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

Hi monks,

I'm trying to understand the behavior of the given-when construct
when I use an array reference in the "when" clause.
I saw a similar example in "Programming Perl - 4th ed."
but it's not doing what I expected.
I have this test case: 

#!/usr/bin/env perl

use v5.14;

my @array = (0, 1000..10_000, 'abcd');

for my $element (qw/a ab abc 100 1000 10000 abcd/) {
   say "With grep:";
   if (grep {/^$element$/} @array) {
      say "Element $element is there";
   } else {
      say "Element $element is not there";
   }
   
   say "With given-when:";
   given ($element) {
      when (\@array) {
         say "Element $element is there";
      }
      default {
         say "Element $element is not there";
      }
   }
   say "";
}

[download]


It yields this result: 

With grep:
Element a is not there
With given-when:
Element a is there

With grep:
Element ab is not there
With given-when:
Element ab is there

With grep:
Element abc is not there
With given-when:
Element abc is there

With grep:
Element 100 is not there
With given-when:
Element 100 is not there

With grep:
Element 1000 is there
With given-when:
Element 1000 is there

With grep:
Element 10000 is there
With given-when:
Element 10000 is there

With grep:
Element abcd is there
With given-when:
Element abcd is there

[download]


The 'grep' gets it right every time.
The given-when construct finds elements 'a', 'ab' and 'abc' even if the're not in @array.

The funny thing is: if I remove the 0 from @array,
elements 'a', 'ab' and 'abc' are reported as "not there",
so the matching element seems to be the 0.
I'm rather puzzled.

Can someone explain this?

Thanks, bye.

Replies are listed 'Best First'.
Re: given-when construct unexpected bahavior wit arrays
by toolic (Bishop) on Jun 07, 2012 at 13:31 UTC
    warnings kicks out some warning messages: 
    use warnings;

...


With grep:
Element a is not there
With given-when:
Argument "a" isn't numeric in smart match at z line 23.
Element a is there

With grep:
Element ab is not there
With given-when:
Argument "ab" isn't numeric in smart match at z line 23.
Element ab is there

    [download]
[reply]
[d/l]

      Thank you for pointing this out, I thought use v5.14 would enforce strict and warnings, but I found out it's not so.

[reply]
[d/l]
        From use, only strict is used, not warnings:
        if the specified Perl version is greater than or equal to 5.11.0, strictures are enabled lexically as with use strict .
[reply]
Re: given-when construct unexpected bahavior wit arrays
by Neighbour (Friar) on Jun 07, 2012 at 13:32 UTC
    With grep, you're using a regular expression to find matches, which means all numeric values are stringified and then compared (as string).
    You're almost there when you conclude that if you remove 0, elements 'a', 'ab' and 'abc' are not found anymore...
    Apparently given-when does 'a' == 0 (which is true, when converting the string 'a' to a numerical value, it yields 0).

    Edit: Aw, beaten to it by toolic :)
[reply]
Re: given-when construct unexpected bahavior wit arrays
by Tanktalus (Canon) on Jun 07, 2012 at 14:38 UTC

    I have to admit, I find this surprising. Looking at the perldocs for perlop, I see that:

            Left      Right      Description and pseudocode
        ==============================================================
+=
[...]
        Any       ARRAY      smartmatch each ARRAY element[3]
                       like: grep { Any ~~ $_ } ARRAY

    [download]
    Ok, that's fine. So now I need to check how Any ~~ $_ works. So, a little further down, I see:
            Left      Right      Description and pseudocode
        ==============================================================
+=
[...]
        Any       Num        numeric equality
                        like: Any == Num
        Num       nummy[4]    numeric equality
                        like: Num == nummy
        undef     Any        check whether undefined
                        like: !defined(Any)
        Any       Any        string equality
                        like: Any eq Any

    [download]
    Now, this is interesting. When we see a number on the right, we compare with ==, even if the left doesn't look like a number. That seems wrong. I would have expected:
            nummy[4]  Num        numeric equality
                        like: nummy == Num

    [download]
    Maybe a bug report should be opened :-) (Though as I understand it, there's a fair bit of consternation about the whole smart-match thing in the first place.) I would also expect that nummy would include objects with == overridden.

[reply]
[d/l]
[select]
Re: given-when construct unexpected bahavior wit arrays
by brx (Pilgrim) on Jun 07, 2012 at 16:03 UTC

    As Tanktalus said in Re: given-when construct unexpected bahavior wit arrays, this seems not natural.

    Here, something doing what you want (I also change first grep with "eq"), but I feel bad with that trick :-)

    . 
    #!/usr/bin/env perl

use v5.14;

my @array = (0, 1000..10_000, 'abcd');

for my $element (qw/a ab abc 100 1000 10000 abcd/) {
   say "With grep:";
   if (grep {$element eq $_} @array) {
      say "Element $element is there";
   } else {
      say "Element $element is not there";
   }
   
   say "With given-when:";
   given ("_$element") {
      when ([map {"_$_"} @array]) {
         say "Element $element is there";
      }
      default {
         say "Element $element is not there";
      }
   }
   say "";
}

    [download]
[reply]
[d/l]

      Thanks you all, monks.
      I tested the regexp vs "eq" thing in grep (using Benchmark qw/timethis/) and "eq" is waaaaaaay faster than the regexp :D

      As for the smart match thing, I found out that too, and it seems quite broken. What I find strange is that the various comments go back even to 2010, and I thought it should had been fixed by now (<-- feel free to fix the verbs in the previous sentence, I usually get lost in hypothetical sentences :P ).
      I'll benchmark the last solution proposed by brx, just out of curiosity.

