Python 2.7全局名稱未定義

Question

編寫一個python程序來執行合並排序，但我得到一個找不到全局名稱的錯誤

### Program for Merge Sort ###
def merge(numbers, l, m, r):
    n1 = m-l-1 ### Size of the left side array ###
    n2 = r-l  ### Size of the right side array ###
    ### Temporary Lists ###
    for i in range(n1):
        L[i] = numbers[l+i]

    for j in range(n2):
        R[j] =  numbers[m+j+1]

    ### The following code eliminates searching if any of the lists have become empty ###
    L[n1] = float("inf")
    R[n1] = float("inf")

    i = 0
    j = 0

    for k in range(l,r+1):
        if(L[i]<=R[j]):
            numbers[k] = L[i]
            i+=1
        else:
            numbers[k] = R[j]
            j+=1

def mergeSort(numbers, l, r):
    if (l<r):
        m = l+(r-1)/2 ### Avoids overflow for large numbers
        mergeSort(numbers, l, m)
        mergeSort(numbers, m+1, r)
        merge(numbers, l, m, r)

我的驅動程序在用戶輸入數字上調用mergeSort函數，運行代碼時出現錯誤，提示未定義全局名稱“ R”

Traceback (most recent call last):
  File "merge_sort.py", line 55, in <module>
    main()
  File "merge_sort.py", line 50, in main
    mergeSort(numbers, 0, size-1)
  File "merge_sort.py", line 32, in mergeSort
    mergeSort(numbers, l, m)
  File "merge_sort.py", line 33, in mergeSort
    mergeSort(numbers, m+1, r)
  File "merge_sort.py", line 34, in mergeSort
    merge(numbers, l, m, r)
  File "merge_sort.py", line 10, in merge
    R[j] =  numbers[m+j+1]
NameError: global name 'R' is not defined`

我究竟做錯了什么？

Answer 1

實際上，您從未定義名稱R 通過替換來解決此問題：

for j in range(n2):
    R[j] =  numbers[m+j+1]

有：

R = [numbers[m+j+1] for j in range(n2)]

（順便說一句，對於L ，同樣的方法－我很驚訝您沒有將其視為錯誤，因為您在名稱R之前訪問了名稱L ）。

Answer 2

您需要先初始化臨時列表L和R ，然后再使用它們。

L = []
R = []
for i in range(n1):
    L[i] = numbers[l+i]

for j in range(n2):
    R[j] =  numbers[m+j+1]