如何替换列表中的每个第二个特定单词

Question

我在 python 中有这个列表：

['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']

我想用Pear 替换每一秒的Banana（这应该是结果）：

['Pear', 'Apple', 'John', 'Banana', 'Food', 'Pear']

我已经有了这个代码：

with open('text,txt') as f:
    words = f.read().split()

words_B = [word if word != 'Banana' else 'Pear' for word in words]

Answer 1

这是一种通用方法，它通过倒数自上次替换后单词的出现次数来工作：

from collections import defaultdict

d = defaultdict(int)
r = {
    'Banana': 'Pear',
}

fruits = ['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']

def replace(fruits, every, first=True):
  for f in fruits:
    # see if current fruit f should be replaced
    if f in r:
        # count down occurence since last change
        # -- start at 1 if first should be changed otherwise 0
        d.setdefault(f, int(not first))
        d[f] -= 1
        # if we have reached count down, replace
        if d[f] < 0:
            yield r[f]
            d[f] = every - 1 
            continue
    # otherwise append fruit as is
    yield f

=> list(replace(fruits, 2, first=True))

['Pear', 'Apple', 'John', 'Banana', 'Food', 'Pear']

=> list(replace(fruits, 2, first=False))

['Banana', 'Apple', 'John', 'Pear', 'Food', 'Banana']

Answer 2

有点令人费解，但我想我会把它扔在那里。您可以使用itertools.cycle 使用辅助函数，该函数在 Banana 和 Pear 之间交替，其中元素是 Banana 或只是原始值，例如：

from itertools import cycle

data = ['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']
b2p = lambda L,c=cycle(['Banana', 'Pear']): next(c) if L == 'Banana' else L
replaced = [b2p(el) for el in data]
# ['Banana', 'Apple', 'John', 'Pear', 'Food', 'Banana']

Answer 3

（只是要指出，您的预期结果并没有显示您用Pear 替换每一秒Banana，它显示您替换第一个和第三个Banana，而不是第二个。如果确实如此你想要什么，你可以在我下面的代码中将shouldReplace = False更改为shouldReplace = True。）

MSeifert 的解决方案很巧妙，对我作为一个 Python 初学者来说非常有用，但只是指出另一种方法来改变你的列表是这样的：

def replaceEverySecondInstance(searchWord, replaceWord):
    shouldReplace = False
    for index, word in enumerate(words):
        if word == searchWord:
            if shouldReplace == True:
                words[index] = replaceWord
            shouldReplace = not shouldReplace

跑步

print(words)
replaceEverySecondInstance('Banana', 'Pear')
print(words)

给出以下输出：

['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']
['Banana', 'Apple', 'John', 'Pear', 'Food', 'Banana']

Answer 4

您可以使用列表推导获取Banana 的所有索引，然后切片以获取这些索引中的每一秒，然后将相应的列表项设置为Pear：

>>> l = ['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']
>>> for idx in [idx for idx, name in enumerate(l) if name == 'Banana'][::2]:
...     l[idx] = 'Pear'
>>> l
['Pear', 'Apple', 'John', 'Banana', 'Food', 'Pear']

您也可以使用生成器表达式和itertools.islice，而不是理解和切片：

>>> from itertools import islice
>>> l = ['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']
>>> for idx in islice((idx for idx, name in enumerate(l) if name == 'Banana'), None, None, 2):
...     l[idx] = 'Pear'
>>> l
['Pear', 'Apple', 'John', 'Banana', 'Food', 'Pear']

---

另一种可能性，特别是如果您不想就地更改列表，则可以创建自己的生成器函数：

def replace_every_second(inp, needle, repl):
    cnt = 0
    for item in inp:
        if item == needle:    # is it a match?
            if cnt % 2 == 0:  # is it a second occurence?
                yield repl
            else: 
                yield item
            cnt += 1          # always increase the counter
        else:
            yield item

>>> l = ['Banana', 'Apple', 'John', 'Banana', 'Food', 'Banana']
>>> list(replace_every_second(l, 'Banana', 'Pear'))
['Pear', 'Apple', 'John', 'Banana', 'Food', 'Pear']