如何在Python中从列表b中减去列表a？

Question

So I have 2 lists, say list a and list b where

a = [9, 8, 7, 6]
b = [1, 2, 3, 4]

How would I go about subtracting the contents of b from a ?

Answer 1

You can zip the two lists and subtract the subelements to create a new list:

zip(b,a) -> [(1, 9), (2, 8), (3, 7), (4, 6)]

a = [9, 8, 7, 6]
b = [1, 2, 3, 4]

print([y-x for x,y in zip(b,a)])
[8, 6, 4, 2]

If you want to change a itself use enumerate subtracting elements at common indexes:

for ind,ele in enumerate(a):
    a[ind] -= b[ind]
print(a)
[8, 6, 4, 2]

Or using numpy:

import numpy as np

a = np.array([9, 8, 7, 6])
b = np.array([1, 2, 3, 4])

print(a - b)
[8 6 4 2]

Answer 2

You can use the map function and it's feature to support more than one iterable (the following assumes Python2):

>>> a = [9, 8, 7, 6]
>>> b = [1, 2, 3, 4]
>>> map(lambda x,y: x-y, a, b)
[8, 6, 4, 2]

map applies the first argument (which has to be a function) on all elements of the following arguments. For example:

>>> from math import sqrt
>>> map(sqrt, [1,2,9])
[1.0, 1.4142135623730951, 3.0]

If you use more than two arguments, the function in the first parameter must take more parameters, because it is called with elements from each list:

>>> from math import pow
>>> map(pow, [2,3,4], [2,3,4])
[4.0, 27.0, 256.0]

The result is 2^2, 3^3 and 4^4.

The lambda in my example is just a shorter way to define the subtraction function, the following code would do the same:

def sub(x,y):
  return x-y

map(sub, a, b)

Answer 3

a=[1,2,3,4,5]
b=[9,8,7,6,4]

t=0
h=[]

lenA=len(a)

while lenA != t:
    x=a[t]-b[t]
    t=t+1
    h.append(x)