' in
the standard Time::Local module. Otherwise, you should look into
the Date::Calc and Date::Manip modules from CPAN.
How can I find the Julian Day?
You could use Date::Calc's Delta_Days function and calculate the
number of days from there. Assuming that's what you really want,
that is.
Before you immerse yourself too deeply in this, be sure to
verify that it is the *Julian* Day you really want. Are they
really just interested in a way of getting serial days so that
they can do date arithmetic? If you are interested in performing
date arithmetic, this can be done using either Date::Manip or
Date::Calc, without converting to Julian Day first.
There is too much confusion on this issue to cover in this FAQ,
but the term is applied (correctly) to a calendar now supplanted
by the Gregorian Calendar, with the Julian Calendar failing to
adjust properly for leap years on centennial years (among other
annoyances). The term is also used (incorrectly) to mean: [1]
days in the Gregorian Calendar; and [2] days since a particular
starting time or `epoch', usually 1970 in the Unix world and
1980 in the MS-DOS/Windows world. If you find that it is not the
first meaning that you really want, then check out the
Date::Manip and Date::Calc modules. (Thanks to David Cassell for
most of this text.)
There is also an example of Julian date calculation that should
help you in
http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Tim
e/JulianDay.pm.gz
How do I find yesterday's date?
The `time()' function returns the current time in seconds since
the epoch. Take twenty-four hours off that:
$yesterday = time() - ( 24 * 60 * 60 );
Then you can pass this to `localtime()' and get the individual
year, month, day, hour, minute, seconds values.
Note very carefully that the code above assumes that your days
are twenty-four hours each. For most people, there are two days
a year when they aren't: the switch to and from summer time
throws this off. A solution to this issue is offered by Russ
Allbery.
sub yesterday {
my $now = defined $_[0] ? $_[0] : time;
my $then = $now - 60 * 60 * 24;
my $ndst = (localtime $now)[8] > 0;
my $tdst = (localtime $then)[8] > 0;
$then - ($tdst - $ndst) * 60 * 60;
}
# Should give you "this time yesterday" in seconds since epoch relative to
# the first argument or the current time if no argument is given and
# suitable for passing to localtime or whatever else you need to do with
# it. $ndst is whether we're currently in daylight savings time; $tdst is
# whether the point 24 hours ago was in daylight savings time. If $tdst
# and $ndst are the same, a boundary wasn't crossed, and the correction
# will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
# from yesterday's time since we gained an extra hour while going off
# daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
# negative hour (add an hour) to yesterday's time since we lost an hour.
#
# All of this is because during those days when one switches off or onto
# DST, a "day" isn't 24 hours long; it's either 23 or 25.
#
# The explicit settings of $ndst and $tdst are necessary because localtime
# only says it returns the system tm struct, and the system tm struct at
# least on Solaris doesn't guarantee any particuliar positive value (like,
# say, 1) for isdst, just a positive value. And that value can
# potentially be negative, if DST information isn't available (this sub
# just treats those cases like no DST).
#
# Note that between 2am and 3am on the day after the time zone switches
# off daylight savings time, the exact hour of "yesterday" corresponding
# to the current hour is not clearly defined. Note also that if used
# between 2am and 3am the day after the change to daylight savings time,
# the result will be between 3am and 4am of the previous day; it's
# arguable whether this is correct.
#
# This sub does not attempt to deal with leap seconds (most things don't).
#
# Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
# This code is in the public domain
Does Perl have a year 2000 problem? Is Perl Y2K compliant?
Short answer: No, Perl does not have a Year 2000 problem. Yes,
Perl is Y2K compliant (whatever that means). The programmers
you've hired to use it, however, probably are not.
Long answer: The question belies a true understanding of the
issue. Perl is just as Y2K compliant as your pencil--no more,
and no less. Can you use your pencil to write a non-Y2K-
compliant memo? Of course you can. Is that the pencil's fault?
Of course it isn't.
The date and time functions supplied with perl (gmtime and
localtime) supply adequate information to determine the year
well beyond 2000 (2038 is when trouble strikes for 32-bit
machines). The year returned by these functions when used in an
array context is the year minus 1900. For years between 1910 and
1999 this *happens* to be a 2-digit decimal number. To avoid the
year 2000 problem simply do not treat the year as a 2-digit
number. It isn't.
When gmtime() and localtime() are used in scalar context they
return a timestamp string that contains a fully-expanded year.
For example, `$timestamp = gmtime(1005613200)' sets $timestamp
to "Tue Nov 13 01:00:00 2001". There's no year 2000 problem
here.
That doesn't mean that Perl can't be used to create non-Y2K
compliant programs. It can. But so can your pencil. It's the
fault of the user, not the language. At the risk of inflaming
the NRA: ``Perl doesn't break Y2K, people do.'' See
http://language.perl.com/news/y2k.html for a longer exposition.
Data: Strings
How do I validate input?
The answer to this question is usually a regular expression,
perhaps with auxiliary logic. See the more specific questions
(numbers, mail addresses, etc.) for details.
How do I unescape a string?
It depends just what you mean by ``escape''. URL escapes are
dealt with in the perlfaq9 manpage. Shell escapes with the
backslash (`\') character are removed with:
s/\\(.)/$1/g;
This won't expand `"\n"' or `"\t"' or any other special escapes.
How do I remove consecutive pairs of characters?
To turn `"abbcccd"' into `"abccd"':
s/(.)\1/$1/g; # add /s to include newlines
Here's a solution that turns "abbcccd" to "abcd":
y///cs; # y == tr, but shorter :-)
How do I expand function calls in a string?
