Python:在字符串中查找子串并返回子串的索引

问题描述

我有:

• 一个函数:def find_str(s, char)

和一个字符串:"Happy Birthday",

我本质上想输入 "py" 并返回 3 但我一直让 2 返回.

I essentially want to input "py" and return 3 but I keep getting 2 to return instead.

代码:

def find_str(s, char):
    index = 0
    if char in s:
        char = char[0]
        for ch in s:
            if ch in s:
                index += 1
            if ch == char:
                return index

    else:
        return -1

print(find_str("Happy birthday", "py"))

不知道怎么了！

推荐答案

理想情况下，您应该使用 str.find 或 str.index 就像疯了的刺猬说的.但你说你不能...

Ideally you would use str.find or str.index like demented hedgehog said. But you said you can't ...

您的问题是您的代码仅搜索搜索字符串的第一个字符(第一个字符)位于索引 2 处.

Your problem is your code searches only for the first character of your search string which(the first one) is at index 2.

你基本上是说如果 char[0] 在 s 中，增加 index 直到 ch == char[0] 在我测试时返回 3 但它仍然是错误的.这是一种方法.

You are basically saying if char[0] is in s, increment index until ch == char[0] which returned 3 when I tested it but it was still wrong. Here's a way to do it.

def find_str(s, char):
    index = 0

    if char in s:
        c = char[0]
        for ch in s:
            if ch == c:
                if s[index:index+len(char)] == char:
                    return index

            index += 1

    return -1

print(find_str("Happy birthday", "py"))
print(find_str("Happy birthday", "rth"))
print(find_str("Happy birthday", "rh"))

它产生了以下输出:

3
8
-1

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