Question

I have a 2 dimensional array

v = [ ["ab","12"], ["ab","31"], ["gh","54"] ]

The first element of the subarray of v will have repeating elements, such as "ab". I want to create a hash that puts the key as the first element of the subarray, and values as an array of corresponding second elements from v.

please advice.

Further, I want this, h={“ab”=>[“12″,”31”],”gh”=>[“54”]} and then I want to return h.values, such that the array [[“12″,”31”],[“54”]] is returned

Answer 1

v.inject(Hash.new{|h,k|h[k]=[]}) { |h, (k, v)| h[k] << v ; h}

What it does:

• inject (also called reduce) is a fold. Wikipedia defines folds like this: “a family of higher-order functions that analyze a recursive data structure and recombine through use of a given combining operation the results of recursively processing its constituent parts, building up a return value“.

• The block form of Hash.new takes two arguments, the hash itself and the key. If your default argument is a mutable object, you have to set the default this way, otherwise all keys will point to the same array instance.

• In inject‘s block, we get two arguments, the hash and the current value of the iteration. Since this is a two element array, (k, v) is used to destructure the latter into two variables.

• Finally we add each value to the array for its key and return the entire hash for the next iteration.