This is documented in the perlref manpage. In general, this is
fraught with quoting and readability problems, but it is
possible. To interpolate a subroutine call (in list context)
into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
If you prefer scalar context, similar chicanery is also useful
for arbitrary expressions:
print "That yields ${\($n + 5)} widgets\n";
Version 5.004 of Perl had a bug that gave list context to the
expression in `${...}', but this is fixed in version 5.005.
See also ``How can I expand variables in text strings?'' in this
section of the FAQ.
How do I find matching/nesting anything?
This isn't something that can be done in one regular expression,
no matter how complicated. To find something between two single
characters, a pattern like `/x([^x]*)x/' will get the
intervening bits in $1. For multiple ones, then something more
like `/alpha(.*?)omega/' would be needed. But none of these
deals with nested patterns, nor can they. For that you'll have
to write a parser.
If you are serious about writing a parser, there are a number of
modules or oddities that will make your life a lot easier. There
is the CPAN module Parse::RecDescent, the standard module
Text::Balanced, the byacc program, the CPAN module Parse::Yapp,
and Mark-Jason Dominus's excellent *py* tool at
http://www.plover.com/~mjd/perl/py/ .
One simple destructive, inside-out approach that you might try
is to pull out the smallest nesting parts one at a time:
while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
# do something with $1
}
A more complicated and sneaky approach is to make Perl's regular
expression engine do it for you. This is courtesy Dean Inada,
and rather has the nature of an Obfuscated Perl Contest entry,
but it really does work:
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
@$ = (eval{/$re/},$@!~/unmatched/);
print join("\n",@$[0..$#$]) if( $$[-1] );
How do I reverse a string?
Use reverse() in scalar context, as documented in the "reverse"
entry in the perlfunc manpage.
$reversed = reverse $string;
How do I expand tabs in a string?
You can do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
Or you can just use the Text::Tabs module (part of the standard
perl distribution).
use Text::Tabs;
@expanded_lines = expand(@lines_with_tabs);
How do I reformat a paragraph?
Use Text::Wrap (part of the standard perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
The paragraphs you give to Text::Wrap should not contain
embedded newlines. Text::Wrap doesn't justify the lines (flush-
right).
How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
substr():
$first_byte = substr($a, 0, 1);
If you want to modify part of a string, the simplest way is
often to use substr() as an lvalue:
substr($a, 0, 3) = "Tom";
Although those with a pattern matching kind of thought process
will likely prefer:
$a =~ s/^.../Tom/;
How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, let's say you
want to change the fifth occurrence of `"whoever"' or
`"whomever"' into `"whosoever"' or `"whomsoever"', case
insensitively. These all assume that $_ contains the string to
be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;
In the more general case, you can use the `/g' modifier in a
`while' loop, keeping count of matches.
$WANT = 3;
$count = 0;
$_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
}
}
That prints out: `"The third fish is a red one."' You can also
use a repetition count and repeated pattern like this:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
How can I count the number of occurrences of a substring within a string?
There are a number of ways, with varying efficiency: If you want
a count of a certain single character (X) within a string, you
can use the `tr///' function like so:
$string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
print "There are $count X characters in the string";
This is fine if you are just looking for a single character.
However, if you are trying to count multiple character
substrings within a larger string, `tr///' won't work. What you
can do is wrap a while() loop around a global pattern match. For
example, let's count negative integers:
$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
How do I capitalize all the words on one line?
To make the first letter of each word upper case:
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning "`don't do it'" into
"`Don'T Do It'". Sometimes you might want this, instead
(Suggested by Brian Foy):
$string =~ s/ (
(^\w) #at the beginning of the line
| # or
(\s\w) #preceded by whitespace
)
/\U$1/xg;
$string =~ /([\w']+)/\u\L$1/g;
To make the whole line upper case:
$line = uc($line);
To force each word to be lower case, with the first letter upper
case:
$line =~ s/(\w+)/\u\L$1/g;
You can (and probably should) enable locale awareness of those
characters by placing a `use locale' pragma in your program. See
the perllocale manpage for endless details on locales.
This is sometimes referred to as putting something into "title
case", but that's not quite accurate. Consider the proper
capitalization of the movie *Dr. Strangelove or: How I Learned
to Stop Worrying and Love the Bomb*, for example.
How can I split a [character] delimited string except when inside
[character]? (Comma-separated files)
Take the example case of trying to split a string that is comma-
separated into its different fields. (We'll pretend you said
comma-separated, not comma-delimited, which is different and
almost never what you mean.) You can't use `split(/,/)' because
you shouldn't split if the comma is inside quotes. For example,
take a data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex
problem. Thankfully, we have Jeffrey Friedl, author of a highly
recommended book on regular expressions, to handle these for us.
He suggests (assuming your string is contained in $text):
@new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a quotation-
mark-delimited field, escape them with backslashes (eg, `"like
\"this\""'. Unescaping them is a task addressed earlier in this
section.
Alternatively, the Text::ParseWords module (part of the standard
perl distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
There's also a Text::CSV module on CPAN.
How do I strip blank space from the beginning/end of a string?
Although the simplest approach would seem to be:
$string =~ s/^\s*(.*?)\s*$/$1/;
Not only is this unnecessarily slow and destructive, it also
fails with embedded newlines. It is much faster to do this
operation in two steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
Or more nicely written as:
for ($string) {
s/^\s+//;
s/\s+$//;
}
This idiom takes advantage of the `foreach' loop's aliasing
behavior to factor out common code. You can do this on several
strings at once, or arrays, or even the values of a hash if you
use a slice:
# trim whitespace in the scalar, the array,
# and all the values in the hash
foreach ($scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+$//;
}
How do I pad a string with blanks or pad a number with zeroes?
