From: Jeffrey Schwab <jeff@...>
Date: 2006-10-13T00:50:14+09:00
Subject: Re: Isolating non-unique items in an array

Jason Burgett wrote:
> I'm basically trying to the opposite of .uniq Let's say I have an array:
> 
> ["a", "a", "a", "b", "c", "d", "d"]
> 
> I would like to boil this down by first tossing the values that only 
> appear once. Which leaves me with:
> 
> ["a", "a", "a", "d", "d"]
> 
> Then I need to somehow determine that "a" appears 3 times and "d" 
> appears 2 times. Any help would be great. Thanks.

# Output:
# ["a", "a", "a", "d", "d"]
# {"a"=>3, "d"=>2}

def get_counts(keys)
     counts = Hash.new(0)
     keys.each {|k| counts[k] += 1 }
     counts
end

def non_uniq(elements)
     counts = get_counts(elements)
     counts.delete_if {|k, v| v < 2 }
     elements.select {|e| counts.key?(e) }
end

elements = ["a", "a", "a", "b", "c", "d", "d"]

p non_uniq(elements)
p get_counts(elements).delete_if {|k, v| v < 2 }