I should've known better!

Well, I am red-faced with embarassment. Having worked around Perl for several years, I found one of those "traps" that must frustrate "newbies" but that I should've seen coming given my time with Perl.

A construct that I (and I suspect otheres) have used is the following:


foreach my $line (@text){
   chomp($line);
   next if($line =~ /^\s*#/);
   #...do something with the non-commented lines
}
[download]

The surprise was not in the basic construct, but rather that I forgot that $line is just an alias for each of the actual elements in the array @text. Of course the array could be anything (doesn't have to be text) and the processing in the loop could be anything.

The important thing is that anything (e.g., chomp($line) in this case) that alters the loop variable (e.g., $line) is actually operating on and affecting the actual values in the array.

Plenty of documentation reminds (and even warns us) of that fact...but I am almost continually forgetting it. I am amazed that this doesn't bit me more frequently...maybe if it did, I'd do a better job of remembering it...he says with a wry grin.

It just now bit me on a little test case I had quickly coded up as I looked at one of the Q&A's about 'arrays' when I went to print out the value of @text after processing it in the above loop and then push()'ing the result into an ouput array which I then went on to print out for comparison.

Much to my dismay, when I printed out @text after the loop I found that all of the new-lines were gone!

I had to stop and fret over it and then I remembered! In chomp()'ing the loop variable, I was actually chomping the array's member! Duhhh! Well, color me 'embarassed'!

I think it is really powerful that the loop variable is an alias and I have actually used that to good advantage in several instnaces. But it must be really mysterious to those new to Perl that using common constructs like the comment-removal code I showed above can result in some pretty mysterious behavior.

Just an observation regarding the power and, sometimes, mystery of Perl.

ack Albuquerque, NM

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Re: I should've known better! by TimToady (Parson) on Sep 11, 2009 at 02:26 UTC
Yes, well, I should have known better too. `:-)` In fact, in Perl 6, the equivalent loop: `for @text -> $line { ... }` [download] makes `$line` a read-only alias to catch just this sort of mixup. You'd write: `for @text <-> $line { ... }` [download] (note the double-ended arrow) to get the read-write semantics. But it's not the default. Perl 6 is all about picking sane defaults while not preventing you from doing the sorts of powerful things you can do in Perl 5.	[reply] [d/l] [select]
Re^2: I should've known better! by GrandFather (Saint) on Sep 11, 2009 at 03:01 UTC
In Perl 5 `my $var; ... for $var (...) {...}` doesn't alter the contents of the $var global to the for loop. Is that true for both the -> and <-> variants of the Perl 6 for loop illustrated above? I presume you can `for @array <-> my $var {...}` in Perl 6 to make the scope of $var obvious. True laziness is hard work	[reply] [d/l] [select]
Re^3: I should've known better! by moritz (Cardinal) on Sep 14, 2009 at 14:16 UTC
In Perl 5 `my $var; ... for $var (...) {...}` doesn't alter the contents of the $var global to the for loop. Is that true for both the -> and <-> variants of the Perl 6 for loop illustrated above? Yes. presume you can `for @array <-> my $var {...},` in Perl 6 to make the scope of $var obvious. No, you can't. Both arrows actually introduce a signature, and `my` in a signature doesn't make sense. I used to write `-> my $x` too, but got used to omitting the my rather quickly :-). Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l] [select]
Re: I should've known better! by vitoco (Hermit) on Sep 10, 2009 at 22:32 UTC
When I got your post, I was reading (again) perlsub to check that it's the same to pass an array by reference than by value to save memory, because of aliasing magic built in `@_`. Fortunately, `shift`ing or directly assigning values into `my` variables, breaks that internal linking, so modifying them won't alter the source of the parameters as in `foreach`, but it exists the ~~possibility~~ power to do that by touching `@_` directly. Unfortunately, assigning `@_` to a local variable just to write clear code, all the parameters array will be duplicated. A test on this topic... Read more... (1329 Bytes)	[reply] [d/l] [select]
Re: I should've known better! by Zen (Deacon) on Sep 14, 2009 at 14:04 UTC
Perl and Perl Jr. on the links: `for @text <-> $line` 24 keystrokes `foreach $line(@text)` 24 keystrokes	[reply] [d/l] [select]


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