From: Stefano Crocco <stefano.crocco@...>
Date: 2007-12-26T18:27:38+09:00
Subject: Re: Index of the n-th occurrence

Alle mercoledì 26 dicembre 2007, Marco ha scritto:
> Hi, some newbie questions...
>
> 1) How can I obtain all the indices of the occurrences of a pattern in a
> string (and not only the index of the first one using "index")? And the
> index of the n-th occurrence?

I'm sure there's a better solution, but I can't think of it right now. For the 
first part of your question, notice that String#index (unlike Array#index) 
accept an optional offset argument. You can then do something like:

str ="a test string"

positions = []

pos = -1

while (pos = s.index(your_pattern, pos + 1))
  positions << pos
end

After running this code, positions will contain the indices of all the 
occurrences of your_pattern (which can be any argument String#index accepts) 
inside str.

To get the index of the n-th occurrence, you can extract the n-th element from 
such an array, or add a counter to the while cycle.


> 2) What's the simplest way to select elements of a vector,on the basis of a
> condition on a second vector of the same length?

It depends on what your needs exactly are. Suppose you have the two arrays 

a1=%w[a b c d]
a2=[2, 3, 5, 6]

and want to get the letters in a1 in the same places of even numbers in a2 
(i.e, ["a", "d"]).

These are some approaches (you need to do require 'enumerator' before all of 
them):

a1.enum_for(:each_with_index).select{|e, idx| a2[idx]%2==0}
=> [["a", 0], ["d", 3]]

This returns an array of the pairs of the letter and its index. You can then 
extract the letters with map.



a1.enum_for(:each_with_index).inject([]){|res, i| 
  a2[i[1]]%2 == 0 ? res << i[0] : res }
=> ["a", "d"]



a1.enum_for(:each_with_index).map{|e, idx| a2[idx]%2==0 ? e : nil}.compact

This will not work if a1 can contain nil elements (because they also would be 
removed by nil).

I hope this helps

Stefano