在 Python 列表中的每个第 n 个元素之后插入元素 [英] Insert element in Python list after every nth element

问题描述

假设我有一个这样的 Python 列表:

letters = ['a','b','c','d','e','f','g','h','i','j']

我想在每个第 n 个元素后插入一个x"，假设该列表中的三个字符.结果应该是:

letters = ['a','b','c','x','d','e','f','x','g','h','我','x','j']

我知道我可以通过循环和插入来做到这一点.我实际上正在寻找的是一种 Python 风格的方式，也许是一种单线方式?

解决方案

我有两个一体式衬垫.

给定:

<预><代码>>>>字母 = ['a','b','c','d','e','f','g','h','i','j']

1. 使用enumerate获取索引，每3个^rd字母添加'x'，eg: mod(n, 3) == 2，然后连接成字符串和list().

   <预><代码>>>>list(''.join(l + 'x' * (n % 3 == 2) for n, l in enumerate(letters)))['a', 'b', 'c', 'x', 'd', 'e', 'f', 'x', 'g', 'h', 'i', 'x', 'j']

   但是作为 @sancho.s 指出如果任何元素有一个以上的字母，这将不起作用.

2. 使用嵌套推导式来展平列表列表^(a)，以 3 个为一组切片，如果距离末尾小于 3，则添加 'x'清单.

   <预><代码>>>>[x for y in (letters[i:i+3] + ['x'] * (i < len(letters) - 2) fori in xrange(0, len(letters), 3)) for x in y]['a', 'b', 'c', 'x', 'd', 'e', 'f', 'x', 'g', 'h', 'i', 'x', 'j']

(a) [item for subgroup in groups for item in subgroup] 展平一个锯齿状的列表列表.

Say I have a Python list like this:

letters = ['a','b','c','d','e','f','g','h','i','j']

I want to insert an 'x' after every nth element, let's say three characters in that list. The result should be:

letters = ['a','b','c','x','d','e','f','x','g','h','i','x','j']

I understand that I can do that with looping and inserting. What I'm actually looking for is a Pythonish-way, a one-liner maybe?

解决方案

I've got two one liners.

Given:

>>> letters = ['a','b','c','d','e','f','g','h','i','j']

1. Use enumerate to get index, add 'x' every 3^rd letter, eg: mod(n, 3) == 2, then concatenate into string and list() it.

   >>> list(''.join(l + 'x' * (n % 3 == 2) for n, l in enumerate(letters)))
   
   ['a', 'b', 'c', 'x', 'd', 'e', 'f', 'x', 'g', 'h', 'i', 'x', 'j']
   

   But as @sancho.s points out this doesn't work if any of the elements have more than one letter.

2. Use nested comprehensions to flatten a list of lists^(a), sliced in groups of 3 with 'x' added if less than 3 from end of list.

   >>> [x for y in (letters[i:i+3] + ['x'] * (i < len(letters) - 2) for
        i in xrange(0, len(letters), 3)) for x in y]
   
   ['a', 'b', 'c', 'x', 'd', 'e', 'f', 'x', 'g', 'h', 'i', 'x', 'j']
   

(a) [item for subgroup in groups for item in subgroup] flattens a jagged list of lists.

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