Re: evaluation strategy of perl

The problem is that interally, "$i\n" becomes $i . "\n" - which demonstrates why the problem occurs. You have to interpolate the value beforehand:

for (my $i = 0; $i<3; $i++) {
    my $str = "$i\n";
    push @d, sub { print $str };
}
[download]

However, that will not work correctly:

0
1
2

Oops. You meant

for (my $i = 1; $i<=3; $i++) {
    my $str = "$i\n";
    push @d, sub { print $str };
}
[download]

Because of this sort of problems, you should use the for(LIST) form.

for my $i (1..3) {
    my $str = "$i\n";
    push @d, sub { print $str };
}
[download]

That eliminates an entire class of mistakes known as "fencepost errors", where the loop limit condition is fudged, usually resulting in one-off errors (one iteration too many or too few).

Makeshifts last the longest.

Comment on Re: evaluation strategy of perl Select or Download Code

Replies are listed 'Best First'.
Re: Re: evaluation strategy of perl by blakem (Monsignor) on Sep 20, 2002 at 15:15 UTC
There is something significantly different between the two `for` styles in your last two examples.... Can you explain why $i binds early and $j binds late? `#!/usr/bin/perl -wT use strict; my (@d,$i,$j); # all vars in same scope for $i (1..3) { push @d, sub { print "$i\n" }; # same as line below } for ($j = 1; $j<=3; $j++) { push @d, sub { print "$j\n" }; # same as line above } $_->() for @d; # execute all anon subs __END__ 1 2 3 4 4 4` [download] -Blake	[reply] [d/l] [select]
Re^3: evaluation strategy of perl by Aristotle (Chancellor) on Sep 20, 2002 at 15:33 UTC
The `for(LIST)` doesn't bind early either. However, `$i` is just an alias to a different value, so rather than `$i` itself, the closure remembers the alias. It's easily visible if we use an array rather than a literal list. `#!/usr/bin/perl -w use strict; my @t = (1,2,3); my @d; for my $i (@t) { push @d, sub { print "$i\n" }; } $_++ for @t; &$_ for @d; __END__ 2 3 4` [download] I hadn't thought about that, though in retrospect it's quite clear. Thanks for making me ponder this. Makeshifts last the longest.	[reply] [d/l]