Passing Reference of a Hash to a method

piyush.shourie has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I am passing the reference of a HASH to a method, which does not produce the desired result.

Refer the following code:

use XML::Simple;
my $config = XMLin("SAMLResponse with attrs.xml");
traverseHash($config);


sub traverseHash
{
    my $hRef=@_;
    print ref($hRef);
}
[download]

The above code produces no output, indicating that the parameter received is not a hash. However, if the type of variable $config is checked using ref() method, it clearly indicates that it is a HASH.

Please clarify this discrepancy in the result.

Thanks in advance,
Piyush

Comment on Passing Reference of a Hash to a method Download Code

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Re: Passing Reference of a Hash to a method by davorg (Chancellor) on Nov 23, 2004 at 10:37 UTC
Your variable $hRef will contain the value '1'. You are evaluating an array (@_) in scalar context, which returns the number of elements in the array (in this case, 1). To get round this, either extract a useful scalar from @_ using one of these approaches: `my $hRef = shift; my $hRef = $_[0];` [download] Or change the assigment to a list assignment: `my ($hRef) = @_;` [download] -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply] [d/l] [select]
Re: Passing Reference of a Hash to a method by dave_the_m (Monsignor) on Nov 23, 2004 at 10:36 UTC
`my $hRef=@_;` That should be `my ($hRef)=@_;` [download] An array in scalar context returns the number of elements, not the first element. Dave.	[reply] [d/l] [select]
Re: Passing Reference of a Hash to a method by edan (Curate) on Nov 23, 2004 at 10:35 UTC
Add some parens: `my ($hRef)=@_;` [download] -- edan	[reply] [d/l]
Re: Passing Reference of a Hash to a method by Zaxo (Archbishop) on Nov 23, 2004 at 10:40 UTC
In, `my $hRef=@_;` you're forcing @_ into scalar context, yielding count of its elements. As called, that is 1, and ref(1) gets you undef. Fix this by making the assignment happen in list context: `sub traverseHash { my ($hRef) = @_; print ref($hRef); }` [download] After Compline, Zaxo	[reply] [d/l] [select]
Re^2: Passing Reference of a Hash to a method by bart (Canon) on Nov 23, 2004 at 11:53 UTC
ref never returns undef. Even `ref(undef)` returns an empty string. `local $\ = "\n"; print defined ref undef; print length ref undef;` [download] prints 1 0	[reply] [d/l] [select]
Re: Passing Reference of a Hash to a method (simpler) by Luca Benini (Scribe) on Nov 23, 2004 at 11:31 UTC
also: my $hRef=shift;	[reply]
Re: Passing Reference of a Hash to a method by rrwo (Friar) on Nov 23, 2004 at 18:25 UTC
You were assigning a list of arguments to a scalar, which was a scalar of the number of arguments ("1"). The ref of a non-reference is blank. Try this: `sub traverseHash { my $hRef=shift; print ref($hRef); }` [download]	[reply] [d/l]
Re^2: Passing Reference of a Hash to a method by davorg (Chancellor) on Nov 24, 2004 at 06:34 UTC
Er... no. If the original poster was assigning a list to a scalar, then the behaviour of the comma operator would ensure that he ended up with the last element in the list in the scalar variable - which would have been what he wanted. `my $scalar = ('this', 'is', 'a', 'list'); print $scalar; # prints 'list'` [download] The problem was caused because he was assigning an array, not a list. Please read What is the difference between a list and an array? from perlfaq4 which explains this difference in more detail. -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply] [d/l]