declaring a common loop variable

pritesh_ugrankar has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

Is it ok to declare a common loop variable if I want to iterate through multiple arrays?

use warnings;
use strict;

my @arr1 = qw (this will print for sure);
my @arr2 = qw (this is second array);
my $things = "nothing";
foreach $things (@arr1) {
    print "$things\n";
};
    print "$things\n";

foreach $things (@arr2) {
    print "$things\n";
};
    print "$things\n";
[download]

I am using "nothing" just to verify if it gets reset to it's original value and do understand that "nothing" is a string, and has "nothing" to do (pun unintended) with undef or empty.

So this will be a single copy of loop variable used for multiple arrays. My question is, is this considered ok in Perl? or am I to have a seperate loop variable for each of the arrays? Even if I have the same name for the loop variable, the following two $things are different right?

use warnings;
use strict;

my @arr1 = qw (this will print for sure);
my @arr2 = qw (this is second array);
foreach my $things (@arr1) {
    print "$things\n";
};

foreach my $things (@arr2) {
    print "$things\n";
};
[download]

Comment on declaring a common loop variable Select or Download Code

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Re: declaring a common loop variable by poj (Abbot) on Feb 28, 2017 at 12:12 UTC
I want to iterate through multiple arrays `foreach my $thing (@arr1,@arr2) { print "$thing\n"; };` [download] poj	[reply] [d/l]
Re: declaring a common loop variable by haukex (Archbishop) on Feb 28, 2017 at 12:02 UTC
Even if I have the same name for the loop variable, the following two $things are different right? Yes, they are two separate variables lexically scoped to the loops. My question is, is this considered ok in Perl? or am I to have a seperate loop variable for each of the arrays? That depends on your definition of "ok" ;-) Techincally, yes, you can do it. I just wouldn't consider it good style. Also, what would be the point of re-using this variable? As you've already seen with your test code, its value will be locally scoped to the `foreach` loop, so you don't seem to be gaining anything by doing it, and the problem is that the code might get confusing by re-using the same variable. Update: See also the first few paragraphs of Foreach Loops.	[reply] [d/l]
Re: declaring a common loop variable by 1nickt (Canon) on Feb 28, 2017 at 13:25 UTC
Hi pritesh_ugrankar, Yes, it's OK to use a single variable to iterate through multiple arrays: `perl -Mstrict -WE ' my @x = (1,2); my @y = (3,4); for my $z ( @x, @y ) { say $z; } '` [download] Output: `1 2 3 4` [download] If you want to iterate through your arrays one at a time, you can still use the same variable name, since it will be local to the `for` loop: `perl -Mstrict -WE ' my @x = (1,2); my @y = (3,4); for my $z ( @x ) { say $z; } for my $z ( @y ) { say $z; } '` [download] Output: `1 2 3 4` [download] For the same reason (locally scoped variable) you can use the same variable name even with nested loops: `perl -Mstrict -WE ' my @x = (1,2); my @y = (3,4); for my $z ( @x ) { say $z; for my $z ( @y ) { say " $z"; } say $z; } '` [download] Output: `1 3 4 1 2 3 4 2` [download] ... although this is rarely a good idea, since it makes it less easy to follow the flow of the code. Variable names are free; go ahead and use another one! Hope this helps! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re: declaring a common loop variable by AnomalousMonk (Archbishop) on Feb 28, 2017 at 22:24 UTC
Further to 1nickt's post: pritesh_ugrankar: Note that the local scoping (or aliasing) of a `for`-loop iteration variable is very strong and holds even if you do not declare a separate lexical for each loop: `c:\@Work\Perl\monks\nysus>perl -wMstrict -le "my @x = (1, 2); my @y = qw(foo bar); ;; my $z = 'original'; for $z (@x) { printf qq{$z }; for $z (@y) { printf qq{'$z' }; } print $z; } print qq{after all loops: '$z'}; " 1 'foo' 'bar' 1 2 'foo' 'bar' 2 after all loops: 'original'` [download] (But yes, certainly use different variable names as a sanity-saver!) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: declaring a common loop variable by pritesh_ugrankar (Monk) on Feb 28, 2017 at 13:04 UTC
Awesome. This is why I love this place. Thank you very much.	[reply]
Re: declaring a common loop variable by pritesh_ugrankar (Monk) on Feb 28, 2017 at 14:08 UTC
Thank you for the detailed description.	[reply]