My favorite looping mechanism in Perl is:

[vroom]

for

 26/20%

foreach

 71/55%

while

 25/19%

goto

 3/2%

what?

 4/3%

129 total votes

[merlyn]

My favorite construct is the naked block, together with redo and last to trigger restart and exit:

{
  blah;
  blah blah;
  last if $some_condition;
  more blah;
  more blah blah;
  redo if $some_other_condition;
}

There... we've got a middle-exiting loop, guaranteeing at least one execution of the first part. Who says we don't have an easy "do-while" loop!

-- Randal L. Schwartz, Perl hacker

[gaudior]

This is not an original observation, but you are one sick man. ;-)

Thanks, Randal, for the insights you have shared in actually USING perl over the years.

We appreciate it greatly.

[Russ]

Don't forget grep!

my @True;
for my $Element (@Array){
  if ($Element){
    push @True, $Element;
  }
}

becomes:

my @True = grep {$_} @Array;

To get every nth element from a list:
(Previously posted by me as Anonymous Monk...)

#(For every fourth element in @List)
grep {not ++$i % 4} @List;

#(To skip every fifth element)
grep {++$i % 5} (1..50)

I suppose we could argue that grep isn't really intended as a looping mechanism, but map seems to be popular around here, and map is implemented on grep code (internally) so...

:-)

Russ

[marcos]

I agree with BBQ: I love foreach, but when I discovered map ... it was love at first sight :) being able to write things like:


@words = ('foo', 'bar', 'and', 'whatelse');
map {print "$_\n"} @words;


is a real pleasure for me!
marcos

[merlyn]

Don't use map in a void context. Now that the backwards foreach is in there, it's never necessary. And it's also wasteful.

[marcos]

Thank you for your note, but could you please explain why using map in void context is "a wasteful"? Is it a problem of code performance and/or optimization?
TIA, marcos

[ChuckularOne]

map! Map! MAP! I love it. I have never used it, and now I'm thinking of all of the extra lines of code I've written over time to do what a simple map does!!!

Thank you!
I think I have a new favorite widget for the week!

Your humble servant,
-Chuck

[marcos]

This is the same reaction I had when I discovered map :-) I'm not sure if it is a real looping mechanism (technically speaking): if you check the manpage it says "map - apply a change to a list to get back a new list with the changes". Anyway I think it is very powerful, and very beautiful, and I use whenever I can!
marcos

[Simplicus]

foreach is cool, but without while you couldn't say while(<FILE>), which is cool, too...
Simplicus

[BBQ]

AHA! But you CAN! One of my 1st posts (back in December) was exactly on this topic. The only reason why its not good to do a

foreach (<FILE>) { }

is because the whole FH gets thrown into memory before the iterations (or so I've been told). But, yeah, practically it wouldn't be that good...

[turnstep]

My favorite is -p

:)

(Yes, it's technically a "while", but a very smooth and subtle "while"...)

[Adam]

for and foreach are synonymous. You can use either to do the same thing.

[BBQ]

Yeah, but foreach is so much more cuddly! Don't you just want to hug it and never let go? I could never go back to for.

[Adam]

You're scaring me, John. :)

[guice]

Well, 'for' and 'foreach' may be the same, but I never use them for it. I use 'foreach' to step through arrays/lists, and I use a 'for' for pattern switching:

for ($var) {
   /test/i  && do { # stuff here
                     last;
              };
   do { # nice default
         last; # not really needed here .. But what the hey :)
   };
}

You can lable it SWITCH and say last SWITCH .. You can even use 'break' instead of 'last' (If I remember right) .. Either way, it works like the C/C++ switch with better matching abilities :) 'foreach' and 'while' do all my loops and walk throughs. 'map' and 'grep' are used for quickly one liners, or two, depening on what I want done ...

The right function for the job. I just don't have use for the formal 'for' loop. Too many things are far better and faster in Perl. Leave 'for(;;;)' for C(++)?, Pascal and (Q|R|V)?Basic.;P

-- philip
Never underestimate the power of very stupid people in large groups.

[Punto]

Well, I come from Pascal, so I don't use those fancy loop things like "foreach", or "map".

If I want to go through an array I do:

for ($i = 0; $i<=$#array; $i++) {
   stuff;
};

I also don't use $_ to get the contents of a file. I do this:

while ($line = <FILE>) {
   stuff;
};

And I put ; at the end of every line..

[BrentDax]

Uh...why?

Personally, I find for-as-foreach more useful:


for my $i(0..$#array) {   #same thing as your C-like for
    stuff;
}

=cut
--Brent Dax


@HPAJ=split("", "rekcaH lreP rentonA tsuJ");
print reverse @HPAJ;  #sucky but who cares?

[Anonymous Monk]

Personally, I find for-as-foreach more useful:

Try make:

for my $i (0 .. $#array) { ... }

efficiently do something like:

for ($i=0; $i < @array; $i += ROWSIZE) { ... } 

[ambrus]

Wait, I've not replied to this yet? Anyway, lots of options seem to be missing. Here are some more possibilities, no doubt there are more.

• recursion with named subs: $_=0;sub f{say;$_++<9and f()}f;
• recursion with subrefs: my$s;$s=sub{say$_[0];$_[0]<9and&$s(1+$_[0])};&$s(0);$s=0;
• recursion via overload, tie, DESTROY, DB, SIG, or other automagical sub calls: $_=0;DESTROY{say$_;$_++<9and bless[]};bless[];
• bounded looping without recursion: $_=0;sub f0{say;$_++<9}eval join"",map{"sub f".(1+$_)."{f$_ and f$_}"}0..7;f8();
• recursion the nice way (ie. without circular references ie. without letrec or mutable variables): my$s=sub{say$_[0];$_[0]<9and&{$_[1]}(1+$_[0],$_[1])};&$s(0,$s);
• redo: $_=0;{say;$_++<9and redo};
• map: map say,0..9;
• grep: grep say,0..9;
• sort: ()=sort{$u{$a}++or say$a;$u{$b}++or say$b;0}0..9;
• repeated substitution: $k=0;$_="a"x10;s/./say$k++/ge;
• (?:{}) with list context g regex or the regex engine itself looping: $k=0;()=("a"x10)=~/.(?{say$k++})/g;
• string eval: $_=0;eval "say(\$_++);"x10; (don't laugh, I heared the developers of the J programming language used this in the very early stage of creating J when it didn't yet have the goto or other control statements)
• own loop library implemented in XS

Update 2010-10-17: see also the other poll If I was forced to use only one kind of loop for the rest of my days it would be a.

[Grok]

What about map?