(This answer contributed by Uri Guttman, with kibitzing from
Bart Lateur.)
In the following examples, `$pad_len' is the length to which you
wish to pad the string, `$text' or `$num' contains the string to
be padded, and `$pad_char' contains the padding character. You
can use a single character string constant instead of the
`$pad_char' variable if you know what it is in advance. And in
the same way you can use an integer in place of `$pad_len' if
you know the pad length in advance.
The simplest method uses the `sprintf' function. It can pad on
the left or right with blanks and on the left with zeroes and it
will not truncate the result. The `pack' function can only pad
strings on the right with blanks and it will truncate the result
to a maximum length of `$pad_len'.
# Left padding a string with blanks (no truncation):
$padded = sprintf("%${pad_len}s", $text);
# Right padding a string with blanks (no truncation):
$padded = sprintf("%-${pad_len}s", $text);
# Left padding a number with 0 (no truncation):
$padded = sprintf("%0${pad_len}d", $num);
# Right padding a string with blanks using pack (will truncate):
$padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you
can use one of the following methods. They all generate a pad
string with the `x' operator and combine that with `$text'.
These methods do not truncate `$text'.
Left and right padding with any character, creating a new
string:
$padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
$padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying `$text'
directly:
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
$text .= $pad_char x ( $pad_len - length( $text ) );
How do I extract selected columns from a string?
Use substr() or unpack(), both documented in the perlfunc
manpage. If you prefer thinking in terms of columns instead of
widths, you can use this kind of thing:
# determine the unpack format needed to split Linux ps output
# arguments are cut columns
my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
sub cut2fmt {
my(@positions) = @_;
my $template = '';
my $lastpos = 1;
for my $place (@positions) {
$template .= "A" . ($place - $lastpos) . " ";
$lastpos = $place;
}
$template .= "A*";
return $template;
}
How do I find the soundex value of a string?
Use the standard Text::Soundex module distributed with perl. But
before you do so, you may want to determine whether `soundex' is
in fact what you think it is. Knuth's soundex algorithm
compresses words into a small space, and so it does not
necessarily distinguish between two words which you might want
to appear separately. For example, the last names `Knuth' and
`Kant' are both mapped to the soundex code K530. If
Text::Soundex does not do what you are looking for, you might
want to consider the String::Approx module available at CPAN.
How can I expand variables in text strings?
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
If those were both global variables, then this would suffice:
$text =~ s/\$(\w+)/${$1}/g; # no /e needed
But since they are probably lexicals, or at least, they could
be, you'd have to do this:
$text =~ s/(\$\w+)/$1/eeg;
die if $@; # needed /ee, not /e
It's probably better in the general case to treat those
variables as entries in some special hash. For example:
%user_defs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
See also ``How do I expand function calls in a string?'' in this
section of the FAQ.
What's wrong with always quoting "$vars"?
The problem is that those double-quotes force stringification,
coercing numbers and references into strings, even when you
don't want them to be. Think of it this way: double-quote
expansion is used to produce new strings. If you already have a
string, why do you need more?
If you get used to writing odd things like these:
print "$var"; # BAD
$new = "$old"; # BAD
somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases) be
the simpler and more direct:
print $var;
$new = $old;
somefunc($var);
Otherwise, besides slowing you down, you're going to break code
when the thing in the scalar is actually neither a string nor a
number, but a reference:
func(\@array);
sub func {
my $aref = shift;
my $oref = "$aref"; # WRONG
}
You can also get into subtle problems on those few operations in
Perl that actually do care about the difference between a string
and a number, such as the magical `++' autoincrement operator or
the syscall() function.
Stringification also destroys arrays.
@lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right
Why don't my <<HERE documents work?
Check for these three things:
1. There must be no space after the << part.
2. There (probably) should be a semicolon at the end.
3. You can't (easily) have any space in front of the tag.
If you want to indent the text in the here document, you can do
this:
# all in one
($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGET
But the HERE_TARGET must still be flush against the margin. If
you want that indented also, you'll have to quote in the
indentation.
($quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s*--/\n--/;
A nice general-purpose fixer-upper function for indented here
documents follows. It expects to be called with a here document
as its argument. It looks to see whether each line begins with a
common substring, and if so, strips that off. Otherwise, it
takes the amount of leading white space found on the first line
and removes that much off each subsequent line.
sub fix {
local $_ = shift;
my ($white, $leader); # common white space and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}
This works with leading special strings, dynamically determined:
$remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
@@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading white space, with remaining
indentation correctly preserved:
$poem = fix<<EVER_ON_AND_ON;
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
EVER_ON_AND_ON
Data: Arrays
What is the difference between a list and an array?
An array has a changeable length. A list does not. An array is
something you can push or pop, while a list is a set of values.
Some people make the distinction that a list is a value while an
array is a variable. Subroutines are passed and return lists,
you put things into list context, you initialize arrays with
lists, and you foreach() across a list. `@' variables are
arrays, anonymous arrays are arrays, arrays in scalar context
behave like the number of elements in them, subroutines access
their arguments through the array `@_', push/pop/shift only work
on arrays.
As a side note, there's no such thing as a list in scalar
context. When you say
$scalar = (2, 5, 7, 9);
you're using the comma operator in scalar context, so it uses
the scalar comma operator. There never was a list there at all!
This causes the last value to be returned: 9.
What is the difference between $array[1] and @array[1]?
