在[:index]上使用动态索引进行列表切片 [英] List slicing with dynamic index on [:index]

问题描述

我需要使用负动态索引([:-index])对列表进行切片.这很容易，直到我意识到如果我的动态索引的值为0，则不返回任何项目，而不是返回整个列表.如何以索引为0时返回整个字符串的方式实现此目的? 我的代码很长很复杂，但是基本上这个例子显示了这个问题:

    arr='test text'
    index=2
    print arr[:-index]
    >>'test te'    #Entire string minus 2 from the right
    index=1
    print arr[:-index]
    >>'test tex'    #Entire string minus 1 from the right
    index=0
    print arr[:-index]
    >>''           #I would like entire string minus 0 from the right

注意:我正在使用Python 2.7.

解决方案

另一个有趣的潜在解决方案.

>>> arr = [1, 2, 3]
>>> index = 0
>>> arr[:-index or None]
[1, 2, 3]
>>> index = 1
>>> arr[:-index or None]
[1, 2]

---

要在字符串等不可变序列类型上获得更高的性能，可以在切片操作之前检查索引 的值，从而避免在索引为0的情况下完全对序列进行切片.

这里有三个要测试的功能:

def shashank1(seq, index):
    return seq[:-index or None]

def shashank2(seq, index):
    return index and seq[:-index] or seq

def shashank3(seq, index):
    return seq[:-index] if index else seq

在索引为0的情况下，后两个应该快得多，而在其他情况下则可能要慢(或快).

---

更新的基准代码: http://repl.it/oA5

注意:结果在很大程度上取决于Python的实现.

I need to slice a list using negative dynamic indexes ([:-index]). This was easy until I realized that if the value of my dynamic index was 0, no items were returned, instead of returning the entire list. How can I implement this in a way that when the index is 0, it returns the entire string? My code is very long and complicated, but basically this example shows the problem:

    arr='test text'
    index=2
    print arr[:-index]
    >>'test te'    #Entire string minus 2 from the right
    index=1
    print arr[:-index]
    >>'test tex'    #Entire string minus 1 from the right
    index=0
    print arr[:-index]
    >>''           #I would like entire string minus 0 from the right

Note: I am using Python 2.7.

解决方案

Another potential solution for fun.

>>> arr = [1, 2, 3]
>>> index = 0
>>> arr[:-index or None]
[1, 2, 3]
>>> index = 1
>>> arr[:-index or None]
[1, 2]

---

For higher performance on immutable sequence types like strings, you can avoid slicing the sequence entirely in the case that index is 0 by checking the value of index before the slice operation.

Here's three functions to test in terms of performance:

def shashank1(seq, index):
    return seq[:-index or None]

def shashank2(seq, index):
    return index and seq[:-index] or seq

def shashank3(seq, index):
    return seq[:-index] if index else seq

The latter two should be much faster in the case where index is 0, but may be slower (or faster) in other cases.

---

Updated benchmark code: http://repl.it/oA5

Note: The results depend quite a bit on the Python implementation.

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