Using hash for counting

When we have an array or a list of items and we want to find out the number of occurrences of a particular item then we generally use the following kind of logic:

 my $count = 0;
 for (@list) {
     $count++ if $_ eq "apple";
 }

This can be made better by using the grep function like:

    $count = grep $_ eq "apple", @list;

Here we use the list returned by grep in a scalar context here by getting the number of elements in the grep -ed list.However if we have to find the count for more than one element in the list, then by this approach we need to make repetitive sentences like:

    $count_apples = grep $_ eq "apple", @list;
    $count_pears  = grep $_ eq "pear",  @list;

A better method will be something like:

    my %histogram;
    $histogram{$_}++ for @list;

This hash has a count of each individual item in the list and it also traverses the list only once.Moreover, to find number of unique elements in the list,all we have to do is:

    $unique = keys %histogram;

In order to find the five most popular items in the list we can do something like:

    @popular = (sort { $histogram{$b} <=> $histogram{$a} } keys %histogram)[0..4];

This sorts the unique elements of the list ie the keys of hash, in a descending order and pulls out only the top five.