在 NumPy 数组中搜索

Numpy 提供了多种方法来搜索不同种类的数值，在本文中，我们将介绍两个重要的方法。

• numpy.where()
• numpy.searchsorted()

1. numpy.where:()它返回满足给定条件的输入数组中元素的索引。

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.

以下示例演示了如何使用where()进行搜索。

Python3

# importing the module
import numpy as np
  
# creating the array
arr = np.array([10, 32, 30, 50, 20, 82, 91, 45])
  
#  printing arr
print("arr = {}".format(arr))
  
#  looking for value 30 in arr and storing its index in i
i = np.where(arr == 30)
print("i = {}".format(i))

输出：

arr = [10 32 30 50 20 82 91 45]
i = (array([2], dtype=int64),)

如您所见，变量 i 是一个可迭代对象，其搜索值的索引作为第一个元素。我们可以通过将最后一个打印语句替换为

print("i = {}".format(i[0]))

这会将最终输出更改为

arr = [10 32 30 50 20 82 91 45]
i = [2]

2. numpy.searchsorted()：该函数用于在排序数组 arr 中查找索引，这样，如果在索引之前插入元素，则 arr 的顺序仍将保留。在这里，使用二分搜索来查找所需的插入索引。

Syntax : numpy.searchsorted(arr, num, side=’left’, sorter=None)

Parameters :

• arr : [array_like] Input array. If sorter is None, then it must be sorted in ascending order, otherwise sorter must be an array of indices that sort it.
• num : [array_like]The Values which we want to insert into arr.
• side : [‘left’, ‘right’], optional.If ‘left’, the index of the first suitable location found is given. If ‘right’, return the last such index. If there is no suitable index, return either 0 or N (where N is the length of a).
• num : [array_like, Optional] array of integer indices that sort array a into ascending order. They are typically the result of argsort.

Return : [indices], Array of insertion points with the same shape as num.

以下示例解释了searchsorted()的使用。

蟒蛇3

# importing the module
import numpy as np
  
# creating the array
arr = [1, 2, 2, 3, 3, 3, 4, 5, 6, 6]
print("arr = {}".format(arr))
  
# left-most 3
print("left-most index = {}".format(np.searchsorted(arr, 3, side="left")))
  
# right-most 3
print("right-most index = {}".format(np.searchsorted(arr, 3, side="right")))

输出：

arr = [1, 2, 2, 3, 3, 3, 4, 5, 6, 6]
left-most index = 3
right-most index = 6