in reply to Internally, how do for() and while() differ?

for iterates over a list, everything in brackets is expanded (the only exception are ranges ). So <> will create a temporary list.

while loops as long a boolean expression is true.

This can be used together with an iterator, like while($x=iter()) .

There is no general automatism to set $_ like your example implies, that's just magic behavior of the file iterator <> in scalar context. that boolean context.

Cheers Rolf
(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!

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Re^2: Internally, how do for() and while() differ?
by Anonymous Monk on Oct 27, 2015 at 01:23 UTC
     There is no general automatism to set $_ like your example implies, that's just magic behavior of the file iterator <> in scalar context.

    Um, not quite, thats part of the magic of while not readline 

    $ perl -MO=Deparse,-p -le "while(<>){print}"
BEGIN { $/ = "\n"; $\ = "\n"; }
while (defined(($_ = <ARGV>))) {
    print($_);
}
-e syntax OK

$ perl -MO=Deparse,-p -le "while($f=<>){print}"
BEGIN { $/ = "\n"; $\ = "\n"; }
while (defined(($f = <ARGV>))) {
    print($_);
}
-e syntax OK

$ echo hi |perl -le "while($f=<>){print}"

    [download]
    You see  <> in scalar context didn't set $_
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      I meant while's scalar context, and only this iterator.

      Should be documented ( perlsyn ? )

      Cheers Rolf
      (addicted to the Perl Programming Language and ☆☆☆☆ :)
      Je suis Charlie!

      UPDATE

      see perlop#IO-Operators :

      > If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a for(;;) loop), the value is automatically assigned to the global variable $_ , destroying whatever was there previously.

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