Question

我必须编写一个程序来反转文件中单词的字母。

例如，如果文件包含单词：

snow   
tree
star
wreath

它会将它们变成：

wons
eert
rats
htaerw

一旦完成，我必须编写一个新文件，它将以相反的顺序写入它们，如下所示：

htaerw
rats
eert
wons

这是我的代码：

def reverse(string):
   #empty list for string
   word = []

   #for each letter in the string obtain the corresponding opposite number
   #first letter to last letter, second letter to second last letter, etc...

   for letter in range(len(string)-1, -1, -1):
       word.append(string[letter])

#create main() function
def main():
    #open file and read words in each line
    input_file = open("words.txt", "r")
    word_file = input_file.readlines()
    #empty list for the words in the file, after their letters have been reversed
    reversed_list = []
    for word in range(len(word_file)):
        reverse_word = reverse(word_file[word])
        reversed_list.append(reverse_word)

    #create new file of the reversed words IN REVERSED ORDER!
    reverse_file = open("reverse.txt","w")
    reverse_file.writelines(reversed_list)
    reverse_file.close()

main()

如何在不使用内置.reverse（）函数的情况下编辑主函数以反转单词的顺序？

Answer 1

with open('path/to/input') as infile:
  words = []
  for line in infile:
    words.append(line.strip()[::-1])

with open('path/to/output', 'w') as outfile:
  for word in words[::-1]:
    outfile.write(word)
    outfile.write('\n')

一个衬里（因为我们都喜欢它们）：

with open('path/to/input') as infile:
  words = [line.strip()[::-1] for line in infile]

with open('path/to/output', 'w') as outfile:
  outfile.write('\n'.join(words[::-1]))

Answer 2

reversey = lambda w: w if len(w) < 2 else reversey(w[1:]) + w[0]

>>> reversey("yellow")

reversex = labda x: x if len(x) < 2 else reversex(x[1:]) + [w[0]]
>>>reversex(["yellow","apple","purple","watermelon"])

是一个递归实现...但是有很多方法可以做到这一点...我写了这个函数，你的老师会知道你可能没有写它...但希望你xcan看看我是什么做并将其改为你教授期望的东西

Answer 3

如果你组合rev | tac（如你的情况那样），那么结果就是一个反向字节顺序的文件（忽略空格中可能存在的差异）。要获得所需的输出，您可以从最后一个字节开始读取并一次移动到文件的开头一个字节。

你可以一次读/写：

with open('words.txt', 'rb') as file, open('reverse.txt', 'wb') as outfile:
    outfile.write(file.read()[::-1])

一次一个字节：

#!/usr/bin/env python
"""Print file in the reverse byte order."""
import os

with open('words.txt', 'rb') as file, open('reverse.txt', 'wb') as outfile:
    file.seek(0, os.SEEK_END) # move to the end
    for position in range(file.tell() - 1, -1, -1): # from last position
        file.seek(position, os.SEEK_SET) # move back
        outfile.write(file.read(1)) # read/write byte

缺点是Python中一次读取一个字节的速度很慢。优点是它还支持不适合内存的文件。

mmap module allows to treat a file as a string：

#!/usr/bin/env python3
from mmap import ACCESS_READ, mmap

with open('words.txt') as f, mmap(f.fileno(), 0, access=ACCESS_READ) as s:
    with open('reverse.txt', 'wb') as outfile:
        for i in range(len(s) - 1, -1, -1):
            outfile.write(s[i:i+1])

您还可以阅读一个区块。请参阅Most efficient way to search the last x lines of a file in python。

是否可以在不使用内置.reverse（）函数的情况下反转列表中项目的顺序？

3 个答案: