Re: Is there a Perl variable to count iterations in a loop?

There is probably no such variable, but you can play with a range operator .. in scalar context:

my @letters = qw(alfa beta gama);
for my $letter (@letters){
   warn "iteration is ", scalar($0 .. !$0);
}
[download]

The $0, resp. !$0 are just expressions which are allways true, resp. allways false. Unfortunately you cannot use 1, 0 because they are given some special meaning by the range operator.

Although this "trick" works, it is not particularly self-explainable.

Comment on Re: Is there a Perl variable to count iterations in a loop? Download Code

Replies are listed 'Best First'.
Re^2: Is there a Perl variable to count iterations in a loop? by ruzam (Curate) on Dec 18, 2008 at 23:00 UTC
Wow! That could be really useful. How the heck does it work and is it documented anywhere? Update - never mind... After some quick research and testing I see that ($0 .. !$0) only increments as it's used. So you can't just pop it into an if statement somewhere to get the current iteration (which would be very useful). You might as well just dedicate a new variable to keep count. Interesting none the less.	[reply]
Re^3: Is there a Perl variable to count iterations in a loop? by roman (Monk) on Dec 19, 2008 at 10:50 UTC
Be aware. I just get burned by this hack - the range operator iterates all over its lifetime. `sub iterate { for my $item ( @_ ){ warn 'iteration '. ($0 .. !$0), "\n"; } } iterate( qw(a b c) ); iterate( 1, 2, 3, 4); iterate( 0 );` [download] yields `iteration 1 iteration 2 iteration 3 iteration 4 iteration 5 iteration 6 iteration 7 iteration 8` [download]	[reply] [d/l] [select]
Re^4: Is there a Perl variable to count iterations in a loop? by pat_mc (Pilgrim) on Dec 19, 2008 at 11:27 UTC
Interesting insight nontheless, roman. Thanks for sharing!	[reply]
Re^4: Is there a Perl variable to count iterations in a loop? by LanX (Saint) on Dec 19, 2008 at 11:29 UTC
But this works `my @letters = qw(alfa beta gama); for my $letter (@letters){ warn "iteration is ", scalar($0 .. !$0); } for my $letter (@letters){ warn "iteration is ", scalar($0 .. !$0); }` [download] I think the range-operator depends on the code position, but your sub is always at the same one. * Cheers Rolf UPDATE: well the documentation could be clearer... perlop ... The sequence number is reset for each range encountered .... ...in the compilation phase!!! (*) that's why the scalar range is known as flip-flop-operator	[reply] [d/l]