Python While 循环 - 变量未更新? [英] Python While Loop - Variable Not Updating?

问题描述

此代码只是在另一个字符串中查找一个字符串，并返回搜索字符串中出现的最后一个位置，如果未找到，则返回 -1.

This code simply looks for a string in another string and returns the last postion of the occurance in the search string or -1 if its is not found.

我不明白为什么我的变量 next_y 没有更新，因为 pos 是计算 next_y 的输入.我的想法是，如果我更新 pos 那么 next_y 也应该更新.取而代之的是 pos 被更新并永远留在循环中.

I don't understand why my variable next_y is not updating considering that pos is an input into computation of next_y. My thought that is that if I update pos then next_y should also update. Instead pos gets updated and remains in the loop forever.

def find_last(x,y):
    if x.find(y) == -1:
        return -1

    pos = x.find(y)
    next_y = x.find(y, pos + 1)

    while next_y != -1:
        pos = pos + next_y

    return pos


search = 'tom ran up but tom fell down'
target = 'tom'

print(find_last(search,target))

推荐答案

您不会在 while 循环中更改 next_y 的值，因此它的值不会更新.next_y 的值计算一次并永远比较(或仅比较一次).要更新此值，您应该在循环中调用 'next_y = x.find(y, pos + 1)'.

You don't change the value of next_y in the while loop, so its value isn't updated. Value of next_y is calculated once and compared for ever (or only once). To update this value you should call 'next_y = x.find(y, pos + 1)' in the loop.

def find_last(x,y):
  if x.find(y) == -1:
    return -1
  pos = x.find(y)
  next_y = x.find(y, pos + 1)
  while next_y != -1:
    pos = pos + next_y
    next_y = x.find(y, pos + 1)
  return pos

search = 'tom ran up but tom fell down'
target = 'tom'

print(find_last(search,target))

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