Subtract two lists in Python without using classes

Question

I have two numeric lists, a, b that I am trying to subtract like this; b - a.

I want this to be easy for a beginner to understand so I don't want to import classes or libraries.

This is what I have tried, and it works:

a = [420, 660, 730, 735] 
b = [450, 675, 770, 930]   
i = 0
j = len(a)
difference = []

while i < j:
    difference.append(b[i] - a[i])
    i += 1
print (difference) 
>>[30, 15, 40, 195]    **the correct result**

However, there must be a simpler way of doing this without importing classes or libraries that I am missing.

Answer 1

A simple way to show this would be:

a = [420, 660, 730, 735] 
b = [450, 675, 770, 930]


print([v2 - v1 for v1, v2 in zip(a, b)])

zip will create a tuple between each of the elements in your list. So if you run zip alone you will have this:

zip(a, b)

[(420, 450), (660, 675), (730, 770), (735, 930)]

Then, to further analyze what is happening in the answer I provided, what you are doing is iterating over each element in your list, and then specifying that v1 and v2 are each item in your tuple. Then the v2 - v1 is pretty much doing your math operation. And all of this is wrapped inside what is called a list comprehension.

If you are still convinced that you still don't want to use zip at all, and if your example is using two equal lists, then what I suggest is to drop the while loop and use a for instead. And your solution will be very similar to what you already have, but as such:

n = []
for i, v in enumerate(a):
    n.append(b[i] - v)
print(n)

So, you have to create a new list that will hold your new data. Use enumerate so you get your index and value through each iteration, and append your math operation to your new list.