The former is a scalar value, the latter an array slice, which
makes it a list with one (scalar) value. You should use $ when
you want a scalar value (most of the time) and @ when you want a
list with one scalar value in it (very, very rarely; nearly
never, in fact).
Sometimes it doesn't make a difference, but sometimes it does.
For example, compare:
$good[0] = `some program that outputs several lines`;
with
@bad[0] = `same program that outputs several lines`;
The -w flag will warn you about these matters.
How can I remove duplicate elements from a list or array?
There are several possible ways, depending on whether the array
is ordered and whether you wish to preserve the ordering.
a) If @in is sorted, and you want @out to be sorted:
(this assumes all true values in the array)
$prev = 'nonesuch';
@out = grep($_ ne $prev && ($prev = $_), @in);
This is nice in that it doesn't use much extra memory,
simulating uniq(1)'s behavior of removing only adjacent
duplicates. It's less nice in that it won't work with false
values like undef, 0, or ""; "0 but true" is ok, though.
b) If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$saw{$_}++, @in);
c) Like (b), but @in contains only small integers:
@out = grep(!$saw[$_]++, @in);
d) A way to do (b) without any loops or greps:
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
e) Like (d), but @in contains only small positive integers:
undef @ary;
@ary[@in] = @in;
@out = @ary;
But perhaps you should have been using a hash all along, eh?
How can I tell whether a list or array contains a certain element?
Hearing the word "in" is an *in*dication that you probably
should have used a hash, not a list or array, to store your
data. Hashes are designed to answer this question quickly and
efficiently. Arrays aren't.
That being said, there are several ways to approach this. If you
are going to make this query many times over arbitrary string
values, the fastest way is probably to invert the original array
and keep an associative array lying about whose keys are the
first array's values.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
undef %is_blue;
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might have
been a good idea to keep the blues all in a hash in the first
place.
If the values are all small integers, you could use a simple
indexed array. This kind of an array will take up less space:
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
undef @is_tiny_prime;
for (@primes) { $is_tiny_prime[$_] = 1 }
# or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings, you
can save quite a lot of space by using bit strings instead:
@articles = ( 1..10, 150..2000, 2017 );
undef $read;
for (@articles) { vec($read,$_,1) = 1 }
Now check whether `vec($read,$n,1)' is true for some `$n'.
Please do not use
$is_there = grep $_ eq $whatever, @array;
or worse yet
$is_there = grep /$whatever/, @array;
These are slow (checks every element even if the first matches),
inefficient (same reason), and potentially buggy (what if there
are regex characters in $whatever?). If you're only testing
once, then use:
$is_there = 0;
foreach $elt (@array) {
if ($elt eq $elt_to_find) {
$is_there = 1;
last;
}
}
if ($is_there) { ... }
How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes that
each element is unique in a given array:
@union = @intersection = @difference = ();
%count = ();
foreach $element (@array1, @array2) { $count{$element}++ }
foreach $element (keys %count) {
push @union, $element;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
Note that this is the *symmetric difference*, that is, all
elements in either A or in B, but not in both. Think of it as an
xor operation.
How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a
stringwise comparison, and does not distinguish defined versus
undefined empty strings. Modify if you have other needs.
$are_equal = compare_arrays(\@frogs, \@toads);
sub compare_arrays {
my ($first, $second) = @_;
local $^W = 0; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
}
return 1;
}
For multilevel structures, you may wish to use an approach more
like this one. It uses the CPAN module FreezeThaw:
use FreezeThaw qw(cmpStr);
@a = @b = ( "this", "that", [ "more", "stuff" ] );
printf "a and b contain %s arrays\n",
cmpStr(\@a, \@b) == 0
? "the same"
: "different";
This approach also works for comparing hashes. Here we'll
demonstrate two different answers:
use FreezeThaw qw(cmpStr cmpStrHard);
%a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
$a{EXTRA} = \%b;
$b{EXTRA} = \%a;
printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
The first reports that both those the hashes contain the same
data, while the second reports that they do not. Which you
prefer is left as an exercise to the reader.
How do I find the first array element for which a condition is true?
You can use this if you care about the index:
for ($i= 0; $i < @array; $i++) {
if ($array[$i] eq "Waldo") {
$found_index = $i;
last;
}
}
Now `$found_index' has what you want.
How do I handle linked lists?
In general, you usually don't need a linked list in Perl, since
with regular arrays, you can push and pop or shift and unshift
at either end, or you can use splice to add and/or remove
arbitrary number of elements at arbitrary points. Both pop and
shift are both O(1) operations on perl's dynamic arrays. In the
absence of shifts and pops, push in general needs to reallocate
on the order every log(N) times, and unshift will need to copy
pointers each time.
If you really, really wanted, you could use structures as
described in the perldsc manpage or the perltoot manpage and do
just what the algorithm book tells you to do. For example,
imagine a list node like this:
$node = {
VALUE => 42,
LINK => undef,
};
You could walk the list this way:
print "List: ";
for ($node = $head; $node; $node = $node->{LINK}) {
print $node->{VALUE}, " ";
}
print "\n";
You could grow the list this way:
my ($head, $tail);
$tail = append($head, 1); # grow a new head
for $value ( 2 .. 10 ) {
$tail = append($tail, $value);
}
sub append {
my($list, $value) = @_;
my $node = { VALUE => $value };
if ($list) {
$node->{LINK} = $list->{LINK};
$list->{LINK} = $node;
} else {
$_[0] = $node; # replace caller's version
}
return $node;
}
But again, Perl's built-in are virtually always good enough.
How do I handle circular lists?
