How Do I Use Labels?

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

hi I obviously read the wrong books because I'm not able to find out how this "construct" is called:

 MAIN_LOOP: 
 for my $number_to_check (3 .. 200)
    {
...
[download]

So my questions about MAIN_LOOP:
- how do you call 'that'?
- how does it work?
- what do you think about its usage? (none of the books I read use it, but I've seen other code that use it often)

thanks
ben

Comment on How Do I Use Labels? Download Code

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Re: How Do I Use Labels? by ikegami (Patriarch) on Sep 13, 2008 at 23:49 UTC
`MAIN_LOOP` is a label. They can be used on flow control structures, bare blocks and any other statements. They can be used with `next`, `last` and `goto`. "`..`" is the range operator. `for my $var (LIST)` is a foreach loop. Together, they are optimized into a counting loop, basically a more readable version of `for (my $number_to_check=3; $number_to_check<=200; ++$number_to_check)` [download]	[reply] [d/l] [select]
Re: How Do I Use Labels? by GrandFather (Saint) on Sep 14, 2008 at 00:04 UTC
MAIN_LOOP: is a label and is the target of constructs like `next MAIN_LOOP;` and `last MAIN_LOOP;`. However, unless you have nested loops and need "next" or "last" to apply to the outer loop, labels very seldom need to be used. Consider: `use strict; use warnings; MAIN_LOOP: for my $number_to_check (1 .. 20) { for my $colour ('red', 'green', 'black') { next MAIN_LOOP if $number_to_check % 3 and $colour eq 'green'; print "$colour-$number_to_check "; } print "\n"; }` [download] prints: `red-1 red-2 red-3 green-3 black-3 red-4 red-5 red-6 green-6 black-6 red-7 red-8 red-9 green-9 black-9 red-10 red-11 red-12 green-12 black-12 red-13 red-14 red-15 green-15 black-15 red-16 red-17 red-18 green-18 black-18 red-19 red-20` [download] Perl reduces RSI - it saves typing	[reply] [d/l] [select]
Re^2: How Do I Use Labels? by baurel (Sexton) on Sep 14, 2008 at 00:38 UTC
thank you all for your help!	[reply]
Re: How Do I Use Labels? by Perlbotics (Archbishop) on Sep 13, 2008 at 23:48 UTC
It's called a label and among other things it can be used for loop control. perlsyn describes its details. Update: It can help to structure complicated nested loops, but it's better avoided in combination with goto.	[reply]