      Thank you again!
      Cheers.

[reply]

        Ok, this is the last testcase:

        #!/usr/bin/env perl
# ex: set tabstop=4 noexpandtab:

use v5.14;
use warnings;
use Benchmark qw/timethis/;

my $count = shift || 10_000;

sub grep_in_array {
   my ($element, @array) = @_;
   grep {$element eq $_} @array and return 1;
   return 0;
}

sub is_in_array {
   my ($element, @array) = @_;
   given ("_$element") {
      when ([map {"_$_"} @array]) { return 1; }
   }
   return 0;
}

my @array = (0..10_000, 'abcd');

for my $element (qw/a ab abc 0 1 10 100 1000 10000 10001 abcd/) {

   say "Test with: $element";

   say "With grep:";
   say sprintf("Element %s %s in array", $element, grep_in_array($elem
+ent, @array) ? "is" : "is not");
   
   say "With given-when:";
   say sprintf("Element %s %s in array", $element,   is_in_array($elem
+ent, @array) ? "is" : "is not");

   say "With grep:";
   timethis($count, sub { grep_in_array($element, @array); });
   
   say "With given-when:";
   timethis($count, sub {   is_in_array($element, @array); });

}

        [download]

        And the winner is: grep

        Test with: a
With grep:
Element a is not in array
With given-when:
Element a is not in array
With grep:
timethis 1000:  2 wallclock secs ( 2.17 usr +  0.00 sys =  2.17 CPU) @
+ 460.83/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.34 usr +  0.00 sys =  7.34 CPU) @
+ 136.24/s (n=1000)
Test with: ab
With grep:
Element ab is not in array
With given-when:
Element ab is not in array
With grep:
timethis 1000:  2 wallclock secs ( 2.20 usr +  0.00 sys =  2.20 CPU) @
+ 454.55/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.66 usr +  0.00 sys =  7.66 CPU) @
+ 130.55/s (n=1000)
Test with: abc
With grep:
Element abc is not in array
With given-when:
Element abc is not in array
With grep:
timethis 1000:  2 wallclock secs ( 2.33 usr +  0.00 sys =  2.33 CPU) @
+ 429.18/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.62 usr +  0.00 sys =  7.62 CPU) @
+ 131.23/s (n=1000)
Test with: 0
With grep:
Element 0 is in array
With given-when:
Element 0 is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.17 usr +  0.00 sys =  2.17 CPU) @
+ 460.83/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.24 usr +  0.00 sys =  7.24 CPU) @
+ 138.12/s (n=1000)
Test with: 1
With grep:
Element 1 is in array
With given-when:
Element 1 is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.34 usr +  0.00 sys =  2.34 CPU) @
+ 427.35/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.44 usr +  0.00 sys =  7.44 CPU) @
+ 134.41/s (n=1000)
Test with: 10
With grep:
Element 10 is in array
With given-when:
Element 10 is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.27 usr +  0.00 sys =  2.27 CPU) @
+ 440.53/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.57 usr +  0.01 sys =  7.58 CPU) @
+ 131.93/s (n=1000)
Test with: 100
With grep:
Element 100 is in array
With given-when:
Element 100 is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.13 usr +  0.00 sys =  2.13 CPU) @
+ 469.48/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.97 usr +  0.02 sys =  7.99 CPU) @
+ 125.16/s (n=1000)
Test with: 1000
With grep:
Element 1000 is in array
With given-when:
Element 1000 is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.29 usr +  0.00 sys =  2.29 CPU) @
+ 436.68/s (n=1000)
With given-when:
timethis 1000:  8 wallclock secs ( 7.69 usr +  0.01 sys =  7.70 CPU) @
+ 129.87/s (n=1000)
Test with: 10000
With grep:
Element 10000 is in array
With given-when:
Element 10000 is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.05 usr +  0.00 sys =  2.05 CPU) @
+ 487.80/s (n=1000)
With given-when:
timethis 1000: 10 wallclock secs ( 9.23 usr +  0.01 sys =  9.24 CPU) @
+ 108.23/s (n=1000)
Test with: 10001
With grep:
Element 10001 is not in array
With given-when:
Element 10001 is not in array
With grep:
timethis 1000:  2 wallclock secs ( 2.29 usr +  0.00 sys =  2.29 CPU) @
+ 436.68/s (n=1000)
With given-when:
timethis 1000:  9 wallclock secs ( 8.54 usr +  0.01 sys =  8.55 CPU) @
+ 116.96/s (n=1000)
Test with: abcd
With grep:
Element abcd is in array
With given-when:
Element abcd is in array
With grep:
timethis 1000:  2 wallclock secs ( 2.28 usr +  0.00 sys =  2.28 CPU) @
+ 438.60/s (n=1000)
With given-when:
timethis 1000: 10 wallclock secs ( 9.53 usr +  0.03 sys =  9.56 CPU) @
+ 104.60/s (n=1000)

        [download]

        Bye.

[reply]
[d/l]
[select]

Back to Seekers of Perl Wisdom