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in reply to Re: What would you do?
in thread What would you do?

Not quite. The first method does not assign to the same $_ that the while loop uses. If it did, you couldn't use $_ in that for loop at all. 
while (<FH>) {
  # localized $_
  for (split ' ') {
    # localized $_
  }
  # previous $_
}

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It's when you start doing silly and dangerous things like: 
sub foo {
  $_ = shift;  # TSK!  local $_
  tr/aeiou/AEIOU/;
  return $_;
}

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that problems occur. It is a bad idea to assign to a global $_ -- where "global" means "not localized".

japhy -- Perl and Regex Hacker

Replies are listed 'Best First'.
Does while localize $_?
by tenya (Beadle) on Mar 18, 2001 at 22:40 UTC
    Your comment and several others I have seen here suggest that while(FH) and foreach automatically localize $_. But the folowing code seems does not revert to the old $_ after leaving the while loop. What am I missing? Thanks. 
    use strict; use diagnostics;
$_ = "Before";
print "Before loop= $_\n";
while (<>){
    print "In while loop= $_\n";
    last;
}
print "After while loop= $_\n";

Output:
Before loop= Before
myinput  (entered from the keyboard)
In while loop= myinput

After while loop= myinput

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[d/l]

      A foreach loop does implicitly localize $_ (or whatever variable is being used as the loop variable), but $_ is not automatically localized for while(<>) loops. Here is the relevant entry from the perlop manpage:

      Ordinarily you must assign the returned value to a vari- able, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a `while' state- ment (even if disguised as a `for(;;)' loop), the value is automatically assigned to the global variable $_, destroy- ing whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) The $_ variables is not implicitly localized. You'll have to put a `local $_;' before the loop if you want that to happen.

      However, that last line is misleading: you'll need to wrap it all in a block, or localize $_ in the while condition to get the proper effect: 

      {
    local $_;
    while(<>){
        print;
    }
}

# or,

while(local $_ = <>) {
    print;
}

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[d/l]
[select]
[reply]
        Which is helpful because this allows while loops inside the foreach loop to use the foreach $_. Correct?
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