could you clarify why this array kmeans_ret[old_count+j] is used? unable to comprehend use of adding j

[yolob21]

I have read through the blog , still didn't understand why j is not used within kmeans_ret

old_count = 0
		for i in range(n_images):
			l = len(descriptor_list[i])
			for j in range(l):
				if kmeans_ret is None:
					idx = self.kmeans_ret[old_count+j]
				else:
					idx = kmeans_ret[old_count+j]
				self.mega_histogram[i][idx] += 1
			old_count += l
		print "Vocabulary Histogram Generated"