in reply to What would you do?

A wise monk once advised me to avoid assigning directly to $_.

He counseled me that Perl uses $_ in a variety of ways that fledgelings such as myself may not understand. When through youthful exuberance we blow away the existing values stored in $_ we create problems for ourselves later.

For instance, what if the code you are writing is inside a loop which already uses $_ for something else? For instance: 

while (<MYFILEHANDLE>){
...your code here...
print;
}

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All of a sudden your first way of doing it, which assigns to $_, breaks the existing program so that it no longer prints out all of the lines in a file.

"Does the second loop give you nasty C flashbacks?"

I would argue that we are programming in Perl. We are not programming in "Not-C". The second method is perfectly proper Perl and is preferable for those of us who are concerned about modifying $_ in a way which may interfere with other parts of a program.

Update: japhy informs me below that for(split ' '){} inside a  while (<FH>){} creates a local copy of $_ and does not use the same one that the while() uses. Hence, the use of the first method does not break the program I posted.

Since posting the above I have also noticed that LW, Merlyn et al in the Camel book sometimes assign directly to $_. For example, on page 103 of the second edition they say: 

while (<>) {
    chomp;
    if (s/\\$//) {
        $_ .= <>;
        redo;
    }
    # now process $_
}

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One ought not to be more Catholic than the Pope. If LW and Merlyn think that assigning to $_ is safe and proper Perl style then so be it. </code>

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Re: Re: What would you do?
by japhy (Canon) on Mar 16, 2001 at 20:45 UTC
    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
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      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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        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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