Circular lists could be handled in the traditional fashion with
linked lists, or you could just do something like this with an
array:
unshift(@array, pop(@array)); # the last shall be first
push(@array, shift(@array)); # and vice versa
How do I shuffle an array randomly?
Use this:
# fisher_yates_shuffle( \@array ) :
# generate a random permutation of @array in place
sub fisher_yates_shuffle {
my $array = shift;
my $i;
for ($i = @$array; --$i; ) {
my $j = int rand ($i+1);
next if $i == $j;
@$array[$i,$j] = @$array[$j,$i];
}
}
fisher_yates_shuffle( \@array ); # permutes @array in place
You've probably seen shuffling algorithms that work using
splice, randomly picking another element to swap the current
element with:
srand;
@new = ();
@old = 1 .. 10; # just a demo
while (@old) {
push(@new, splice(@old, rand @old, 1));
}
This is bad because splice is already O(N), and since you do it
N times, you just invented a quadratic algorithm; that is,
O(N**2). This does not scale, although Perl is so efficient that
you probably won't notice this until you have rather largish
arrays.
How do I process/modify each element of an array?
Use `for'/`foreach':
for (@lines) {
s/foo/bar/; # change that word
y/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
for (@volumes = @radii) { # @volumes has changed parts
$_ **= 3;
$_ *= (4/3) * 3.14159; # this will be constant folded
}
If you want to do the same thing to modify the values of the
hash, you may not use the `values' function, oddly enough. You
need a slice:
for $orbit ( @orbits{keys %orbits} ) {
($orbit **= 3) *= (4/3) * 3.14159;
}
How do I select a random element from an array?
Use the rand() function (see the "rand" entry in the perlfunc
manpage):
# at the top of the program:
srand; # not needed for 5.004 and later
# then later on
$index = rand @array;
$element = $array[$index];
Make sure you *only call srand once per program, if then*. If
you are calling it more than once (such as before each call to
rand), you're almost certainly doing something wrong.
How do I permute N elements of a list?
Here's a little program that generates all permutations of all
the words on each line of input. The algorithm embodied in the
permute() function should work on any list:
#!/usr/bin/perl -n
# tsc-permute: permute each word of input
permute([split], []);
sub permute {
my @items = @{ $_[0] };
my @perms = @{ $_[1] };
unless (@items) {
print "@perms\n";
} else {
my(@newitems,@newperms,$i);
foreach $i (0 .. $#items) {
@newitems = @items;
@newperms = @perms;
unshift(@newperms, splice(@newitems, $i, 1));
permute([@newitems], [@newperms]);
}
}
}
How do I sort an array by (anything)?
Supply a comparison function to sort() (described in the "sort"
entry in the perlfunc manpage):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would
sort `(1, 2, 10)' into `(1, 10, 2)'. `<=>', used above, is the
numerical comparison operator.
If you have a complicated function needed to pull out the part
you want to sort on, then don't do it inside the sort function.
Pull it out first, because the sort BLOCK can be called many
times for the same element. Here's an example of how to pull out
the first word after the first number on each item, and then
sort those words case-insensitively.
@idx = ();
for (@data) {
($item) = /\d+\s*(\S+)/;
push @idx, uc($item);
}
@sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
Which could also be written this way, using a trick that's come
to be known as the Schwartzian Transform:
@sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is
useful.
@sorted = sort { field1($a) <=> field1($b) ||
field2($a) cmp field2($b) ||
field3($a) cmp field3($b)
} @data;
This can be conveniently combined with precalculation of keys as
given above.
See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more
about this approach.
See also the question below on sorting hashes.
How do I manipulate arrays of bits?
Use pack() and unpack(), or else vec() and the bitwise
operations.
For example, this sets $vec to have bit N set if $ints[N] was
set:
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
And here's how, given a vector in $vec, you can get those bits
into your @ints array:
sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/\0// / length $vec > 0.95) {
use integer;
my $i;
# This method is faster with mostly null-bytes
while($vec =~ /[^\0]/g ) {
$i = -9 + 8 * pos $vec;
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
}
} else {
# This method is a fast general algorithm
use integer;
my $bits = unpack "b*", $vec;
push @ints, 0 if $bits =~ s/^(\d)// && $1;
push @ints, pos $bits while($bits =~ /1/g);
}
return \@ints;
}
This method gets faster the more sparse the bit vector is.
(Courtesy of Tim Bunce and Winfried Koenig.)
Here's a demo on how to use vec():
# vec demo
$vector = "\xff\x0f\xef\xfe";
print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", $vector), "\n";
$is_set = vec($vector, 23, 1);
print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
pvec($vector);
set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);
set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);
set_vec(0,32,17);
set_vec(1,32,17);
sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print "offset=$offset width=$width value=$value\n";
pvec($vector);
}
sub pvec {
my $vector = shift;
my $bits = unpack("b*", $vector);
my $i = 0;
my $BASE = 8;
print "vector length in bytes: ", length($vector), "\n";
@bytes = unpack("A8" x length($vector), $bits);
print "bits are: @bytes\n\n";
}
Why does defined() return true on empty arrays and hashes?
The short story is that you should probably only use defined on
scalars or functions, not on aggregates (arrays and hashes). See
the "defined" entry in the perlfunc manpage in the 5.004 release
or later of Perl for more detail.
Data: Hashes (Associative Arrays)
How do I process an entire hash?
Use the each() function (see the "each" entry in the perlfunc
manpage) if you don't care whether it's sorted:
while ( ($key, $value) = each %hash) {
print "$key = $value\n";
}
If you want it sorted, you'll have to use foreach() on the
result of sorting the keys as shown in an earlier question.
