From strings to arrays to hashes and how to view the values in Perl

Written by Claudio Kuenzler - 0 comments

Published on June 4th 2019 - Listed in Perl Coding

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So I have several key/value pairs stored in a string:

my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';

From string to array

Of course I want to make an array out of these pair. The comma is used as the separator between the pairs:

my @list = split /,/, $list;

Now the array called @list contains two values. They can be shown with a foreach loop:

foreach my $element (@list) {
  print "Aaa: $element\n";
}

Which gives the following output:

# cat bla.pl
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
foreach my $element (@list) {
  print "Aaa: $element\n";
}

# perl bla.pl
Aaa: Reallocated_Sector_Ct=17
Aaa: Uncorrectable_Error_Cnt=34000

Another possibility is to use the data dumper module:

# cat bla.pl
use Data::Dumper qw(Dumper);
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
print Dumper \@list;

# perl bla.pl
$VAR1 = [
          'Reallocated_Sector_Ct=17',
          'Uncorrectable_Error_Cnt=34000'
        ];

The data dumper should only be used for debugging purposes.

From array to hash

As both values in this array are a key/value pair, it makes sense to put these into a hash. The easiest method to create a hash from an existing array would be:

my %list = @list;

But this works only if the array was prepared in the following way: "key1, value1, key2, value2, key3, value3, ...". 
As the array currently looks like "key1=value1,key2=value2" this won't work and results in this:

# cat bla.pl
use Data::Dumper qw(Dumper);
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
my %list = @list;
print Dumper \%list;

# perl bla.pl
$VAR1 = {
          'Reallocated_Sector_Ct=17' => 'Uncorrectable_Error_Cnt=34000'
        };

Not really what I wanted.

A solution for this is to go through each key value pair (through each value in our @list array), splitting up the pair using the equal sign (=) as delimiter, creating a key and a value and push them into the hash:

# cat bla.pl
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
my %list;

foreach my $pair (@list) {
  print "Putting the pair $pair into the hash\n";
  ($key, $value) = split /=/, $pair;
  print "Look at that key: $key\n";
  print "Look at that value $value\n";
  $list{ $key } = $value;
}

# perl bla.pl
Putting the pair Reallocated_Sector_Ct=17 into the hash
Look at that key: Reallocated_Sector_Ct
Look at that value 17
Putting the pair Uncorrectable_Error_Cnt=34000 into the hash
Look at that key: Uncorrectable_Error_Cnt
Look at that value 34000

Using the data dumper again, the hash looks good:

# cat bla.pl
use Data::Dumper qw(Dumper);
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
my %list;

foreach my $pair (@list) {
  ($key, $value) = split /=/, $pair;
  $list{ $key } = $value;
}

print Dumper \%list;

# perl bla.pl
$VAR1 = {
          'Reallocated_Sector_Ct' => '17',
          'Uncorrectable_Error_Cnt' => '34000'
        };

Or, using a for loop, going through the whole hash and print each key value pair:

# cat bla.pl
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
my %list;

foreach my $pair (@list) {
  ($key, $value) = split /=/, $pair;
  $list{ $key } = $value;
}

for my $attribute ( keys %list ) {
      my $value = $list{$attribute};
      print "$attribute => $value\n";
}

# perl bla.pl
Reallocated_Sector_Ct => 17
Uncorrectable_Error_Cnt => 34000

It's of course also possible to print a value of a single key instead of printing the whole hash:

$ cat bla.pl
my $list = 'Reallocated_Sector_Ct=17,Uncorrectable_Error_Cnt=34000';
my @list = split /,/, $list;
my %list;

foreach my $pair (@list) {
  ($key, $value) = split /=/, $pair;
  $list{ $key } = $value;
}

print "Value of Uncorrectable_Error_Cnt is: $list{'Uncorrectable_Error_Cnt'}\n";

$ perl bla.pl
Value of Uncorrectable_Error_Cnt is: 34000

Helpful links and further references:

• Creating hash from an array on Perlmaven
• Perl Hashes on Perlmaven
• Perl Split on Perlmaven
  
• Perl Hash Howto
• Array to hash conversation at Perlmonks

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