Count elements in array ref

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

Hi Monks!

Trying here to verify if an array ref is empty, but this error:

Can't use an undefined value as an ARRAY reference
[download]

Trying like this:

Sample 1:
It works if the array ref has elements

 my $data = data();
 my $count = scalar(@$data); #  $#$data;
print $count;
[download]

Sample 2:

It does not work if the array ref returns nothings
 my $data = data();
 my $count = scalar(@$data); #  $#$data;
print $count;
[download]

Sample 3:
Works but gives me count of 1, since "undef" is been counted I would like to see 0.

my @count = $data;
my $count = @count;
print $count;
[download]

Thanks for helping!

Comment on Count elements in array ref Select or Download Code

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Re: Count elements in array ref by Your Mother (Archbishop) on Mar 29, 2016 at 18:10 UTC
`scalar` is unnecessary due to context. This is an idiom I like. `my $data = data(); my $count = @{ $data \|\| [] };` [download]	[reply] [d/l] [select]
Re^2: Count elements in array ref by Anonymous Monk on Mar 29, 2016 at 18:27 UTC
Just got it, but you were faster, thank you!	[reply]
Re: Count elements in array ref by Corion (Patriarch) on Mar 29, 2016 at 18:01 UTC
Why don't you just check whether `data()` returned something? Alternatively, have `data()` return an empty arrayref when there is nothing to return.	[reply] [d/l] [select]
Re: Count elements in array ref by JohnCub (Beadle) on Mar 29, 2016 at 18:06 UTC
You could try: `my @Data = (); @Data + 0; my $Count = scalar(@Data); print $Count;` [download] Outputs: 0 whereas: `my @Data = ("asdf","345"); @Data + 0; my $Count = scalar(@Data); print $Count;` [download] Outputs: 2	[reply] [d/l] [select]
Re^2: Count elements in array ref by AnomalousMonk (Archbishop) on Mar 29, 2016 at 19:29 UTC
I don't understand the purpose of the `@Data + 0;` statement in your code examples. Could you please explain? (Note that they produce "`Useless use of addition (+) in void context at ...`" warnings if warnings are enabled.) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Count elements in array ref by JohnCub (Beadle) on Mar 29, 2016 at 20:03 UTC
According to the camel book, that's a way to force an array to a scalar context. Their example is `@days + 0; # implicitly force @days into a scalar context` I'm afraid I don't understand it any more than that.	[reply] [d/l]
Re^4: Count elements in array ref by AnomalousMonk (Archbishop) on Mar 29, 2016 at 20:41 UTC
Re: Count elements in array ref by hippo (Archbishop) on Mar 30, 2016 at 09:16 UTC
Maybe I'm missing the point here, but is there some reason that you don't use the $#$ construct? You even have it in your comments. `#!/usr/bin/env perl use strict; use warnings; use Test::More tests => 3; my $data = ['alpha', 'beta']; my $count = $#$data + 1; is ($count, 2, 'With elements'); $data = []; $count = $#$data + 1; is ($count, 0, 'Without elements'); $data = undef; $count = $#$data + 1; is ($count, 0, 'As undef');` [download] By my reading of your post this seems to satisfy all your criteria and is both clear and concise. HTH.	[reply] [d/l]