check array reference empty

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

I wanted to know what is the best method to check if an array reference exists or not. The following is a basic test i have done.

#!/usr/bin/perl

use strict;
use warnings;


my @arr1 = (1,2,3,4);
my @arr2 = ();

print ((@arr1) ? "arr1 is defined\n" : "arr1 is not defined\n");
print ((@arr2) ? "arr2 is defined\n" : "arr2 is not defined\n");


my $arr_ref1 = [11,22,33,44];
my $arr_ref2 = [];


print (($arr_ref1) ? "arr_ref1 is defined\n" : "arr_ref1 is not define
+d\n");
print (($arr_ref2) ? "arr_ref2 is defined\n" : "arr_ref2 is not define
+d\n");


print (($arr_ref1->[0]) ? "arr_ref1 is defined\n" : "arr_ref1 is not d
+efined\n");
print (($arr_ref2->[0]) ? "arr_ref2 is defined\n" : "arr_ref2 is not d
+efined\n");
[download]

Output:

arr1 is defined
arr2 is not defined
arr_ref1 is defined
arr_ref2 is defined
arr_ref1 is defined
arr_ref2 is not defined
[download]

The above method ie $arr_ref->[0] will fail in conditions when the first element is set to undef and the rest of the elements exist.
Your thoughts please

Comment on check array reference empty Select or Download Code

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Re: check array reference empty by jwkrahn (Abbot) on Sep 02, 2011 at 02:06 UTC
Although your message says "defined" you need to use the defined function to test whether a scalar is defined. To determine if a scalar contains a reference to an array use the ref function: `ref( $arr_ref1 ) eq 'ARRAY'`. To determine if `$arr_ref1->[0]` exists then test the size of the array `@$arr_ref1 >= 1`	[reply] [d/l] [select]
Re: check array reference empty by ikegami (Patriarch) on Sep 02, 2011 at 03:31 UTC
It's unclear to me what you want, but then again, I can't seen to focus right now. If `$arr_ref` is an array reference, and you want to check if there are any elements in the referenced array, the following will do: `if (@$arr_ref)` [download] If `$arr_ref` can be either undef or an array reference, and you want to check if there are any elements in the referenced array (if any), the following will do: `if ($arr_ref && @$arr_ref)` [download] If `$arr_ref` can be either undef or an array reference, and you want to check if it's an array reference, the following will do: `if ($arr_ref)` [download]	[reply] [d/l] [select]
Re: check array reference empty by Marshall (Canon) on Sep 03, 2011 at 01:12 UTC
Any element of any array can be "undefined". A reference to an array is defined if the array exists. That array may contain Zero (No) elements. #!/usr/bin/perl -w use strict; my $arr_ref2 = []; print "".defined $arr_ref2 ? "arr_ref2 is defined\n" : "arr_ref2 is not defined\n"; print "".defined $arr_ref2->[0] ? "arr_ref2->[0] is defined\n" : "arr_ref2->[0] is not defined\n"; my $arr_ref3 = [1,2,undef,4]; # [2] is undefined! print "\n"; print "".defined $arr_ref3->[2] ? "arr_ref3->[2] is defined\n" : "arr_ref3->[2] is not defined\n"; print "".defined $arr_ref3->[1] ? "arr_ref3->[1] is defined\n" : "arr_ref3->[1] is not defined\n"; __END__ arr_ref2 is defined arr_ref2->[0] is not defined arr_ref3->[2] is not defined arr_ref3->[1] is defined [download]	[reply] [d/l]
Re: check array reference empty (Please Ignore) by luis.roca (Deacon) on Sep 02, 2011 at 02:11 UTC
Please Ignore: I realized after adding a note that this skips empty values, essentially all but answering the OP's question. One easy way would be to use Data::Dumper `#!/usr/bin/perl use strict; use warnings; use diagnostics; use Data::Dumper; my @arr1 = (1,2,3,4); my @arr2 = (); my $arr_ref1 = [11,22,33,44]; my $arr_ref2 = []; # The last scalar ($VAR8 when printed) does not exist so will return ' +undef' print Dumper( @arr1, @arr2, $arr_ref1, $arr_ref2, $$arr_ref1[0], $$arr_ref2[1] ); __END__ # RETURNS $VAR1 = 1; $VAR2 = 2; $VAR3 = 3; $VAR4 = 4; $VAR5 = [ 11, 22, 33, 44 ]; $VAR6 = []; $VAR7 = 11; $VAR8 = undef;` [download] Note that @arr2 has been skipped but $arr_ref2 has not. Remember that while the first has not been assigned a value the second has: It just so happens to be an empty array. "...the adversities born of well-placed thoughts should be considered mercies rather than misfortunes." — Don Quixote	[reply] [d/l]