Python中的 numpy.where()

numpy.where()函数返回输入数组中满足给定条件的元素的索引。

Syntax :numpy.where(condition[, x, y])
Parameters:
condition : When True, yield x, otherwise yield y.
x, y : Values from which to choose. x, y and condition need to be broadcastable to some shape.

Returns:
out : [ndarray or tuple of ndarrays] If both x and y are specified, the output array contains elements of x where condition is True, and elements from y elsewhere.

If only condition is given, return the tuple condition.nonzero(), the indices where condition is True.

代码#1：

# Python program explaining 
# where() function 
  
import numpy as np
  
np.where([[True, False], [True, True]],
         [[1, 2], [3, 4]], [[5, 6], [7, 8]])

输出 ：

array([[1, 6],
       [3, 4]])

代码#2：

# Python program explaining 
# where() function 
  
import numpy as np
  
# a is an array of integers.
a = np.array([[1, 2, 3], [4, 5, 6]])
  
print(a)
  
print ('Indices of elements <4')
  
b = np.where(a<4)
print(b)
  
print("Elements which are <4")
print(a[b])

输出 ：

[[1 2 3]
 [4 5 6]]

Indices of elements <4
(array([0, 0, 0], dtype=int64), array([0, 1, 2], dtype=int64))

Elements which are <4
array([1, 2, 3])