I have two arrays: a = numpy.array([0,1,2,3,4,5,6,7,8,9]) b = numpy.array([0,0,1,1,2,2,0,1,2,3]) I would like to get the part of a that corresponds to where b is equal to i. For example: i = 0 => ([0,1,6]) i = 1 => ([2,3,7]) Cheers Tommy