Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Automatically get loop index in foreach loop in Perl

Tags:

foreach

perl

If I have the following array in Perl:

@x = qw(a b c);

and I iterate over it with foreach, then $_ will refer to the current element in the array:

foreach (@x) {
    print;
}

will print:

abc

Is there a similar way to get the index of the current element, without manually updating a counter? Something such as:

foreach (@x) {
    print $index;
}

where $index is updated like $_ to yield the output:

012
like image 593
Nathan Fellman Avatar asked Jun 10 '09 09:06

Nathan Fellman


4 Answers

Like codehead said, you'd have to iterate over the array indices instead of its elements. I prefer this variant over the C-style for loop:

for my $i (0 .. $#x) {
    print "$i: $x[$i]\n";
}
like image 151
trendels Avatar answered Nov 16 '22 09:11

trendels


In Perl prior to 5.10, you can say

#!/usr/bin/perl

use strict;
use warnings;

my @a = qw/a b c d e/;

my $index;
for my $elem (@a) {
    print "At index ", $index++, ", I saw $elem\n";
}

#or

for my $index (0 .. $#a) {
    print "At index $index I saw $a[$index]\n";
}

In Perl 5.10, you use state to declare a variable that never gets reinitialized (unlike ones created with my). This lets you keep the $index variable in a smaller scope, but it can lead to bugs (if you enter the loop a second time it will still have the last value):

#!/usr/bin/perl

use 5.010;
use strict;
use warnings;

my @a = qw/a b c d e/;

for my $elem (@a) {
    state $index;
    say "At index ", $index++, ", I saw $elem";
}

In Perl 5.12 you can say

#!/usr/bin/perl

use 5.012; # This enables strict
use warnings;

my @a = qw/a b c d e/;

while (my ($index, $elem) = each @a) {
    say "At index $index I saw $elem";
}

But be warned: you there are restrictions to what you are allowed to do with @a while iterating over it with each.

It won't help you now, but in Perl 6 you will be able to say

#!/usr/bin/perl6

my @a = <a b c d e>;
for @a Z 0 .. Inf -> $elem, $index {
    say "at index $index, I saw $elem"
}

The Z operator zips the two lists together (i.e. it takes one element from the first list, then one element from the second, then one element from the first, and so on). The second list is a lazy list that contains every integer from 0 to infinity (at least theoretically). The -> $elem, $index says that we are taking two values at a time from the result of the zip. The rest should look normal to you (unless you are not familiar with the say function from 5.10 yet).

like image 44
Chas. Owens Avatar answered Nov 16 '22 09:11

Chas. Owens


perldoc perlvar does not seem to suggest any such variable.

like image 21
Alan Haggai Alavi Avatar answered Nov 16 '22 10:11

Alan Haggai Alavi


It can be done with a while loop (foreach doesn't support this):

my @arr = (1111, 2222, 3333);

while (my ($index, $element) = each(@arr))
{
   # You may need to "use feature 'say';"
   say "Index: $index, Element: $element";
}

Output:

Index: 0, Element: 1111
Index: 1, Element: 2222
Index: 2, Element: 3333

Perl version: 5.14.4

like image 10
Venkata Raju Avatar answered Nov 16 '22 09:11

Venkata Raju