What happens if I add or remove keys from a hash while iterating over it?
Don't do that. :-)
[lwall] In Perl 4, you were not allowed to modify a hash at all
while interating over it. In Perl 5 you can delete from it, but
you still can't add to it, because that might cause a doubling
of the hash table, in which half the entries get copied up to
the new top half of the table, at which point you've totally
bamboozled the interator code. Even if the table doesn't double,
there's no telling whether your new entry will be inserted
before or after the current iterator position.
Either treasure up your changes and make them after the iterator
finishes, or use keys to fetch all the old keys at once, and
iterate over the list of keys.
How do I look up a hash element by value?
Create a reverse hash:
%by_value = reverse %by_key;
$key = $by_value{$value};
That's not particularly efficient. It would be more space-
efficient to use:
while (($key, $value) = each %by_key) {
$by_value{$value} = $key;
}
If your hash could have repeated values, the methods above will
only find one of the associated keys. This may or may not worry
you. If it does worry you, you can always reverse the hash into
a hash of arrays instead:
while (($key, $value) = each %by_key) {
push @{$key_list_by_value{$value}}, $key;
}
How can I know how many entries are in a hash?
If you mean how many keys, then all you have to do is take the
scalar sense of the keys() function:
$num_keys = scalar keys %hash;
In void context, the keys() function just resets the iterator,
which is faster for tied hashes than would be iterating through
the whole hash, one key-value pair at a time.
How do I sort a hash (optionally by value instead of key)?
Internally, hashes are stored in a way that prevents you from
imposing an order on key-value pairs. Instead, you have to sort
a list of the keys or values:
@keys = sort keys %hash; # sorted by key
@keys = sort {
$hash{$a} cmp $hash{$b}
} keys %hash; # and by value
Here we'll do a reverse numeric sort by value, and if two keys
are identical, sort by length of key, and if that fails, by
straight ASCII comparison of the keys (well, possibly modified
by your locale -- see the perllocale manpage).
@keys = sort {
$hash{$b} <=> $hash{$a}
||
length($b) <=> length($a)
||
$a cmp $b
} keys %hash;
How can I always keep my hash sorted?
You can look into using the DB_File module and tie() using the
$DB_BTREE hash bindings as documented in the section on "In
Memory Databases" in the DB_File manpage. The Tie::IxHash module
from CPAN might also be instructive.
What's the difference between "delete" and "undef" with hashes?
Hashes are pairs of scalars: the first is the key, the second is
the value. The key will be coerced to a string, although the
value can be any kind of scalar: string, number, or reference.
If a key `$key' is present in the array, `exists($key)' will
return true. The value for a given key can be `undef', in which
case `$array{$key}' will be `undef' while `$exists{$key}' will
return true. This corresponds to (`$key', `undef') being in the
hash.
Pictures help... here's the `%ary' table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
And these conditions hold
$ary{'a'} is true
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is true
exists $ary{'a'} is true (perl5 only)
grep ($_ eq 'a', keys %ary) is true
If you now say
undef $ary{'a'}
your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is FALSE
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is FALSE
exists $ary{'a'} is true (perl5 only)
grep ($_ eq 'a', keys %ary) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $ary{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is false
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is false
exists $ary{'a'} is FALSE (perl5 only)
grep ($_ eq 'a', keys %ary) is FALSE
See, the whole entry is gone!
Why don't my tied hashes make the defined/exists distinction?
They may or may not implement the EXISTS() and DEFINED() methods
differently. For example, there isn't the concept of undef with
hashes that are tied to DBM* files. This means the true/false
tables above will give different results when used on such a
hash. It also means that exists and defined do the same thing
with a DBM* file, and what they end up doing is not what they do
with ordinary hashes.
How do I reset an each() operation part-way through?
Using `keys %hash' in scalar context returns the number of keys
in the hash *and* resets the iterator associated with the hash.
You may need to do this if you use `last' to exit a loop early
so that when you re-enter it, the hash iterator has been reset.
How can I get the unique keys from two hashes?
First you extract the keys from the hashes into lists, then
solve the "removing duplicates" problem described above. For
example:
%seen = ();
for $element (keys(%foo), keys(%bar)) {
$seen{$element}++;
}
@uniq = keys %seen;
Or more succinctly:
@uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
%seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
@uniq = keys %seen;
How can I store a multidimensional array in a DBM file?
Either stringify the structure yourself (no fun), or else get
the MLDBM (which uses Data::Dumper) module from CPAN and layer
it on top of either DB_File or GDBM_File.
How can I make my hash remember the order I put elements into it?
Use the Tie::IxHash from CPAN.
use Tie::IxHash;
tie(%myhash, Tie::IxHash);
for ($i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
@keys = keys %myhash;
# @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a hash create it?
If you say something like:
somefunc($hash{"nonesuch key here"});
Then that element "autovivifies"; that is, it springs into
existence whether you store something there or not. That's
because functions get scalars passed in by reference. If
somefunc() modifies `$_[0]', it has to be ready to write it back
into the caller's version.
This has been fixed as of perl5.004.
Normally, merely accessing a key's value for a nonexistent key
does *not* cause that key to be forever there. This is different
than awk's behavior.
How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Usually a hash ref, perhaps like this:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};
References are documented in the perlref manpage and the
upcoming the perlreftut manpage. Examples of complex data
structures are given in the perldsc manpage and the perllol
manpage. Examples of structures and object-oriented classes are
in the perltoot manpage.
How can I use a reference as a hash key?
