Python:从文件中选择随机行,然后删除该行

时间:2015-09-25 18:20:58

标签: python algorithm random

我是Python的新手(我通过CodeAcademy课程学习了它)并且可以使用一些帮助来解决这个问题。

我有一个'TestingDeleteLines.txt'文件,大约有300行文字。现在,我试图让它从该文件中打印出10条随机行,然后删除这些行。

所以如果我的文件有10行:

Carrot
Banana
Strawberry
Canteloupe
Blueberry
Snacks
Apple
Raspberry
Papaya
Watermelon

我需要从这些行中随机挑出,告诉我它是随机挑选的蓝莓,胡萝卜,西瓜和香蕉,然后删除这些行。

问题是,当Python读取文件时,它会读取该文件,一旦它到达最后,它就不会返回并删除这些行。我目前的想法是,我可以将行写入列表,然后重新打开文件,将列表与文本文件匹配,如果找到匹配项,则删除行。

我目前的问题有两个:

  1. 它复制了随机元素。如果它选择一条线,我需要它不再选择相同的线。但是,使用random.sample似乎不起作用,因为当我稍后使用每一行附加到URL时,我需要将这些行分开。
  2. 我不觉得我的逻辑(写入数组 - >在文本文件中找到匹配 - >删除)是最理想的逻辑。有没有更好的方法来写这个?

    import webbrowser
    import random
    
    """url= 'http://www.google.com'
    webbrowser.open_new_tab(url+myline)""" Eventually, I need a base URL + my 10 random lines opening in each new tab
    
    def ShowMeTheRandoms():
        x=1
        DeleteList= []
        lines=open('TestingDeleteLines.txt').read().splitlines()
    for x in range(0,10):
        myline=random.choice(lines)
        print(myline) """debugging, remove later"""
        DeleteList.append(myline)
        x=x+1
        print DeleteList """debugging, remove later"""
    ShowMeTheRandoms()
    

6 个答案:

答案 0 :(得分:4)

要点是:你不要从文件中“删除”,而是用新内容重写整个文件(或另一个文件)。规范的方法是逐行读取原始文件,将要保留的行写回临时文件,然后用新文件替换旧文件。

with open("/path/to/source.txt") as src, open("/path/to/temp.txt", "w") as dest:
    for line in src:
        if should_we_keep_this_line(line):
            dest.write(line)
os.rename("/path/to/temp.txt", "/path/to/source.txt")

答案 1 :(得分:1)

list.pop怎么样 - 它会为您提供项目并一步更新列表。

--hot

答案 2 :(得分:1)

让我们假设您有一个存储在mywebsite.com/blog-post.html?post=post-alias

中的文件行列表
items

在此处,您将覆盖以前的文本文件,其中>>> items = ['a', 'b', 'c', 'd', 'e', 'f'] >>> choices = random.sample(items, 2) # select 2 items >>> choices # here are the two ['b', 'c'] >>> for i in choices: ... items.remove(i) ... >>> items # tee daa, no more b or c ['a', 'd', 'e', 'f'] 的内容会以您的首选行结尾\ r \ n或\ n加入。 items不会删除行结尾,因此如果您使用该方法,则无需添加自己的行结尾。

答案 3 :(得分:1)

  

我有一个文件,' TestingDeleteLines.txt',大概有300行文字。现在,我试图让它从该文件中打印出10条随机行,然后删除这些行。

#!/usr/bin/env python
import random

k = 10
filename = 'TestingDeleteLines.txt'
with open(filename) as file:
    lines = file.read().splitlines()

if len(lines) > k:
    random_lines = random.sample(lines, k)
    print("\n".join(random_lines)) # print random lines

    with open(filename, 'w') as output_file:
        output_file.writelines(line + "\n"
                               for line in lines if line not in random_lines)
elif lines: # file is too small
    print("\n".join(lines)) # print all lines
    with open(filename, 'wb', 0): # empty the file
        pass

O(n**2)算法can be improved如有必要(您不需要它来处理输入等小文件)

答案 4 :(得分:1)

要从文件中选择随机行,您可以使用节省空间的单遍reservoir-sampling algorithm。要删除该行,您可以打印除所选行之外的所有内容:

#!/usr/bin/env python3
import fileinput

with open(filename) as file:
    k = select_random_it(enumerate(file), default=[-1])[0]

if k >= 0: # file is not empty
    with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
        for i, line in enumerate(file):
            if i != k: # keep line
                print(line, end='') # stdout is redirected to filename

其中select_random_it() implements the reservoir-sampling algorithm

import random

def select_random_it(iterator, default=None, randrange=random.randrange):
    """Return a random element from iterator.

    Return default if iterator is empty.
    iterator is exhausted.
    O(n)-time, O(1)-space algorithm.
    """
    # from https://stackoverflow.com/a/1456750/4279
    # select 1st item with probability 100% (if input is one item, return it)
    # select 2nd item with probability 50% (or 50% the selection stays the 1st)
    # select 3rd item with probability 33.(3)%
    # select nth item with probability 1/n
    selection = default
    for i, item in enumerate(iterator, start=1):
        if randrange(i) == 0: # random [0..i)
            selection = item
    return selection

从文件中打印k个随机行并删除它们:

#!/usr/bin/env python3
import random
import sys

k = 10
filename = 'TestingDeleteLines.txt'
with open(filename) as file:
    random_lines = reservoir_sample(file, k) # get k random lines

if not random_lines: # file is empty
    sys.exit() # do nothing, exit immediately

print("\n".join(map(str.strip, random_lines))) # print random lines
delete_lines(filename, random_lines) # delete them from the file

其中reservoir_sample()使用与select_random_it()相同的算法,但允许选择k个项而不是一个:

import random

def reservoir_sample(iterable, k,
                     randrange=random.randrange, shuffle=random.shuffle):
    """Select *k* random elements from *iterable*.

    Use O(n) Algorithm R https://en.wikipedia.org/wiki/Reservoir_sampling

    If number of items less then *k* then return all items in random order.
    """
    it = iter(iterable)
    if not (k > 0):
        raise ValueError("sample size must be positive")

    sample = list(islice(it, k)) # fill the reservoir
    shuffle(sample)
    for i, item in enumerate(it, start=k+1):
        j = randrange(i) # random [0..i)
        if j < k:
            sample[j] = item # replace item with gradually decreasing probability
    return sample

delete_lines()效用函数删除文件中选择的随机行:

import fileinput
import os

def delete_lines(filename, lines):
    """Delete *lines* from *filename*."""
    lines = set(lines) # for amortized O(1) lookup
    with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
        for line in file:
            if line not in lines:
                print(line, end='')
    os.unlink(filename + '.bak') # remove backup if there is no exception

reservoir_sample()delete_lines()功能不会将整个文件加载到内存中,因此它们可以用于任意大文件。

答案 5 :(得分:0)

也许您可以尝试使用

生成0到300之间的10个随机数
deleteLineNums = random.sample(xrange(len(lines)), 10)

然后通过制作列表推导的副本从行数组中删除:

linesCopy = [line for idx, line in enumerate(lines) if idx not in deleteLineNums]
lines[:] = linesCopy

然后将行写回&#39; TestingDeleteLines.txt&#39;。

要了解上述复制代码的工作原理,这篇文章可能会有所帮助:

Remove items from a list while iterating

编辑:要获取随机生成的索引的行,只需执行以下操作:

actualLines = []
for n in deleteLineNums:
    actualLines.append(lines[n])

然后,actualLines包含随机生成的行索引的实际行文本。

编辑:或者甚至更好,使用列表理解:

actualLines = [lines[n] for n in deleteLineNums]