Nested list comprehension with two lists
The reason it has 9 numbers is because python treats
[x + y for x in l2 for y in l1 ]similarly to
for x in l2: for y in l1: x + yie, it is a nested loop
List comprehensions are equivalent to for-loops. Therefore, [x + y for x in l2 for y in l1 ] would become:
new_list = []for x in l2: for y in l1: new_list.append(x + y)Whereas zip returns tuples containing one element from each list. Therefore [x + y for x,y in zip(l1,l2)] is equivalent to:
new_list = []assert len(l1) == len(l2)for index in xrange(len(l1)): new_list.append(l1[index] + l2[index])
The above answers will suffice for your question but I wanted to provide you with a list comprehension solution for reference (seeing as that was your initial code and what you're trying to understand).
Assuming the length of both lists are the same, you could do:
[l1[i] + l2[i] for i in range(0, len(l1))]