finding the differences in 2 hash tables

Posted on 2003-11-04
Last Modified: 2010-05-18
I have a hash table that contains all possible names (called hashtable1)
I also have another hash table (called hashtable2) created by reading a data file (the data file contains names and values)

the only difference is that hashtable1 contains all possibilities and that hashtable2 only contains certain names and values

I want to display a list of the names that are in hashtable1 that are not in hashtable2 (NOTE: the values in the list from hashtable1 that are not in hashtable2 should be set to zero)

i also want to write to a file all the names and values that are in hashtable2 and the names from hashtable1 that aren't in hashtable2

does this make sense ?

Question by:plennon
  • 2
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  • 2
  • +1

Expert Comment

ID: 9680398
# HERE - Complete tested solution:

my(%Hash01) = undef;
my(%Hash02) = undef;

$Hash01{el01} = "01";
$Hash01{el02} = "02";
$Hash01{el03} = "03";
$Hash01{el04} = "04";
$Hash01{el05} = "05";
$Hash01{el06} = "06";

$Hash02{el01} = "01";
$Hash02{el02} = "02";
$Hash02{el06} = "06";

my($key) = undef;
my($OutputHash2) = undef;
my($OutputDifference) = undef;
my($OutFileHash02) = "namesandvaluesofhash02.txt";
my($OutFileHash01) = "notpresentinhash01.txt";

foreach $key (keys %Hash01)
      if (defined $key)
            if (defined $Hash02{$key})
            # If key name from %Hash01 exists in %Hash02
            $OutputHash2 .= "Name $key, Value $Hash02{$key}\n";
            } else {
                  # If key name from %Hash01 Does Not exists in %Hash02
                  # Set the value of this key name to zero
                  $Hash01{$key} = 0;
                  $OutputDifference .= "$key\n";

      open(OUT,  "> $OutFileHash01") or die $!;  
      print OUT $OutputDifference;
      close OUT;

      open(OUT,  "> $OutFileHash02") or die $!;  
      print OUT $OutputHash2;
      close OUT;

Expert Comment

ID: 9680422

Just use the exists keyword to check if each key in hash 1 exists in hash 2 - print out any that aren't.

open LOGFILE, ">mylogfile";

foreach ( keys ( %hashtable1 ) {

   if ( !exists $hashtable{$_} ) {
         print "$_ does not exist in hash2\n";
         print LOGFILE, "$_\n";


close LOGFILE;

Expert Comment

ID: 9680433
forgot a ) on the foreach: should be

open LOGFILE, ">mylogfile";

foreach ( keys ( %hashtable1 ) ){

  if ( !exists $hashtable{$_} ) {
        print "$_ does not exist in hash2\n";
        print LOGFILE, "$_\n";


close LOGFILE;
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LVL 18

Expert Comment

ID: 9681652
How about using grep?

# get the missing names
@missing_names = grep { ! exists $hash2{$_} } keys %hash1;

# display them
print "Missing names: ", join(', ', @missing_names) , $/;

# set values in hash1 to 0
$hash1{$_} = 0 foreach @missing_names;

What I don't really understand is why you would want to write the contents of hash2 to a file, when you got them from a file in the frst place. Could you explain that?

Author Comment

ID: 9687538
The reason i'm writing to a file is because

hash2 is reading info about equipment from a file (the file contains about 20 bits of data per line (i am only reading 2 bits of data  per line, the name and a value)

hash1 contains all the bits of equipment available (only storing the name in hash1)

hash2 stores the name and value in the hash table

all the names and values in hash2 are written to another file with

all the names in hash1 minus the names that are in hash2 (when the names from hash1 are written to file they are given the value zero)

When all info for the equipment is written to a file the information is used to produce a graph

the data is stores as follows

%hash1 = ("001"=>"L-15", "002"=>"L-18", "003"=>"L-25", "004"=>"L-8", );

%hash2 = ("L-15"=>"100", "L-25"=>"55", );

i want to compare the two names

does this make things clearer ???? beacuse i'm confusing myself here!!!!

LVL 18

Accepted Solution

kandura earned 150 total points
ID: 9687808
That does clear up things a bit, yes.
It seems to me that hash1 contains the items in the wrong format: the keys of hash2 are values in hash1.

So if I understand you correctly, you would want the following written out to the file:
L-15, 100
L-25, 55
L-18, 0
L-8, 0


So you do not really need the keys in hash1, only the values.
Now, you could loop over the values in hash1 (that's the elements on the right side of the '=>'),
and setting the value in hash2 to 0 for all the ones that are missing.
After that you print out the contents of hash2 in the format that you want.

# get the missing names
@missing_names = grep { ! exists $hash2{$_} } values %hash1; # note that I use 'values' here

# set their value to 0 in hash2
$hash2{$_} = 0 foreach @missing_names;

# print them out
# first open the file
open F, ">yourfilenamehere" or die $!;

# loop over the hash2
while( my ($k, $v) = each %hash2) {
    print F qq($k, $v), $/;                      # this prints e.g. L-15, 55 on a line
close F;



Author Comment

ID: 9693149

thanks kandura


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