Re: counting the number of repititions in an array

Alle Wednesday 30 January 2008, Adam Akhtar ha scritto:
> Hi if i want to count the number of times values are repeated in an
> array how do i go about it...is there a method?
>
> My solution is
>
> list.sort
> loop through list and compare one element to the next and count
> repetitions.
>
> is there a simpler way?

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.

This can be written much more concisely as:

p a.inject(Hash.new(0)){|res, i| res[i]+=1; res}


> in C i used the for loop a lot to loop through arrays but i notice that
> in ruby using
> list.each do |element|
> ..
> ..
> end
>
> is quite popular.
>
>  If i wanted to do someting like this:
>
> if list[currElement] == list[currElement + 1]

Array#each_index or Array#each_with_index:

list.each_index do |i|
  if list[i] == list[i+1]
  ...
  endif
end

or

list.each_with_index do |element, i|
 if e == list[i+1]
  ...
  endif
end

Of course, in both cases you'd need to handle yourself the case of the last 
element (in which case, list[i+1] will return nil).

Another possibility, if you only need to access the item after the current one 
is to use each_cons:

require 'enumerator'
a = [1,2,3,4]
a.each_cons(2){|i| p i}
=>
[1, 2]
[2, 3]
[3, 4]

I hope this helps

Stefano

> ...
>
> How would i go about writing that using list.each do |element|?
>
> Any help greatly appreciated.