在循环文件时，文件操作从第一次开始 [英] File operation starts again from first while looping through the file

问题描述

 <$ c $ 
我试图在文件中找到某个单词，并希望在满足条件时打印下一行。 c> f = open（'/ path / to / file.txt'，'r'）
 lines = f.readlines（）
对于行：
 if'P / E 'in line：
n = lines.index（line）#get当前行的索引
 print（lines [n + 1]）#print the next line 
 a.close（）
  

字符串P / E将在文件中出现4次，每次换行。

执行时，代码通常会在前两次出现P / E后打印下一行。然后再次返回并再次打印相同的前两个事件并退出。在前两次出现之后循环没有进行;

我检查了数据文件，看看我的输出是否是实际结果，但所有的下一行在P / E '。

我该如何解决这个问题？感谢。

解决方案

list.index（）只有一个参数找到第一个事件。你必须给它一个起点来找到超过前一个索引的元素， list.index（）取第二个参数，告诉它从哪里开始搜索。 / p>

但是，您不需要使用 lines.index（）;这是非常低效的;它需要一个完整的扫描列表，测试每一行，直到找到匹配。

只要使用 enumerate（）函数在循环中添加索引

pre $ 对于索引，行中枚举（行）：
如果'P / E'在行中：
print（lines [index + 1]）$ ​​b $ b

小心， code> index + 1 不是一个有效的索引;如果你在行列表的最后一行找到'P / E'，你会得到一个 IndexError 。您可能需要添加和index + 1 < len（lines） test。

请注意，使用 file.readlines（）把所有的文件一次性存入内存。尽量避免这种情况;您可以直接在文件中循环，并记住前一行行：

  with open（' /path/to/file.txt','r'）as f：
 previous =''
 for line in f：
 if'P / E'in previous：
 print（line）＃print this line 
 previous = line＃记住下一次迭代
  

I'm trying to find a certain word in a file and want to print the next line when a condition is met.

f = open('/path/to/file.txt','r')
lines = f.readlines()
for line in lines:
    if 'P/E' in line:
        n = lines.index(line) #get index of current line
        print(lines[n+1])     #print the next line
a.close()

The string 'P/E' will be present 4 times in the file, each time in a different line.

When executed, the code prints the next line after the first 2 occurrences of 'P/E' normally. It then again goes back and prints the same first 2 occurrences again and exits. The loop is not proceeding after those first 2 occurrences; it kind of repeats the process and exits.

I checked the data file to see if my output is the actual result, but all next lines are different after 'P/E'.

How can I resolve this? Thanks.

解决方案

list.index() with just one argument only finds the first occurrence. You'd have to give it a starting point to find elements past the previous index, list.index() takes a second argument that tells it where to start searching from.

However, you don't need to use lines.index(); that's very inefficient; it requires a full scan through the list, testing each line until a match is found.

Just use the enumerate() function to add indices as you loop:

for index, line in enumerate(lines):
    if 'P/E' in line:
        print(lines[index + 1])

Be careful, there is a chance index + 1 is not a valid index; if you find 'P/E' in the very last line of the lines list you'll get an IndexError. You may have to add a and index + 1 < len(lines) test.

Note that using file.readlines() reads all of the file into memory in one go. Try to avoid this; you could loop directly over the file, and remember the previous line instead:

with open('/path/to/file.txt','r') as f:
    previous = ''
    for line in f:
        if 'P/E' in previous:
            print(line)  # print this line
        previous = line  # remember for the next iteration

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