CODEWITHSUNDEEP
pythonnumpy
Saeed Reza Kheradpisheh
Saeed Reza Kheradpisheh

Reputation: 55

How to subtract each element of an array from another array?

I have two Numpy arrays A (n x 1) and B (m x 1) of different sizes. I want to subtract each element of B from all the elements of A. Thus the elements of the result matrix C (m x n) should be computed as c(i,j) = A(i)-B(j). Is there any direct loop-less computation using Numpy?

Upvotes: 4

Views: 8753

Answers (3)

Vicrobot
Vicrobot

Reputation: 3988

You can use np.meshgrid

A = np.array([1,2,3,4,5])
B = np.array([5,4,2,7])
a, b= np.meshgrid(A,B)
print(a - b)

#output:- 
array([[-4, -3, -2, -1,  0],
       [-3, -2, -1,  0,  1],
       [-1,  0,  1,  2,  3],
       [-6, -5, -4, -3, -2]])

Second method:- 

C = A - np.array([B]).T
print(C)

#output:- 
array([[-4, -3, -2, -1,  0],
       [-3, -2, -1,  0,  1],
       [-1,  0,  1,  2,  3],
       [-6, -5, -4, -3, -2]])

Upvotes: 4

Mark Bakker
Mark Bakker

Reputation: 821

Broadcasting:

A = np.array([1,2,3,4,5])
B = np.array([5,4,2,7])
A - B[:, np.newaxis]

Output:

array([[-4, -3, -2, -1,  0],
       [-3, -2, -1,  0,  1],
       [-1,  0,  1,  2,  3],
       [-6, -5, -4, -3, -2]])

Upvotes: 7

neko
neko

Reputation: 389

You can be a little more efficient than a straightforward loop if you use list comprehension:

import numpy as np

a = np.array([10, 20, 30, 40])
b = np.array([1, 2])


c = np.array([a - b[j] for j in range(len(b))])
print(c)

output:

[[ 9 19 29 39]
 [ 8 18 28 38]]

Upvotes: -1

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