perlfaq4
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NAME
perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date:
1997/04/24 22:43:57 $)
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)?
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.
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?
For rounding to a certain number of digits, sprintf() or
printf() is usually the easiest route.
The POSIX module (part of the standard perl distribution)
implements ceil(), floor(), and a number of other
mathematical and trigonometric functions.
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.
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()
function (documented in the section on pack in the
perlfunc manpage):
$decimal = pack('B8', '10110110');
Here's an example of going the other way:
$binary_string = join('', unpack('B*', "\x29"));
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules
(available from CPAN) or the PDL extension (also available
from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and
collect the results, use:
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore
the results:
foreach $iterator (@array) {
&my_func($iterator);
}
To call a function on each integer in a (small) range, you
can use:
@results = map { &my_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; $i++) {
push(@results, &my_func($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?
The short explanation is that you're getting pseudorandom
numbers, not random ones, because that's how these things
work. A longer explanation is available on
http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of
Tom Phoenix.
You should also check out the Math::TrulyRandom module
from CPAN.
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.
How can I compare two date strings?
Use the Date::Manip or Date::DateCalc modules from CPAN.
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 one of the Date modules from CPAN.
How can I find the Julian Day?
Neither Date::Manip nor Date::DateCalc deal with Julian
days. Instead, there is an example of Julian date
calculation in
http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz,
which should help.
Does Perl have a year 2000 problem?
Not unless you use Perl to create one. 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). 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 a 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.
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, email 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;
Note that 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;
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 a
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";
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 tackled 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.
How do I reverse a string?
Use reverse() in a 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 the old-fashioned way:
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 may 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 regexp kind of thought process will
likely prefer
$a =~ s/^.../Tom/;
How do I change the Nth occurrence of something?
You have to keep track. For example, let's say you want
to change the fifth occurrence of "whoever" or "whomever"
into "whosoever" or "whomsoever", case insensitively.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}igex;
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 charcters 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 <comdog@computerdog.com>):
$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;
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);
How do I strip blank space from the beginning/end of a
string?
The simplest approach, albeit not the fastest, is probably
like this:
$string =~ s/^\s*(.*?)\s*$/$1/;
It would be faster to do this in two steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
Or more nicely written as:
for ($string) {
s/^\s+//;
s/\s+$//;
}
How do I extract selected columns from a string?
Use substr() or unpack(), both documented in the perlfunc
manpage.
How do I find the soundex value of a string?
Use the standard Text::Soundex module distributed with
perl.
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';
$text =~ s/\$(\w+)/${$1}/g;
Before version 5 of perl, this had to be done with a
double-eval substitution:
$text =~ s/(\$\w+)/$1/eeg;
Which is bizarre enough that you'll probably actually need
an EEG afterwards. :-)
See also "How do I expand function calls in a string?" in
this section of the FAQ.
What's wrong with always quoting ""$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.
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.
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.
Data: Arrays
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 extract just the unique elements of an 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:
$prev = 'nonesuch';
@out = grep($_ ne $prev && ($prev = $_), @in);
This is nice in that it doesn't use much extra memory,
simulating uniq(1)'s behavior of removing only
adjacent duplicates.
b) If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$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;
How can I tell whether an array contains a certain
element?
There are several ways to approach this. If you are going
to make this query many times and the values are arbitrary
strings, 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; }
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 $read;
grep (vec($read,$_,1) = 1, @articles);
Now check whether vec($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 regexp characters in $whatever?).
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 }, $element;
}
How do I find the first array element for which a
condition is true?
You can use this if you care about the index:
for ($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.
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.
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?
Here's a shuffling algorithm which works its way through
the list, 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));
}
For large arrays, this avoids a lot of the reshuffling:
srand;
@new = ();
@old = 1 .. 10000; # just a demo
for( @old ){
my $r = rand @new+1;
push(@new,$new[$r]);
$new[$r] = $_;
}
How do I process/modify each element of an array?
Use for/foreach:
for (@lines) {
s/foo/bar/;
tr[a-z][A-Z];
}
Here's another; let's compute spherical volumes:
for (@radii) {
$_ **= 3;
$_ *= (4/3) * 3.14159; # this will be constant folded
}
How do I select a random element from an array?
Use the rand() function (see the rand entry in the
perlfunc manpage):
srand; # not needed for 5.004 and later
$index = rand @array;
$element = $array[$index];
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 permut() function should work on any list:
#!/usr/bin/perl -n
# permute - tchrist@perl.com
permut([split], []);
sub permut {
my @head = @{ $_[0] };
my @tail = @{ $_[1] };
unless (@head) {
# stop recursing when there are no elements in the head
print "@tail\n";
} else {
# for all elements in @head, move one from @head to @tail
# and call permut() on the new @head and @tail
my(@newhead,@newtail,$i);
foreach $i (0 .. $#head) {
@newhead = @head;
@newtail = @tail;
unshift(@newtail, splice(@newhead, $i, 1));
permut([@newhead], [@newtail]);
}
}
}
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.)
Why does defined() return true on empty arrays and hashes?
See the defined entry in the perlfunc manpage in the 5.004
release or later of Perl.
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.
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.
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 it just resets the iterator, which is
faster for tied hashes.
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.
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 a 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 arrays,
and then solve the uniquifying the array 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 $_[0], it has to be
ready to write it back into the caller's version.
This has been fixed as of perl5.004.
Normally, merely accessing a key's value for a nonexistent
key does not cause that key to be forever there. This is
different than awk's behavior.
How can I make the Perl equivalent of a C structure/C++
class/hash or array of hashes or arrays?
Use references (documented in the perlref manpage).
Examples of complex data structures are given in the
perldsc manpage and the perllol manpage. Examples of
structures and object-oriented classes are in the perltoot
manpage.
How can I use a reference as a hash key?
You can't do this directly, but you could use the standard
Tie::Refhash module distributed with perl.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so this shouldn't be a problem. For
example, this works fine (assuming the files are found):
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On some systems, however, you have to play tedious games
with "text" versus "binary" files. See the section on
binmode in the perlfunc manpage.
If you're concerned about 8-bit ASCII data, then see the
perllocale manpage.
If you want to deal with multibyte characters, however,
there are some gotchas. See the section on Regular
Expressions.
How do I determine whether a scalar is a
number/whole/integer/float?
Assuming that you don't care about IEEE notations like
"NaN" or "Infinity", you probably just want to use a
regular expression.
warn "has nondigits" if /\D/;
warn "not a whole number" unless /^\d+$/;
warn "not an integer" unless /^-?\d+$/; # reject +3
warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
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.
How do I print out or copy a recursive data structure?
The Data::Dumper module on CPAN is nice for printing out
data structures, and FreezeThaw for copying them. For
example:
use FreezeThaw qw(freeze thaw);
$new = thaw freeze $old;
Where $old can be (a reference to) any kind of data
structure you'd like. It will be deeply copied.
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.
AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See the perlfaq manpage for
distribution information.
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Copyright (C) 1998
Hurricane Electric.
All Rights Reserved.