You can't do this directly, but you could use the standard
Tie::Refhash module distributed with perl.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so this shouldn't be a problem. For
example, this works fine (assuming the files are found):
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On less elegant (read: Byzantine) systems, however, you have to
play tedious games with "text" versus "binary" files. See the
section on "binmode" in the perlfunc manpage or the perlopentut
manpage. Most of these ancient-thinking systems are curses out
of Microsoft, who seem to be committed to putting the backward
into backward compatibility.
If you're concerned about 8-bit ASCII data, then see the
perllocale manpage.
If you want to deal with multibyte characters, however, there
are some gotchas. See the section on Regular Expressions.
How do I determine whether a scalar is a number/whole/integer/float?
Assuming that you don't care about IEEE notations like "NaN" or
"Infinity", you probably just want to use a regular expression.
if (/\D/) { print "has nondigits\n" }
if (/^\d+$/) { print "is a whole number\n" }
if (/^-?\d+$/) { print "is an integer\n" }
if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
{ print "a C float" }
If you're on a POSIX system, Perl's supports the `POSIX::strtod'
function. Its semantics are somewhat cumbersome, so here's a
`getnum' wrapper function for more convenient access. This
function takes a string and returns the number it found, or
`undef' for input that isn't a C float. The `is_numeric'
function is a front end to `getnum' if you just want to say,
``Is this a float?''
sub getnum {
use POSIX qw(strtod);
my $str = shift;
$str =~ s/^\s+//;
$str =~ s/\s+$//;
$! = 0;
my($num, $unparsed) = strtod($str);
if (($str eq '') || ($unparsed != 0) || $!) {
return undef;
} else {
return $num;
}
}
sub is_numeric { defined &getnum }
Or you could check out http://www.perl.com/CPAN/modules/by-
module/String/String-Scanf-1.1.tar.gz instead. The POSIX module
(part of the standard Perl distribution) provides the `strtol'
and `strtod' for converting strings to double and longs,
respectively.
How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM
modules. See the AnyDBM_File manpage. More generically, you
should consult the FreezeThaw, Storable, or Class::Eroot modules
from CPAN. Here's one example using Storable's `store' and
`retrieve' functions:
use Storable;
store(\%hash, "filename");
# later on...
$href = retrieve("filename"); # by ref
%hash = %{ retrieve("filename") }; # direct to hash
How do I print out or copy a recursive data structure?
The Data::Dumper module on CPAN (or the 5.005 release of Perl)
is great for printing out data structures. The Storable module,
found on CPAN, provides a function called `dclone' that
recursively copies its argument.
use Storable qw(dclone);
$r2 = dclone($r1);
Where $r1 can be a reference to any kind of data structure you'd
like. It will be deeply copied. Because `dclone' takes and
returns references, you'd have to add extra punctuation if you
had a hash of arrays that you wanted to copy.
%newhash = %{ dclone(\%oldhash) };
How do I define methods for every class/object?
Use the UNIVERSAL class (see the UNIVERSAL manpage).
How do I verify a credit card checksum?
Get the Business::CreditCard module from CPAN.
How do I pack arrays of doubles or floats for XS code?
The kgbpack.c code in the PGPLOT module on CPAN does just this.
If you're doing a lot of float or double processing, consider
using the PDL module from CPAN instead--it makes number-
crunching easy.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
When included as part of the Standard Version of Perl, or as
part of its complete documentation whether printed or otherwise,
this work may be distributed only under the terms of Perl's
Artistic License. Any distribution of this file or derivatives
thereof *outside* of that package require that special
arrangements be made with copyright holder.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun or for
profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.
--
But you have to allow a little for the desire to evangelize when you
think you have good news.
--Larry Wall in <1992Aug26.184221.29627@netlabs.com>
------------------------------
Date: Wed, 09 Jun 1999 18:59:29 GMT
From: dragnet@internalysis.com (Marc Bissonnette)
Subject: Separating array into alphabetical array of arrays
Message-Id: <lQy73.7511$ga.7054@news21.bellglobal.com>
I've recently completed a script that reads in a pipe deparate file of names,
addresses, etc, sorts them, formats them and prints to file, thanks to help
recieved from this group (thanks again!). Now here's what I need to do:
Rather than print one long file sorted alphabetically, I need to have a
separate file for each letter (i.e. all names starting with 'a' then 'b', etc)
Can someone point me in the right direction to do this? (Either separating the
array into separate arrays by letter, or printing to a file depending on the
first letter of an array)
Many thanks,
--
----------------------------
Marc Bissonnette
InternAlysis
Corporate Internet Research and Results!
http://www.internalysis.com
------------------------------
Date: Wed, 9 Jun 1999 19:03:47 GMT
From: Darrin Edwards <d-edwards@nospam.uchicago.edu>
Subject: Re: Using a hash slice with references?
Message-Id: <tgemjltcvg.fsf@noise.bsd.uchicago.edu>
klassa@aur.alcatel.com (John Klassa) writes:
> On 8 Jun 1999 09:05:47 -0700, Tom Christiansen <tchrist@mox.perl.com> wrote:
> > In the general case, @$hashref{K1,K2,K3}.
>
> Is there anyone else for whom this notation seems strange? I'm not saying
> it's incorrect; I'm just saying that it's not intuitive to me.
The way I dealt with learning reference syntax (in addition to
"read(perlref, perllol, perldsc) while 1;" :)) was to learn one form,
then gradually learn the forms that were equivalent to that, and then
only eventually learn to deal with situations in which the
"not-so-equivalent" forms were useful. TMTOWTDI meant I could be
writing programs that did what I (thought I) wanted even at the
beginning of this process.
