From: Stefano Crocco <stefano.crocco@...>
Date: 2008-01-31T07:12:42+09:00
Subject: Re: counting the number of repititions in an array

Alle Wednesday 30 January 2008, Adam Akhtar ha scritto:
> Stefano Crocco wrote:
> > a = [2,4,6,2,1,3,4,6,4,1,2,3,1]
> > res = Hash.new(0)
> > a.each do |i|
> >   res[i]+=1
> > end
> > p res
> >
> > This creates a hash, which has a default value of 0 (that is, if a key
> > isn't
> > included in the hash, the [] method returns 0). Then, there's an
> > iteration on
> > all items of the array. For each element, the value of the hash item
> > corresponding to the array element is increased by one.
>
> If the hash is empty to begin with, when you try to look up the key
> using res[i]
> wont it just return 0. I cant see where the hash res is assigned with
> the unique values from a.

writing

var += something

is the same as writing

var = var + something

In fact, ruby actually translate the first form into the second. So, when I 
write

res[i] += 1

I mean:

res[i] = res[i] + 1

When the hash is emtpy (or it doesn't contain i), the call to res[i] on the 
right hand gives 0, so that res[i] + 1 becomes 1. Then you have the 
assignment:

res[i] = 1

> Also is it possible to add more keys to a hash - i checked the instance
> method section in the pick axe but there was nothign like pop or push.

To add a new key to a hash, you should use the []= method:

hash =  Hash.new #this creates an empty hash
hash['a'] = 1 # sets the value corresponding to the key 'a' to 1

You need to be careful: if the hash already contained the key 'a', the 
previous value will be replaced by the new:

hash['b'] = 2
hash['b'] = 19

puts hash['b']
=> 19

Stefano