Fall through loop

[b4swine]

helloed monks,

Often I need to distinguish between a loop that ran to completion, and one that was aborted due to a last statement. For example, if I want to check if every element in a list is less than 0.5, I might write

my @list = (rand(1), rand(1), rand(1));
my $less=1;
for (@list) { $less=0, last if $_>=0.5; }

At the end of this, $less will be true if all the members of the list were small, and false otherwise. I understand that this could have been done via grep in clever ways, but this is just an example. If we are doing a loop, is there a cleaner way to distinguish running a loop to completion from aborting via a last. If not, it would be a nice feature to have a system variable (or some better way), where one could write

for (@list) { last if $_ < 0.5; }
my $less = $loop_ran_to_completion;

---

After thinking about it some more, here is an ugly looking solution, but without any temporary variables:

use Data::Dump;
@list = (rand(1), rand(1), rand(1));
dd \@list;
OUT:
{    ANY:
    {
        for (@list)
        {    next ANY if $_ > 0.5;
        }
        # if all elements are small we come here
        print "All Small\n";
        next OUT
    }
    # if any element is big we come here
    print "Some Big\n";
}

[Athanasius]

Hello b4swine,

I think you’re going to have to add a variable of your own; e.g.:

#! perl
use strict;
use warnings;
use Data::Dump;

my   $count = 0;
my   @list  = (rand(1), rand(1), rand(1));
dd  \@list;

for (@list)
{
    last if $_ < 0.5;
    ++$count;
}

if ($count < @list)
{
    print "exited via last\n";
}
else
{
    print "ran to completion\n";
}

Example output:

23:49 >perl 1610_SoPW.pl
[
  0.894318423773260207,
  0.645644715844781558,
  0.544934475599927026,
]
ran to completion

23:51 >perl 1610_SoPW.pl
[
  0.0333455559602988671,
  0.597882601755799215,
  0.918452691557956058,
]
exited via last

23:51 >

Hope that helps,

[hippo]

You could throw an exception instead of using last:

#!/usr/bin/env perl 
use strict;
use warnings;

use Try::Tiny;

my @list = (rand(1), rand(1), rand(1));
my $less = 1;
try {
    for (@list) { die if $_ >= 0.5; }
} catch {
    $less = 0;
};
print "$less\n";

[AnomalousMonk]

... this could have been done via grep in clever ways ...

You may be thinking of something like:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @list = (rand(1), rand(1), rand(1));
 dd \@list;
 ;;
 my $less = 1;
 for (@list) { $less = 0, last if $_ >= 0.5; }
 print 'for: all satisfy the condition' if $less;
 ;;
 my $any = grep { $_ >= 0.5 } @list;
 print 'grep: none fail the condition' if not $any;
"
[0.860137939453125, 0.796112060546875, 0.875457763671875]


c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @list = (rand(1), rand(1), rand(1));
 dd \@list;
 ;;
 my $less = 1;
 for (@list) { $less = 0, last if $_ >= 0.5; }
 print 'for: all satisfy the condition' if $less;
 ;;
 my $any = grep { $_ >= 0.5 } @list;
 print 'grep: none fail the condition' if not $any;
"
[0.134674072265625, 0.332855224609375, 0.4288330078125]

for: all satisfy the condition
grep: none fail the condition

Because grep must continue to grind through all remaining elements of the list even after it is determined that the overall condition has failed, I'm not sure I would consider this "clever." Quick and dirty, yes, and useful as such.

The for-loop approach has the advantage that it stops immediately when the overall condition is determined to have failed. The guts of List::MoreUtils  any() and  all() and their ilk are based on for-loops. An extra variable is a small price to pay.