Mathematical modeling Python numpy common operations summary

1. Replace elements greater than or less than or equal to a certain value in an array or matrix

The fastest and most concise way is to use Numpy's built-in index.

import numpy as np

a = np.array([[1,2,3],
              [1,2,3],
              [1,2,3]])
a[a > 2] = 0
print(a)

result:

[[1 2 0]
 [1 2 0]
 [1 2 0]]

2. Add the values ​​of rows or columns in a two-dimensional array (matrix)

1), add all

import numpy as np
a = np.array([[1,2,3],
              [1,2,3],
              [1,2,3]])
print(a.sum())

result

18

2), row or column addition

print("行相加：")
print(a.sum(axis=1))
print("列相加：")
print(a.sum(axis=0))

result

行相加：
[6 6 6]
列相加：
[3 6 9]

3), add the specified row or column

print("倒数第一行相加：")
print(a[-1].sum())
print("倒数第一列相加：")
print(a[:, -1].sum())

result

倒数第一行相加：
6
倒数第一列相加：
9