How to get hashes out of arrays in Perl?


I want to write a little "DBQuery" function in perl so I can have one-liners which send an SQL statement and receive back and an array of hashes, i.e. a recordset. However, I'm running into an issue with Perl syntax (and probably some odd pointer/reference issue) which is preventing me from packing out the information from the hash that I'm getting from the database. The sample code below demonstrates the issue.

I can get the data "Jim" out of a hash inside an array with this syntax:

print $records[$index]{'firstName'}

returns "Jim"

but if I copy the hash record in the array to its own hash variable first, then I strangely can't access the data anymore in that hash:

    %row = $records[$index];

returns "" (blank)

Here is the full sample code showing the problem. Any help is appreciated:

my @records = (
   {'id' => 1, 'firstName' => 'Jim'},
   {'id' => 2, 'firstName' => 'Joe'}
my @records2 = ();

$numberOfRecords = scalar(@records);
print "number of records: " . $numberOfRecords . "\n";
for(my $index=0; $index < $numberOfRecords; $index++) {

   print 'you can print the records like this: ' . $records[$index]{'firstName'} . "\n";

   #does NOT work
   %row = $records[$index];
   print 'but not like this: ' . $row{'firstName'} . "\n";



Also note a good perl idiom to use is

for my $rowHR ( @records ) {
   my %row = %$rowHR; 
   #or whatever...

to iterate through the list.

By : jonfm

To add to the lovely answers above, let me add that you should always, always, always (yes, three "always"es) use "use warnings" at the top of your code. If you had done so, you would have gotten the warning "Reference found where even-sized list expected at -e line 1."

By : theorbtwo

Others have commented on hashes vs hashrefs. One other thing that I feel should be mentioned is your DBQuery function - it seems you're trying to do something that's already built into the DBI? If I understand your question correctly, you're trying to replicate something like selectall_arrayref:

This utility method combines "prepare", "execute" and "fetchall_arrayref" into a single call. It returns a reference to an array containing a reference to an array (or hash, see below) for each row of data fetched.

By : zigdon

This video can help you solving your question :)
By: admin