Empty arrays returned in Ruby recursion

Question

I am trying to solve a recursion problem.

Code:

def divide(array)
  half = array.length/2
  return array if array.length == 1
  print ary1 = divide(array[0..half - 1])
  print ary2 = divide(array[half..-1])
  merge(ary1, ary2)
end
def merge(ary1, ary2)
  sorted = []
end

divide([10,9,8,-300,250,1,7,6,5,4,3,2])

If I don't comment out the sorted variable, it returns a few blank arrays mixed with numbers:

[10][9][8][][][-300][250][1][][][][7][6][5][][][4][3][2][][][]

I have no idea why naming a variable (but not calling it) would result in a different output than otherwise. Any insight into this would be appreciated.

Schwern · Accepted Answer · 2015-08-18 03:22:41Z

4

Ruby functions which don't have an explicit return statement return the last evaluated expression. In your case that's sorted = [] which evaluates to just []. So merge will always return [].

A call to merge is the last expression in divide, so it too will return [] if it reaches that point.

For this reason, and for others, it's a good idea to always end functions with a return statement. Even if you intend to return the last evaluated expression, it makes your intent clear. If you intend to return nothing, just write return.

answered Aug 18, 2015 at 3:22

Schwern

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