Question

我尝试将CLRS算法简介中给出的Maximum-Subarray问题的伪代码转换为Python中的完整工作代码。

代码：

def cross(A,low,mid,high):
    left_sum = -float("inf")
    s = 0
    i = mid

    while (i >= low):
        s = s + A[i]
        if s > left_sum:
            left_sum = s
            max_left = i
        i = i - 1

    right_sum = -float("inf")
    s = 0
    j = mid + 1

    while (j < high):
        s = s + A[j]
        if s > right_sum:
            right_sum = s
            max_right = j
        j = j + 1

    return (max_left,max_right,left_sum+right_sum)

def maxSubarray(A,low,high):
    if high == low: return (low,high,A[low])
    else:
        mid = (low+high)/2
        (left_low,left_high,left_sum) = maxSubarray(A,low,mid)
        (right_low,right_high,right_sum) = maxSubarray(A,mid+1,high)
        (cross_low,cross_high,cross_sum) = cross(A,low,mid,high)

        if (left_sum >= right_sum & left_sum >= cross_sum):return (left_low,left_high,left_sum)
        elif (right_sum >= left_sum & right_sum >= cross_sum):return (right_low,right_high,right_sum)
        else: return (cross_low,cross_high,cross_sum)
t = [13,-3,-25,20,-3,-16,-23,18,20,-7,12,-5,-22,15,-4,7]

print maxSubarray(t,0,16)

当我尝试跑步时，我收到此错误。

错误：

Traceback (most recent call last):
  File "/home/suyash/Downloads/python/max_subarray.py", line 64, in <module>
    print maxSubarray(t,0,16)
  File "/home/suyash/Downloads/python/max_subarray.py", line 49, in maxSubarray
    (left_low,left_high,left_sum) = maxSubarray(A,low,mid)
  File "/home/suyash/Downloads/python/max_subarray.py", line 49, in maxSubarray
    (left_low,left_high,left_sum) = maxSubarray(A,low,mid)
  File "/home/suyash/Downloads/python/max_subarray.py", line 49, in maxSubarray
    (left_low,left_high,left_sum) = maxSubarray(A,low,mid)
  File "/home/suyash/Downloads/python/max_subarray.py", line 49, in maxSubarray
    (left_low,left_high,left_sum) = maxSubarray(A,low,mid)
  File "/home/suyash/Downloads/python/max_subarray.py", line 53, in maxSubarray
    (cross_low,cross_high,cross_sum) = cross(A,low,mid,high)
  File "/home/suyash/Downloads/python/max_subarray.py", line 39, in cross
    return (max_left,max_right,left_sum+right_sum)
UnboundLocalError: local variable 'max_right' referenced before assignment

我该如何解决这个问题？我哪里出错？

Answer 1

两个非常小的错误：

您的列表，即t的长度为16.这意味着最后一个索引为15.因此请致电maxSubarray(t,0,15)而不是maxSubarray(t,0,16)
while (j <= high)。循环到j<= high直到j<high

同样通过这两项修复，您无需将任何默认值设置为max_right或max_right。 <{1}}任何while条件在每次递归调用中始终为True。

演示：

if

Answer 2

你忘了考虑边界条件。

当通常成功的条件突然失败时，可能会出现一个问题，使变量保持未绑定状态。无论如何，您都需要确保变量具有合理的默认值。

Answer 3

您只需在max_right循环内的if语句中指定while，这意味着如果您收到异常，则s > right_sum或j < high是从不是真的：

while (j < high):
    s = s + A[j]
    if s > right_sum:
        right_sum = s
        max_right = j
    j = j + 1

您应该max_right while一个默认值在之外max_right = high循环; {{1}}在这里是合适的。

如何修复由于Python中的递归函数调用而导致的UnboundLocalError？

3 个答案: