Python numpy.where()用法及代碼示例

numpy.where(condition [，x，y])函數返回滿足給定條件的輸入數組中元素的索引。

參數:
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.

返回:
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.

代碼1：

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

輸出：

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

代碼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])

輸出：

[[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])