The trick is to use a hash to store each value as a key and increment the hash value for each instance of the array value. Take a look at Hash Keys (strings or numbers?), Not Exactly a Hash Tutorial and The Uniqueness of hashes. in the Tutorials section for stuff on hashes.

And if it's not a homework assignment, it would be a good idea to provide some code first, to show how you've attempted to go about it solving it.  (But why are you trying to do this in Perl in the first place, if you have not encountered hashes before??)

In any event, try reading up on hashes first.  A good start might be perlintro.

Here's one way to do it. This way doesn't preserve original order, but it does sort the frequency list based on the numerical order of the input list, which seems to be how your example data was sorted. Given your example input and output, my script produces the output list in the same order your example demonstrated.

I would think it would be more useful to maintain a key/value relationship so that instead of only ending up with a list of frequencies, you would end up with a set of item/frequency pairs. My solution could be easily adapted to do that instead, should your specification change to include that requirment.

use strict;
use warnings;

my @items = ( 1, 3, 3, 3, 5, 5, 5, 5 );
my @frequency = @{ frequency( \@items ) };

print "@frequency\n";

sub frequency {
    my $items_ref = shift;
    my %counter;
    $counter{$_}++ for @{$items_ref};
    return [ map { $counter{$_} } sort { $a <=> $b } keys %counter ];
}
[download]

  my @array = (1,3,3,3,5,5,5,5);
  my @array = (1,3,3,5,5,5,5,3);
[download]

$ perl -Mwarnings -Mstrict -le'
my @array = qw( 1 3 3 3 5 5 5 5 );
my %count;
my @new_array = map {
    ++$count{ $array[ $_ ] };
    no warnings q/uninitialized/;
    $array[ $_ ] eq $array[ $_ + 1 ] ? () : $count{ $array[ $_ ] }
    } 0 .. $#array;
print "@new_array";
'
1 3 4
[download]

#made this events_array up... but the concept remains

It will always be in a particular order, least to greatest in this cas
+e.  
@events_aray = ("1","3","3","4","5","5");

my %days;
my @appears;
my @date_count;
my $c = 0;

foreach my $v (@events_aray) {

#this foreach will split the values of the array I have populated
#it will then produce a hash with keys as a date.
#another array will be generated containing each dates of the event.  
+ 

 my ($date, $subject, $text) = split(/,/, $events_array);
     $days{$date} .= "<b>$subject($symbolx)<b><br><br>$text<br><br>";
    $date_count[$c] = "$date"; #this stores the date of the event, thi
+s will be the array I need to figure out how many times each date app
+ears
    $c++;

}


#some subroutine to figure out how to retrieve the amount.  #The array
+ that I said I needed to generate will be @appears.
#this is where I need a little help on.  I will continue #looking thou
+gh and if resolved I will post back asap.



$c = 0;
foreach my $u (keys %days) {


print "date appeared $appears[$c] times";
print "<br><br>$days{$u}";

$c++;
}
[download]

The count part of the problem I'd solve thus:

use strict;
use warnings;

my @events_aray = (1, 3, 3, 4, 5, 5);
my %days;

++$days{$_} for @events_aray;

print "date $_ appeared $days{$_} times\n"
    for sort {$a <=> $b} keys %days;
[download]

Prints:

date 1 appeared 1 times
date 3 appeared 2 times
date 4 appeared 1 times
date 5 appeared 2 times
[download]

You may need a different sort if your values are not numbers.

---

DWIM is Perl's answer to Gödel

#!/usr/bin/perl -w
use strict;

my @items = ( 1, 3, 3, 1,3, 5, 5, 5, 5,3 );
my %count;
$count{$_}++ for (sort @items);
print "day ".$_." appeared ".$count{$_}." times\n" for (keys %count);
[download]

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my $count={};
my @a =qw(1 2 3 4 5 5 5 5);

map { $count->{$_}++ } @a;

print Dumper($count);
[download]

grep and map in perl

Whilst recent versions of Perl do away with the memory problems of using map in a void context, it still seems strange to me to use map purely for its side effects. Particularly when for usually works just as well.

$count->{$_}++ for @a;
[download]

--
<http://dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg