只将所有分割行的一个字符串写入文件

Question

I have a text file with around 80000 lines. I am trying to do the following steps :-

1. I am trying to find lines based on its first character. I am able to do that.
2. Now I am left with 7500 lines.
3. I am splitting the lines and displaying its line.split()[-2] string.
4. I want to write those strings to a text file, and I am having difficulty with it.

Below is my code, can anybody figure out what am I doing wrong.

f4 = open("lines.txt")    #subroutine to find out number of intervals
for i in range(header_lines):
    for line in f4:
     with open("satsid.txt","w") as f5:
       if line.startswith(" 12"):
          print line.split()[-2]
          f5.write('line.split()[-2]')
          numints = numints+1;
       if not line:
           break      

Answer 1

Ignoring the loop over header_lines the following would replicate your code without re-opening the f5 file each time (which clears it each time you reopen):

with open("lines.txt") as f4, open("satsid.txt","w") as f5:
    for line in f4:
        if line.startswith(" 12"):
            f5.write(line.split()[-2] + '\n')

This writes the 1-but-last element from the line to f5 if the line starts with ' 12' (a space, then a 1, then a 2). I add a newline between these values in the output file.

If you need to skip a number of lines (say a number stored in header_lines ) in f4 , use itertools.islice() to limit the loop:

from itertools import islice

with open("lines.txt") as f4, open("satsid.txt","w") as f5:
    for line in islice(f4, header_lines, None):
        if line.startswith(" 12"):
            f5.write(line.split()[-2] + '\n')

islice() will skip over header_lines lines, then pass through the rest of the file until the end.