如何查找字符串中任何一组字符的第一个索引 [英] How to find the first index of any of a set of characters in a string

问题描述

我想找到字符串中第一次出现任何特殊字符的索引，如下所示：

I'd like to find the index of the first occurrence of any "special" character in a string, like so:

>>> "Hello world!".index([' ', '!'])
5

...除了那些无效的Python语法。当然，我可以编写一个模拟这种行为的函数：

…except that's not valid Python syntax. Of course, I can write a function that emulates this behavior:

def first_index(s, characters):
    i = []
    for c in characters:
        try:
            i.append(s.index(c))
        except ValueError:
            pass
    if not i:
        raise ValueError
    return min(i)

我也可以使用正则表达式，但两种解决方案似乎都有点矫枉过正。在Python中有没有理智的方法吗？

I could also use regular expressions, but both solutions seem to be a bit overkill. Is there any "sane" way to do this in Python?

推荐答案

你可以使用枚举和下一步使用生成器表达式，获取第一个匹配项或返回否（如果否）字符出现在s：

You can use enumerate and next with a generator expression, getting the first match or returning None if no character appears in s:

s = "Hello world!"

st = {"!"," "}
ind = next((i for i, ch  in enumerate(s) if ch in st),None)
print(ind)

如果没有，你可以将你想要的任何值作为默认返回值传递匹配。

You can pass any value you want to next as a default return value if there is no match.

如果你想使用一个函数并引发一个ValueError：

If you want to use a function and raise a ValueError:

def first_index(s, characters):
    st = set(characters)
    ind = next((i for i, ch in enumerate(s) if ch in st), None)
    if ind is not None:
        return ind
    raise ValueError

For如果有任何差异，使用集合的较小输入将不会产生太大的影响，但对于大字符串，它将更有效。

For smaller inputs using a set won't make much if any difference but for large strings it will be a more efficient.

一些时间：

在字符串中，字符集的最后一个字符：

In the string, last character of character set:

In [40]: s = "Hello world!" * 100    
In [41]: string = s    
In [42]: %%timeit
st = {"x","y","!"}
next((i for i, ch in enumerate(s) if ch in st), None)
   ....: 
1000000 loops, best of 3: 1.71 µs per loop    
In [43]: %%timeit
specials = ['x', 'y', '!']
min(map(lambda x: (string.index(x) if (x in string) else len(string)), specials))
   ....: 
100000 loops, best of 3: 2.64 µs per loop

不在字符串中，字符集更大：

Not in the string, larger character set:

In [44]: %%timeit
st = {"u","v","w","x","y","z"}
next((i for i, ch in enumerate(s) if ch in st), None)
   ....: 
1000000 loops, best of 3: 1.49 µs per loop

In [45]: %%timeit
specials = ["u","v","w","x","y","z"]
min(map(lambda x: (string.index(x) if (x in string) else len(string)), specials))
   ....: 
100000 loops, best of 3: 5.48 µs per loop

字符串中的第一个字符r字符集：

In string an very first character of character set:

In [47]: %%timeit
specials = ['H', 'y', '!']
min(map(lambda x: (string.index(x) if (x in string) else len(string)), specials))
   ....: 
100000 loops, best of 3: 2.02 µs per loop

In [48]: %%timeit
st = {"H","y","!"}
next((i for i, ch in enumerate(s) if ch in st), None)
   ....: 
1000000 loops, best of 3: 903 ns per loop

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