while or for?

Skeeve has asked for the wisdom of the Perl Monks concerning the following question:

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Re: while or for? by broquaint (Abbot) on Jul 21, 2003 at 10:24 UTC
The construct `while(<>)` is special as it iterates on the magical `ARGV` filehandle and sets `$ARGV` to the current filename, checking for `defined`ness all the while. Whereas `for(<>)` just slurps `ARGV` and iterates over that (which is similar to the `while` construct, but not quite the same). Also you probably mean to use `$_` instead of `$ARGV` (unless of course you've cheekily abused the global nature of `ARGV` and used the scalar slot)¹. The `while` behaviour is well documented in Programming Perl* (ver 3, page 82) and in the I/O Operators section of `perlop`. HTH `_________ broquaint` ¹ addendum - as Skeeve points out `$ARGV` holds the name of the current filename that is being iterated over, so is not in fact abuse of the scalar slot of `ARGV` :) update*: added clarifications and fixed mistakes	[reply]
Re: Re: while or for? by Skeeve (Parson) on Jul 21, 2003 at 10:31 UTC
Okay. Understood. > Also you probably mean to use $_ instead of $ARGV I really meant $ARGV. I wanted to store, in a hash, for each line the names of the files they occure in.	[reply]
Re: while or for? by edan (Curate) on Jul 21, 2003 at 10:57 UTC
Okay, here's what I found: `$ perl -mO=Deparse -le 'print "$ARGV $_" for (<>)' foreach $_ (<ARGV>) { print "$ARGV $_"; }` [download] COMPARE: `$ perl -mO=Deparse -le 'print "$ARGV $_" while (<>)' print "$ARGV $_" while defined($_ = <ARGV>);` [download] Since `$ARGV` is set every time `<ARGV>` is evaluated, when you do it in a while loop, `$ARGV` is set correctly for each file in the loop. But using a for loop, you slurp `<ARGV>` in one shot, then iterate over `$_`, so `$ARGV` is set to the last file that was iterated over the last time `<ARGV>` was evaluated... Hope that makes sense... Update: I guess I just repeated what broquaint already said, but since I didn't understand what he wrote, I posted this, thinking I had something new to offer. Perhaps someone else will find it easier to understand, too... -- 3dan	[reply] [d/l] [select]
(jeffa) Re: while or for? by jeffa (Bishop) on Jul 21, 2003 at 13:34 UTC
This does not relate directly to your question, but i noticed that you use a hash named `%para`. Are you wanting to seperate the paragraphs? If so then you simply change `$/` to 2 newlines. Here is an example that you may modify to meet your needs: `use Data::Dumper; my @para = do {local $/ = "\n\n"; <DATA>}; print Dumper \@para; __DATA__ blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah` [download] jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re: (jeffa) Re: while or for? by Skeeve (Parson) on Jul 22, 2003 at 05:38 UTC
Thanks for the hint, but no, I'm not trying to seperate paragraphs. And yes, %para is because I meant paragraph, or to say it completely: ParagraphDef. My files contain a list of (XML-)ParagraphDef names, which I wanted to sort out: Which one is used in which file(s).	[reply]
Re: while or for? by antirice (Priest) on Jul 21, 2003 at 13:23 UTC
There isn't any special case as some may have argued. It is merely the context of <> when it's called. In your for statement, the contents of ARGV are slurped (the list foreach iterates over is built when the loop is constructed...don't believe me? `perl -le '$_=4;$_++,print foreach (1..$_)'`). Thus $ARGV is set to $ARGV[-1] for the duration of the loop. If you were to change your for statement to the following: `for (;<>;) { chomp; push(@{$para{$_}}, $ARGV); }` [download] It will DWYM and work as expected. Now before anyone with B::Deparse tries to deparse it and notices that it optimizes this loop to `while (defined($_ = <>)) ...`, if you change the loop to `for (my $i=0;<>;$i++) ...` it will deparse to a for statement but still DWYM. Hope this helps. antirice The first rule of Perl club is - use Perl The ith rule of Perl club is - follow rule i - 1 for i > 1	[reply] [d/l] [select]
Re: while or for? by tilly (Archbishop) on Jul 21, 2003 at 21:11 UTC
Doing something because it's shorter is not a good reason outside of a golf game. Particularly if the two versions have the same number of operations. If you are performing operations, why not make it understandable and take 3 lines to do it? Whitespace is cheap - in fact cheaper than the extra time it takes to mentally parse an unnecessarily complicated line. In this vein, see The path to mastery.	[reply]
Re: Re: while or for? by Skeeve (Parson) on Jul 22, 2003 at 05:43 UTC
Hey... I'm on training for the next minigolf ;-) If I do scripts that should last longer than, say 2 days, I normaly write a clean, commented loop with use strict and use warnings and all the bells and whistles ;-) But this was just a quick shot :-) I won't use the script any longer as soon as it did it's work. But you are right and I hope that some perl beginners will read this and your comment and learn from it.	[reply]