Check for existance & definedness for Hash and Array references

[kalyanrajsista]

Monks,

I'm confused of how to use the references in conditional statements like

#!/usr/bin/perl
use strict;

my $hash_ref = &getHashRef();

# Check if hash reference has really values in it....
if ( $hash_ref ) {
    .....# do something
}

I want to do the processing only if there is data inside the $hash_ref. eventhough there is no data inside the hashref, it is going into the loop, which it should not be.

I'm trying the following ways. ..

# Which one is correct .....
if ( %$hash_ref ) {
    ......
}

# OR

if ( $hash_ref ) {
    ........
}

Also please provide me check for existence of values inside Array references.....

[ikegami]

When dealing with references, it's often possible for the reference to be undefined. If that's possible here, you want to make sure you got a hash ref, and that the reference hash has some elements.

if ($hash_ref && %$hash_ref)

On the other hand, if you know getHashRef never returns undef, then you don't need to check that.

if (%$hash_ref)

[moritz]

Also please provide me check for existence of values inside Array references.....

It works just like with hash references, but with a @ instead of %. See also: ikegami's reply, perlreftut, perlref, perldsc.

[planetscape]

See also the References section of our very fine Tutorials.

[BrowserUk]

I'm trying the following ways. ..

One of which you've already dismissed as not working. So what happens when you try the second way?

Also, you might consider fixing getHashRef() so that it doesn't return references to empty hashes.

What have you tried for arrays?

[TomDLux]

If you are confused about what's in a variable at a certain point, the best solution is to run the script in the debugger, and examine the variables.

perl -d myscript.pl

while start Perl, load and compile the script, and wait for you to type a command. Try h as your first command, it will print a summary of available commands. l to list sections of your script; n to execute a line of code; s to step into a subroutine; r to continue until the current subroutine returns; x to display the value of a variable ... note that you need to backslash true arrays and hashes, to examine a reference to it, to properly see what's happening:

  DB<1> $h1;

  DB<2> $h2 = {}

  DB<3> $h3 = {has => 'value'}

  DB<4> x $h1
0  undef
  DB<5> x $h2
0  HASH(0x8406f78)
     empty hash
  DB<6> x $h3
0  HASH(0x8406d08)
   'has' => 'value'

h1 is a scalar with no value; h2 is a scalar storing a reference to a hash, but the hash contains no data; h3 is a scalar containing a reference to a hash which has a tag/value pair ... but note that both key and value can be true or false in a boolean sense, and they can both even be undef.