abhay180 has asked for the wisdom of the Perl Monks concerning the following question:

Hello, I am trying to create an array of hashes in my code. The key value pairs are pretty repetetive...hence i thought i can generate them as a string and assign it the array. But this doesnt seem to work! 
This is how the array should look like:

$xentry[0] = {BUCK_0 => JUNK, BUCK_1 => JUNK, BUCK_2 => JUNK, BUCK_3 =
+> JUNK} ;
$xentry[1] = {BUCK_0 => JUNK, BUCK_1 => JUNK, BUCK_2 => JUNK, BUCK_3 =
+> JUNK} ;
So on....

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############################################### For this i did the following: 
$str="";
for(my $i=0; $j<100;$i+=1)  {  
   for(my $j=0; $j<4;$j+=1)  {  
     $str .=  "BUCK"."_"."$j"." => "."JUNK".",";  
   }
   #Removes the Comma at the end
   $str = substr($str,0,length($str)-1);
   #Appending flower braces.
   $str = "{".$str."}";
   $xentry[$i] = $str; 
   $str="";

}

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Now if i try accessing "$xentry[0]->{BUCK_0}"...It doesnt seem to work. Can somebody let me know the right thing to do?

Replies are listed 'Best First'.
Re: Perl Array of Hashes
by moritz (Cardinal) on Jul 24, 2009 at 13:11 UTC
    You're not creating a hash, but a string that looks like a hash to you.

    Try something along these lines: 

    my @xentry;
for (0..100) {
   my %h;
   for my $j (0..4) {
       $h{"BUCK_$j"} = 'JUNK';
   }
   push @xentry, \%h;
}

    [download]

    Read perldata and perlreftut for more introductory material.

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[d/l]
      That was indeed very quick. Thanks a lot. It works.
[reply]
        Hi, I wanted to do little more here... 
        my $k=0,$str="";
 for(my $i=0; $i<4;$i+=1)  { 

    @entry=undef;
    for my $j (0..7) { 
    $h{"BUCK_$j"} = $bucket[$k+$j]; 
    push @entry,\%h;
    }
    $hash_table[$i] = { ENTRY  => [@entry]};
    $k+=8;
 } #end of for i

for(my $i=0; $i<4;$i+=1)  {
    print "ENTRY-0-BUCK-0::", 
        $hash_table[$i]->{ENTRY}->{BUCK_0}, "\n"; 
}

        [download]
        The print of "ENTRY-0-BUCK-0" doesnt work....what is that i am doing wrong here?
[reply]
[d/l]

      ...and the short version:

      my @xentry;
push @xentry, { map { ("BUCK_$_" => 'JUNK') } (0 .. 4) } for 0 .. 100;

      [download]
[reply]
[d/l]
Re: Perl Array of Hashes
by davido (Cardinal) on Jul 25, 2009 at 06:41 UTC

    Is it necessary to create "BUCK_1", "BUCK_2", etc? That seems like an array.

    Does "BUCK" ever become something else? What if the datastructure looked more like this: 

    $xentry[0] = { BUCK => [ qw/JUNK JUNK JUNK JUNK/ ],
                  TUBS => [ qw/THUMP THUMP THUMP THUMP/ ] };

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    Then instead of $xentry[0]->{BUCK_1} it would be $xentry[0]->{BUCK}[1], for example. I believe that not trying to create in-fix indexes in a hash key, and instead, letting arrays do your work will prove to be less headaches in the longrun.


    Dave

[reply]
[d/l]
[select]
      David, Thanks for ur suggestion. I wanted to make number of "BUCK_*" as programmable. With every run of prog i want to have either "4/8/16/.." BUCKs in each entry. In a nut shell i am trying to mimic one kind of hash-table implementation,with bucket,entries e.t.c . I need the number of entries and buckets to be configurable from command line. Hence this approach i guess. Any better way of doing it would help. Thanks Abhay
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Re: Perl Array of Hashes
by stevemayes (Scribe) on Jul 25, 2009 at 11:13 UTC

    One thing that helped me when building / understanding hashes, arrays, hashes of hashes, arrays of arrays, etc, was Data::Dumper as in...

    use strict;
use warnings;
use Data::Dumper;

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    This would have shown you the structure of your "array of hashes" immediately.

[reply]
[d/l]
      Hello, Thnaks a ton for all ur suggestions. I wanted to do little more here... 
      
my $k=0,$str="";
 for(my $i=0; $i<4;$i+=1)  { 

    @entry=undef;
    for my $j (0..7) { 
    $h{"BUCK_$j"} = $bucket[$k+$j]; 
    push @entry,\%h;
    }
    $hash_table[$i] = { ENTRY  => [@entry]};
    $k+=8;
 } #end of for i

for(my $i=0; $i<4;$i+=1)  {
    print "ENTRY-0-BUCK-0::", 
        $hash_table[$i]->{ENTRY}->{BUCK_0}, "\n"; 
}

      [download]
      The print of "ENTRY-0-BUCK-0" doesnt work....what is that i am doing wrong here?
[reply]
[d/l]

        Did you use warnings and use strict when you ran the code?

        But that aside, you are doing some strange things:

        my $k=0,$str="";
 for(my $i=0; $i<4;$i+=1)  { 

    @entry=undef;
    for my $j (0..7) { 
        $h{"BUCK_$j"} = $bucket[$k+$j]; 
        push @entry,\%h;
    }

        [download]

        The loop above will add 8 identical references to %h to @entry.

            $hash_table[$i] = { ENTRY  => [@entry]};
    $k+=8;
 } #end of for i

for(my $i=0; $i<4;$i+=1)  {
    print "ENTRY-0-BUCK-0::", 
        $hash_table[$i]->{ENTRY}->{BUCK_0}, "\n";

        [download]

        You can't address $hash_table like that since it is an array of hash references containing just the key ENTRY (which is a warning sign) that has the value array reference containing hash references.

        }

        [download]

        Did you try following the advice and tried Data::Dumper on, say, (updated) @hash_table?

[reply]
[d/l]
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