替换某个索引中的字符 [英] Replacing a character from a certain index

问题描述

如何替换来自某个索引的字符串中的字符?比如我想从一个字符串中获取中间的字符，比如abc，如果这个字符不等于用户指定的字符，那么我想替换它.

How can I replace a character in a string from a certain index? For example, I want to get the middle character from a string, like abc, and if the character is not equal to the character the user specifies, then I want to replace it.

可能是这样的吗?

middle = ? # (I don't know how to get the middle of a string)

if str[middle] != char:
    str[middle].replace('')

推荐答案

因为字符串是 不可变在 Python 中，只需创建一个新字符串，其中包含所需索引处的值.

As strings are immutable in Python, just create a new string which includes the value at the desired index.

假设你有一个字符串 s，也许 s = "mystring"

Assuming you have a string s, perhaps s = "mystring"

您可以快速(并且显然)通过将其放置在原始切片"之间来替换所需索引处的部分.

You can quickly (and obviously) replace a portion at a desired index by placing it between "slices" of the original.

s = s[:index] + newstring + s[index + 1:]

您可以通过将字符串长度除以 2 来找到中间值 len(s)/2

You can find the middle by dividing your string length by 2 len(s)/2

如果您收到神秘输入，您应该注意处理超出预期范围的索引

If you're getting mystery inputs, you should take care to handle indices outside the expected range

def replacer(s, newstring, index, nofail=False):
    # raise an error if index is outside of the string
    if not nofail and index not in range(len(s)):
        raise ValueError("index outside given string")

    # if not erroring, but the index is still not in the correct range..
    if index < 0:  # add it to the beginning
        return newstring + s
    if index > len(s):  # add it to the end
        return s + newstring

    # insert the new string between "slices" of the original
    return s[:index] + newstring + s[index + 1:]

这将作为

replacer("mystring", "12", 4)
'myst12ing'

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