using shift and $

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

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Re: using shift and $_ in loop by Corion (Patriarch) on Oct 21, 2009 at 15:30 UTC
I'm not sure that what you want to do is sane, but that might be because I don't know your purpose. The following works but leaves `@_` unmodified until after your loop: `sub scribble { for (@_) { ... }; @_ = (); };` [download] Alternatively, with the assignment: `sub scribble { while ($_ = shift) { ... }; };` [download] My second approach and your `while(...)` approach suffer from the same problem that they will end the loop if a false value is contained in `@_`	[reply] [d/l] [select]
Re^2: using shift and $_ in loop by leocharre (Priest) on Oct 21, 2009 at 15:50 UTC
Yes they do suffer that- it tests for truth. Maybe I was too vague in my posting, if so I apologize. I expect my sub to take an unforeseen number of arguments. My sub will take the arguments, discard some, and the ones that are not discarded, will be altered- and a list of the altered arguments are returned back. An example.. `use Lingua::Names 'is_name'; my @a = qw/andy lo1 beca cris/; my @A = alter(@a); sub alter { my @newlist; for ( @_ ){ my $name = is_name($_) or next; push @newlist, uc($name); } @newlist; }` [download] Now, how can I make this so I can use shift, maybe even a for loop with undef and last??? What might be apropriate here to make it a little more minimal? This is a little closer.. `sub alter { my @newlist; while ( @_ ){ $_ = shift @_; my $name = is_name($_) or next; push @newlist, uc($name); } @newlist; }` [download] I'm wondering if I can just shift and push from @_ and then return the modified @_ .. using $_ ??? Maybe this is just too much coffee speaking and I need to chill out?	[reply] [d/l] [select]
Re^3: using shift and $_ in loop by moritz (Cardinal) on Oct 21, 2009 at 16:09 UTC
`sub alter { my @newlist; for ( @_ ){ my $name = is_name($_) or next; push @newlist, uc($name); } @newlist; }` [download] I'd write that as `sub alter { grep $_, map { uc is_name($_) } @_; }` [download] instead. In Perl 6 I'd maybe write something more similar to what you wrote: `sub alter(@a) { gather for @a { my $name = is_name($_); take uc $name if $name; } }` [download] Or even `sub alter(@a) { gather for @a.map({ is_name($_)}) { take .uc if $_; } }` [download] Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l] [select]
Re^4: using shift and $_ in loop by leocharre (Priest) on Oct 21, 2009 at 21:01 UTC
Re^5: using shift and $_ in loop by moritz (Cardinal) on Oct 21, 2009 at 21:51 UTC
Re^5: using shift and $_ in loop by ikegami (Patriarch) on Oct 21, 2009 at 21:10 UTC
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Re^3: using shift and $_ in loop by ikegami (Patriarch) on Oct 21, 2009 at 16:28 UTC
Now, how can I make this so I can use shift, What's wrong with the `for` version? `map` and `grep` lend themselves well here: `sub alter { return map uc, grep is_name($_), @_; }` [download]	[reply] [d/l] [select]
Re^3: using shift and $_ in loop by keszler (Priest) on Oct 21, 2009 at 16:06 UTC
If I'm reading this correctly, you want @A as the elements of @a that pass the is_name(x) test? `@A = grep { is_name($_) } @a;` [download] perldoc -f grep is your friend. Update: I just noticed the uc() you used. In that case: `@A = map { uc($_) } grep { is_name($_) } @a;` [download] perldoc -f map is also your friend.	[reply] [d/l] [select]
Re^4: using shift and $_ in loop by moritz (Cardinal) on Oct 21, 2009 at 16:13 UTC
Re^5: using shift and $_ in loop by keszler (Priest) on Oct 21, 2009 at 17:57 UTC
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Re: using shift and $_ in loop by ikegami (Patriarch) on Oct 21, 2009 at 15:45 UTC
You never assigned to `$_`. The equivalent of `sub scribble { while ( $a = shift ){ $a; # has arg } }` [download] that assigns to `$_` is `sub scribble { while ( $_ = shift ){ $_; # has arg } }` [download] In both cases, you are using global variables. `$_` is particularly dangerous to clobber. Fix: `sub scribble { local $_; while ( $_ = shift ){ $_; # has arg } }` [download]	[reply] [d/l] [select]
Re^2: using shift and $_ in loop by leocharre (Priest) on Oct 21, 2009 at 15:59 UTC
Thank you, this is really useful. So, you suggest localizing $_ as a precaution?	[reply]
Re^3: using shift and $_ in loop by ikegami (Patriarch) on Oct 21, 2009 at 16:37 UTC
Yeah, don't clobber your caller's variables.	[reply]
Re^4: using shift and $_ in loop by leocharre (Priest) on Oct 21, 2009 at 20:26 UTC
Re^5: using shift and $_ in loop by ikegami (Patriarch) on Oct 21, 2009 at 20:34 UTC
Re: using shift and $_ in loop by moritz (Cardinal) on Oct 21, 2009 at 15:27 UTC
If deconstruction of the `@_` array is not required, just use `for (@_) { print; # using $_ here }` [download] See shift, @_. Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l] [select]
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Re: using shift and $_ in loop by jakobi (Pilgrim) on Oct 21, 2009 at 16:08 UTC
Leocharre, you're right on in being cautious of the truth of the loop condition. Which is necessary due to the lack of while(<>)-style magic in a mere `while($_=shift){}`. Consider how the usual looping variants work or mostly fail, or just ask Perl with a simple, cute and short one-liner: `perl -e 'use Data::Dumper; @a=(1,0,undef,2); print Dumper(@a); print " +1=$_=\n" foreach (@a); @ARGV=@a; print "2=$_=\n" while($_=shift @a); +@a=@ARGV; print "3=$_=\n" while($_=shift @a,defined $_); @a=@ARGV; pr +int "4=$_=\n" while($_=shift);'` [download] <Reveal this spoiler or all in this thread> PS: ad local: local($_) helps to not hurt the caller (but isn't e.g. necessary in a `foreach(@a)` scope), while my($_) (since 5.10) helps the current scope to not be hurt by called routines (and signal handlers, etc). \Peter, who is wondering whether the line-eater did like the taste of his attempted .sig	[reply] [d/l] [select]
Re: using shift and $_ in loop by johngg (Canon) on Oct 21, 2009 at 16:59 UTC
No need to test the shift in a loop. `$ perl -le ' > sub isname { return 1 if $_[ 0 ] =~ m{(?i)^(?:bill\|fred)$} } > @a = qw{ apple peach fred }; > sub alter > { > my @new = > map { uc } > grep { isname( $_ ) } > map { my $w = shift } > 0 .. $#_; > } > @A = alter( @a ); > print for @A;' FRED $` [download] I hope this is of interest. Cheers, JohnGG	[reply] [d/l]
Re: using shift and $_ in loop by pileofrogs (Priest) on Oct 21, 2009 at 19:47 UTC
( And I mean, seriously... where the #$!@& is shift output going?? Is its scope only in () ??? ) Actually, it's scope is the whole while loop. For instance.. `for my $foo (@wibble) { print $foo; }` [download] Oh, and $_ is global, as mentioned above (said Pileofrog as though he already knew that...) so the scope won't matter. UpdateD'oh! Removed stupid '=' sign. Thanks moritz.	[reply] [d/l]
Re^2: using shift and $_ in loop by Anonymous Monk on Oct 22, 2009 at 14:10 UTC
... so the scope won't matter. Famous last words :-)	[reply]