使用python的合并排序功能中的RunTimeError

Question

I am trying to write a merge sort function using python 2 but i get a RunTimeError

Input: unsorted list (ex:[10,9,8,7,6,5,4,3,2,1])

Output: [1,2,3,4,5,6,7,8,9,10]

My code is shown below:

lst = [10,9,8,7,6,5,4,3,2,1]
def mergesort(array):
if len(array) == 1:
    return array
left = []
right = []
for i in array:
    if i <= len(array)/2:
        left.append(i)
    else:
        right.append(i)
left = mergesort(left)
right = mergesort(right)
return merge(left,right)

def merge(left,right):
    sortedlist= []
    while left and right:
        if left[0]>right[0]:
            sortedlist.append(right[0])
            right.pop(0)
        else:
            sortedlist.append(left[0])
            left.pop(0)
    while left:
        sortedlist.append(left[0])
        left.pop(0)
    while right:
        sortedlist.append(right[0])
        right.pop(0)

    return sortedlist


print mergesort(lst)

The RunTimeError is: maximum recursion depth exceeded . Does anybody know the cause of this error?

Answer 1

You're comparing list values to list indices:

for i in array: # i is a list value
    if i <= len(array)/2: # len(array)/2 is a list index

Change this to if i <= array[len(array)/2]: and it should work.

Answer 2

This code works:

lst = [10,9,8,7,6,5,4,3,2,1]

def mergesort(array):
    if len(array) == 1:
        return array
    left = []
    right = []
    for i in range(len(array)):
        if i < len(array)/2:
            left.append(array[i])
        else:
            right.append(array[i])



    left = mergesort(left)
    right = mergesort(right)
    return merge(left,right)

def merge(left,right):
    sortedlist= []
    while left and right:
        if left[0]>right[0]:
            sortedlist.append(right[0])
            right.pop(0)
        else:
            sortedlist.append(left[0])
            left.pop(0)
    while left:
        sortedlist.append(left[0])
        left.pop(0)
    while right:
        sortedlist.append(right[0])
        right.pop(0)

    return sortedlist


print mergesort(lst)

There was more than one error. You can compare it with your code. Hope it help. Major error was in this line:

if i < len(array)/2 which led right to be empty when len(array)==2 which caused the error.