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size of an array ref

by fionbarr (Friar)
on Feb 24, 2012 at 12:46 UTC ( #955907=perlquestion: print w/replies, xml ) Need Help??
fionbarr has asked for the wisdom of the Perl Monks concerning the following question:

How do I know the size of a reference to an array....
DB<22> x @a 0 ARRAY(0x8d1814) 0 123 1 456 2 789
if I say scalar @a....I get '1'...I want to get '3'. Thanks

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Re: size of an array ref
by roboticus (Chancellor) on Feb 24, 2012 at 13:03 UTC

    fionbarr:

    You need to use scalar(@{$a[0]}) to get 3. Your array contains only one element, the array reference. So if you want 3, you need to ask the referenced array for its size, not the container. Perhaps this example will help:

    $ cat t.pl #/usr/bin/perl use strict; use warnings; use Data::Dumper; my @a = (1, 2, 3); print "Size of a: ", scalar(@a), "\n"; print Dumper(\@a),"\n"; my @b = [0, 1, 2]; print "Size of b: ", scalar(@b), "\n"; print Dumper(\@b),"\n"; print "Size of b[0]: ", scalar(@{$b[0]}), "\n"; print Dumper($b[0]),"\n";

    which gives us:

    $ perl t.pl Size of a: 3 $VAR1 = [ 1, 2, 3 ]; Size of b: 1 $VAR1 = [ [ 0, 1, 2 ] ]; Size of b[0]: 3 $VAR1 = [ 0, 1, 2 ];

    ...roboticus

    When your only tool is a hammer, all problems look like your thumb.

      that's it! Thanks guys! # array: $ perl -wE 'my @a = qw/a b c/; say scalar @a' 3 # array ref: $ perl -wE 'my $a = qw/a b c/; say scalar @$a'perfect!
Re: size of an array ref
by moritz (Cardinal) on Feb 24, 2012 at 12:52 UTC
      that makes sense...I might have a reference to a reference to an array...I've tried a lot of permutations and not gotten '3'...is it possible to get the size of a ref to a ref?
        @array; @$arrayref; @$$arrayrefref; @$$$arrayrefrefref; @$$$$arrayrefrefrefref; @$$$$$arrayrefrefrefrefref; @$$$$$$arrayrefrefrefrefrefref; ... etc ...
        $a= [1..10];$b=\$a;$c=\$b; $d=\$c;$e=\$d; $e=$$e while (ref($e) eq "REF"); print scalar @$e;
Re: size of an array ref
by swampyankee (Parson) on Feb 24, 2012 at 12:58 UTC

    A question I can still answer!

    The size of a reference to an array is a scalar. If you're looking for the size of the array to which the references is pointing, you may try this:

    print scalar(@{a});

    The curly braces are the dereference operator. Now, if I misunderstood, and you want to know something like the total size of a array of references to arrays, my answer is likely useless.


    Information about American English usage here and here. Floating point issues? Please read this before posting. — emc

      Sorry to disappoint you but the curly braces do not dereference, @{a} is equivalent to @a. The curly braces only allow more complicated constructs with unambiguous precedence, for example ${$a[3]}[4] versus ${$a}[3][4]. If you want dereference, @{$a} or simply @$a is the right choice.

      Update: Thanks to choroba and SuicideJunkie for spotting a $ where a @ should have been

        Insert random cursing here. I should have kept my virtual mouth shut.


        Information about American English usage here and here. Floating point issues? Please read this before posting. — emc

      #!/usr/bin/perl -w use strict; my $aref = [5,6,7,8]; print @$aref,"\n"; #5678 print "@$aref","\n"; #5 6 7 8 print scalar @$aref,"\n"; #4 print ''.@$aref,"\n"; #4 # using string concatenation in the last example # puts @$aref in a scalar context my $ref = [ [qw(a b c)], [qw(x y zzy)] ]; print ''.map{@$_}@$ref; #6 # $ref is a ref to an array of references # @$ref makes a list of those references # the map expands out each array ref into all of its elements # putting this into a scalar context reports the total number print @{$ref->[1]},"\n"; #xyzzy #basically, you need extra {} when a subscript is involved.
Re: size of an array ref
by Anonymous Monk on Feb 24, 2012 at 19:02 UTC
Re: size of an array ref
by TomDLux (Vicar) on Feb 27, 2012 at 19:49 UTC

    @a is an array. The first element of @a is $a[0], which stores a reference to an array of three elements. You can extract the inner array using @{ $a[0] }. Unlike the other suggestions, this works for the (fictional) array stored in $a[1] or other positions, and it can be extended for the third-level, fourth-level, etc.

    As Occam said: Entia non sunt multiplicanda praeter necessitatem.

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[Eily]: kinda sounds like father Christmas is late
[hippo]: (asking because I'm sure you said before about learning English) Did you know there are 2 different words for smaller quantities in English? They are "less" and "fewer". Fewer applies to a number of discrete things like days.
Eily notes also
[hippo]: So if you don't drink as much milk and not as often you are said to drink less milk on fewer days. Confusing, eh?
[Eily]: same as many/much, few/little

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