> The whole
> notion of:
>
> $$hashref{$key}
>
> is weird, too.
Let's back up a bit. From perldata:
As in some shells, you can put curly brackets around the
name to delimit it from following alphanumerics. In fact,
an identifier within such curlies is forced to be a string,
as is any single identifier within a hash subscript.
So if I had a hash called %Nonref_hash, I could refer to an element as
my $value = $Nonref_hash{$key};
or
my $value = ${Nonref_hash}{$key};
Now, instead of putting a string of text inside the curlies giving the
name of the variable, we can put a variable with a (hard) reference
in it:
$Ref_hash = \%Nonref_hash;
my $value = ${$Ref_hash}{$key};
Some people consider _that_ weird, and so in simple cases like this
you're allowed to drop the curlies:
my $value = $$Ref_hash{$key};
Note that perlref actually discusses these in the opposite order;
i.e., the curlies are recommended for cases in which $$ @$ or %$
becomes cumbersome.
> I much prefer:
>
> $hashref->{$key}
>
> because it makes the fact that $hashref is a reference more obvious.
Note though that the right side of the arrow must be such that a
_scalar_ is returned by the expression; you can't do array or hash
slices this way. (I oughtta know, I tried to do just that once, and
was quite startled when perl whacked me in the nose with a rolled-up
newspaper. 'Bad user! Bad user! Read my docs again!' :)) That's why
perlref describes this as "syntactic sugar" and not something
_completely_ equivalent to the first two forms.
> To wit, if I say:
>
> @$hashref{$key1, $key2}
>
> then later decide I should use a single element rather than a slice, my
> inclination is to lop off the front and back (more or less):
>
> $hashref{$key1}
>
> (i.e. simply remove the "@" and the second key). However, at this point,
> I've got problems because there is no hash named %hashref.
Remember the above generalization though. Hopefully you wouldn't have
the inclination to take
@{Nonref_hash}{$key1, $key2};
and "lop it" to get
Nonref_hash{$key1}; #yikes! Something's missing
So your inclination "should" :) have been the same as in the nonref
case, not to _drop_ the @ when going from a list to a single value,
but to _change_ from @ to $.
> As I said, I understand what the notation is... I'm just not sure *why* it
> is, or why someone might think it's intuitive. :-)
It's the same way quantum mechanics is intuitive, i.e., _after_ you've
locked all your old intuitions in a cabinet in the basement and done
enough problems to get some nice, shiny, new intuitions. :)
Cheers,
Darrin
------------------------------
Date: 9 Jun 1999 18:17:22 GMT
From: gbacon@itsc.uah.edu (Greg Bacon)
Subject: Re: writing to a gzip file
Message-Id: <7jmb3i$gi0$1@info2.uah.edu>
In article <Pine.GSO.3.96.990609110044.23755F-100000@stripe.colorado.edu>,
Sean Sidelko <sidelko@stripe.Colorado.EDU> writes:
: i know you can read from a gzipped file, but is there a way to write to
: file while it is gzipped?
Take a look at the Compress::Zlib module. It's available on the CPAN.
Greg
--
Only two things are infinite, the universe and human stupidity, and I'm not
sure about the former.
-- Albert Einstein
------------------------------
Date: Wed, 9 Jun 1999 12:45:10 -0600
From: Sean Sidelko <sidelko@stripe.Colorado.EDU>
Subject: Re: writing to a gzip file
Message-Id: <Pine.GSO.3.96.990609124506.23755H-100000@stripe.Colorado.EDU>
thanks.
Sean
On 9 Jun 1999, Greg Bacon wrote:
: ) In article <Pine.GSO.3.96.990609110044.23755F-100000@stripe.colorado.edu>,
: ) Sean Sidelko <sidelko@stripe.Colorado.EDU> writes:
: ) : i know you can read from a gzipped file, but is there a way to write to
: ) : file while it is gzipped?
: )
: ) Take a look at the Compress::Zlib module. It's available on the CPAN.
: )
: ) Greg
: ) --
: ) Only two things are infinite, the universe and human stupidity, and I'm not
: ) sure about the former.
: ) -- Albert Einstein
: )
: )
------------------------------
Date: 9 Jun 1999 18:31:15 GMT
From: ilya@math.ohio-state.edu (Ilya Zakharevich)
Subject: Re: XS: newSVpv() and malloc
Message-Id: <7jmbtj$4sk$1@mathserv.mps.ohio-state.edu>
[A complimentary Cc of this posting was sent to Dan Sugalski
<sugalskd@netserve.ous.edu>],
who wrote in article <7jm6to$et9$1@news.NERO.NET>:
> :> Yep. All the functions that set an SV's value make copies of the
> :> data you pass.
>
> : Nope. There are .*use.* functions which make a copy. They may
> ^
> |
> I think you're missing the word "don't" here, right Ilya?
Oups, right!
Ilya
------------------------------
Date: 12 Dec 98 21:33:47 GMT (Last modified)
From: Perl-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Special: Digest Administrivia (Last modified: 12 Dec 98)
Message-Id: <null>
Administrivia:
Well, after 6 months, here's the answer to the quiz: what do we do about
comp.lang.perl.moderated. Answer: nothing.
]From: Russ Allbery <rra@stanford.edu>
]Date: 21 Sep 1998 19:53:43 -0700
]Subject: comp.lang.perl.moderated available via e-mail
]
]It is possible to subscribe to comp.lang.perl.moderated as a mailing list.
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------------------------------
End of Perl-Users Digest V8 Issue 5935
**************************************
