in reply to 'for' array being altered - scope

While I don't see where $_ is changed, keep in mind that it is global. It could be format_column_range or any other code invoked from this that changes it -- even though $_ is not explicitly passed to it.

I once accidently clobbered $_ because I had a method that, upon first execution, would read a config file. while(<FILE>){} does not look like it would clobber a global, does it? Took me longer than I liked to spot it ...

The Sidhekin
print "Just another Perl ${\(trickster and hacker)},"

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Re: Re: 'for' array being altered - scope
by diotalevi (Canon) on Sep 26, 2002 at 12:18 UTC
    Two things here: Sidhekin's problem can be solved by localizing $_ to the current sub instead of just using it. As for the original problem, when you iterate over an array with for() the $_ variable is aliased to the right spot in @array. It *is* your array entry so if you assign to $_ then you are assigning to that spot in your array. It allows for clever things like avoiding an assign operation where map{} would require you to assign the value back into place. This is a micro optimization but it' still a good method to know of. 
    @mg = qw(Sam sat on the ground and put his head in his hands.  'I wish
+ I had never come here, and I don't want to see no more magic,' he sa
+id, and fell silent.);

## standard way
# create a new array and assign back into place

@mg = map ucfirst, @mg;

# OR...

## work in place
# alter existing array
$_ = ucfirst() for @mg;

    [download]

    Update And this reply misses the entire point of the original question. Oopsie!

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[d/l]

      That's all very correct1 and appreciated, but it tells neither me nor fireartist anything we did not already know.

      See, in my case localizing $_ had to be done in the sub that assigned to it (local does dynamic scoping, not lexical), and my problem was finding which sub did such a nasty thing.

      find -name '*.pm' | xargs grep '$_ =' did nothing to help me ...

      fireartist's problem is similar. He knows that assigning to $_ has this effect. He just does not see where this assignment takes place.

      1 <nitpick>... except that you are not so much avoiding assign operations as avoiding building a temporary array. The map version performs one assignment (to an array) total, while the for version performs one assignment (to a scalar) each time through the loop. Still, it is an optimization.</nitpick>

      The Sidhekin
      print "Just another Perl ${\(trickster and hacker)},"

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[d/l]
[select]

        Duh! smacks forehead oops! Ok in that case it would be interesting to know where column_num lives to examine that source.

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