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How do you test for return type($,@,%)?

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Contributed by draco_iii on Jun 23, 2000 at 19:53 UTC
Q&A  > subroutines


Description:

I have a routine that can return different information depending on what kind of varible is asked of it.
If asking for a $ it returns a single set of data, if you ask for an @ it will return the array. But I can also have it return the information in a hash format. I've thought about passing the varible as a reference thinking that this would be a better way to determine it.

Is this possible?

Answer: How do you test for return type($,@,%)?
contributed by davorg

A subroutine can only return a scalar or a list. What the caller does with the result is completely out of the subroutine's control.

For example:
sub array { return 0 .. 5; } my %hash = array();

However, you _could_ do something like the code below, but it's pretty strange stuff and I'm not sure how useful it is :-)

#!/usr/bin/perl -w use strict; sub get_stuff { my $ref = shift; if (ref $ref) { if (ref $ref eq 'SCALAR') { $$ref = 'This is a scalar'; } elsif (ref $ref eq 'ARRAY') { @$ref = qw(This is an array); } elsif (ref $ref eq 'HASH' ) { %$ref = (1 => 'This', 2 => 'is', 3 => 'a', 4 => 'Hash'); } else { die "Invalid reference type passed to get_stuff.\n"; } } else { die "Non-reference passed to get_stuff.\n"; } } my ($scalar, @array, %hash); get_stuff(\$scalar); get_stuff(\@array); get_stuff(\%hash); print "$scalar\n"; $"='|'; print "@array\n"; print map { $hash{$_} . ' ' } sort keys %hash;
Answer: How do you test for return type($,@,%)?
contributed by QandAEditors

A subroutine may only know whether its caller wants a SCALAR or an ARRAY. The wantarray function handles this.

sub test { if (wantarray){ return 'One String'; } else { return ('A', 'List', 'of', 'Strings'); } }
There is no built-in way to tell if a subroutine's caller wants a hash, but davorg has posted a sample of how you would do this (see his answer).

The DBI module uses a similar technique (non-relevant code removed for brevity):

sub fetchall_arrayref { my $slice= shift || []; my $mode = ref $slice; if ($mode eq 'ARRAY') { push @rows, [ @$row ] while($row = $sth->fetch); } elsif ($mode eq 'HASH') { push @rows, $row while ($row = $sth->fetchrow_hashref); } else { Carp::croak("fetchall_arrayref($mode) invalid") } return \@rows; }
If you call fetchall_arrayref with an array reference (or no reference), it will return array references. If you call it with a hash reference, it will return hash references.
$sth->fetchall_arrayref( [] ) # returns Array references $sth->fetchall_arrayref( {} ) # returns Hash references

Cool stuff!

Russ

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