Question

前一段时间，当我遇到这种奇怪的行为时，我一直在测试几种不同的python排序算法。

使用合并排序实现copied from GeeksforGeeks，我注意到我的Numpy数组被数组中的最后一个元素覆盖，而内置Python数组却被正确排序。

此行为的原因是什么？我有一种直觉，它是由引用传递vs值传递错误引起的，但是我不能完全确定原因是什么，因为它适用于内置的python数组。 / p>

注意：此行为也发生在某些其他算法上，而不仅仅是合并排序。

以下是可重现的示例：

import numpy as np

def mergeSort(arr):
    if len(arr) >1:
        mid = len(arr)//2 #Finding the mid of the array 
        L = arr[:mid] # Dividing the array elements  
        R = arr[mid:] # into 2 halves 

        mergeSort(L) # Sorting the first half 
        mergeSort(R) # Sorting the second half 

        i = j = k = 0
        # Copy data to temp arrays L[] and R[] 
        while i < len(L) and j < len(R): 
            if L[i] < R[j]: 
#                 print (arr, L, R)
                arr[k] = L[i]
#                 print (arr, L, R)
                i+=1
            else:
#                 print (arr, L, R)
                arr[k] = R[j] # Problem happens here, R replaces L and messes up calculations for numpy
#                 print (arr, L, R)
                j+=1
            k+=1

        # Checking if any element was left 
        while i < len(L): 
            arr[k] = L[i] 
            i+=1
            k+=1

        while j < len(R): 
            arr[k] = R[j] 
            j+=1
            k+=1

array = ([1,2,3,0])
print (f'python array before {array}')
mergeSort(array)
print (f'python array after {array}')

print ('----------')

array = np.array([1,2,3,0])
print (f'numpy array before {array}')
mergeSort(array)
print (f'numpy array after {array}')

这是控制台输出：

python array before [1, 2, 3, 0]
python array after [0, 1, 2, 3]
----------
numpy array before [1 2 3 0]
numpy array after [0 0 0 0]

我将覆盖的位置缩小到该行：

arr[k] = R[j]

您可以取消注释打印语句，并查看覆盖情况：

python array before [1, 2, 3, 0]
[1, 2] [1] [2]
[1, 2] [1] [2]
[3, 0] [3] [0] <-- Before arr[k] = R[j]
[0, 0] [3] [0] <-- After arr[k] = R[j] -- OK
[1, 2, 3, 0] [1, 2] [0, 3]
[0, 2, 3, 0] [1, 2] [0, 3]
[0, 2, 3, 0] [1, 2] [0, 3]
[0, 1, 3, 0] [1, 2] [0, 3]
[0, 1, 3, 0] [1, 2] [0, 3]
[0, 1, 2, 0] [1, 2] [0, 3]
python array after [0, 1, 2, 3]
----------
numpy array before [1 2 3 0]
[1 2] [1] [2]
[1 2] [1] [2]
[3 0] [3] [0] <-- Before arr[k] = R[j]
[0 0] [0] [0] <-- After arr[k] = R[j] -- BAD
[1 2 0 0] [1 2] [0 0]
[0 2 0 0] [0 2] [0 0]
[0 2 0 0] [0 2] [0 0]
[0 0 0 0] [0 0] [0 0]
numpy array after [0 0 0 0]

转载于：

Python 3.8.3 + Numpy 1.19.0

Jupyter Notebook 5.7.9 + Python 3.6.5 + Numpy 1.14.3

算法会覆盖Numpy数组元素，但不会覆盖Python列表元素

0 个答案: