Loops in Perl

[carlriz]

I was looking at this expression in perl:

@ranks[@sorted_positions] = (0 .. $#sorted_positions);

Does perl increment the index for both arrays automatically? Is it shorthand for a 'foreach' loop?

[LanX]

that's a list assignment to an array slice on the LHS.

> Is it shorthand for a foreach loop?

ehm ... well the effect is analogous to many scalar assignments

$ranks[ $sorted_positions[$_] ] = $_ for 0 .. $#sorted_positions

but actually it's an assignment of list to list

> Does perl increment the index for both arrays automatically?

there is no increment after the range operator created a list on the RHS.

[kcott]

G'day carlriz,

Let's say @sorted_positions has five elements (1, 3, 5, 7, 9). (So, $#sorted_positions will be 4.)

The code you posted:

@ranks[@sorted_positions] = (0 .. $#sorted_positions);

will be evaluated like this:

@ranks[1, 3, 5, 7, 9] = (0, 1, 2, 3, 4);

The element $ranks[1] will be set to 0, the element $ranks[3] will be set to 1 and so on.

Here's a short piece of code demonstrating this.

#!/usr/bin/env perl -l

use strict;
use warnings;

my @ranks = qw{a b c d e f g h i j}; 
my @sorted_positions = (1, 3, 5, 7, 9);

print "@ranks";
@ranks[@sorted_positions] = (0 .. $#sorted_positions);
print "@ranks";

Output:

a b c d e f g h i j
a 0 c 1 e 2 g 3 i 4

The elements with indices 0, 2, 4, 6 and 8 are unchanged. The remaining elements (with indices 1, 3, 5, 7 and 9) are set to the values 0, 1, 2, 3 and 4 respectively.

[carlriz]

Awesome example. Thanks!

[Laurent_R]

Yes, what you see on the left side of the equal sign is called an array slice. You can do something like this:

my @array = 0..5;
my @ranks[@array] = 5..10; # @ranks now contains 5, 6, 7, 8, 9, 10

The code you posted is just the same idea.

Update:

As noted by AnomalousMonk in the post just below, there is an error in the code above: it should be:

my @array = 0..5;
my @ranks;
@ranks[@array] = 5..10; # @ranks now contains 5, 6, 7, 8, 9, 10

We can't use "my" on an array with subscripts accessing to part of the array, the array must be declared before. I tested the code directly under the debugger, where it is not pôssible to use "my", and added the my on the line afterwards, creating erroneous code.

[AnomalousMonk]

my @ranks[@array] = 5..10; is a syntax error.

c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my @array = 0..5;
 my @ranks;
 @ranks[@array] = 5..10;
 dd \@ranks;
"
[5 .. 10]

Update: However, the following works for initializing a new lexical (reverse used just to add some variety):

c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my @array = reverse 0 .. 5;
 my @ranks = (5 .. 10)[@array];
 dd \@ranks;
"
[10, 9, 8, 7, 6, 5]

[Laurent_R]

Thank you for your comment, AnomalousMonk, you are right. I added the "my" after having tested and that was a mistake. I have updated my post accordingly.

[Anonymous Monk]

ppi_dumper

Read more... (857 Bytes)

So the .. is an operator , and operators are described in perlop

You can also see the structure of programs with wxPPIxregexplain.pl

[2teez]

Hi carlriz,
Also check Slices Just a "kobo" note!

[Marshall]

This is actually a rather strange idea in Perl.
And I figure syntactically incorrect.

Normal would be:

 
@ranks= sort { my ($A) = (split($a,'')))[0];
               my ($B) =  split($b,'')))[0];
               $A[0] <=> $B[0]
              }@ranks; 
               #sorts on numeric comparison
               #between item's [0] in the @ranks array
             }