[Tutor] How to print the next line in python

Dave Angel davea at ieee.org
Sat Sep 12 11:45:03 CEST 2009

ranjan das wrote:
> Hi,
> I am new to python and i wrote this piece of code which is ofcourse not
> serving my purpose:
> Aim of the code:
> To read a file and look for lines which contain the string 'CL'. When found,
> print the entry of the next line (positioned directly below the string 'CL')
> ....continue to do this till the end of the file (since there are more than
> one occurrences of 'CL' in the file)
> My piece of code (which just prints lines which contain the string 'CL')
> f=open('somefile.txt','r')
> for line in f.readlines():
>      if 'CL' in line:
>               print line
> please suggest how do i print the entry right below the string 'CL'
Easiest way is probably to introduce another local, "previous_line" 
containing the immediately previous line each time through the loop.  
Then if "CL" is in the previous_line, you print current_line.



previous_line = ""

for current_line in infile:
    if 'CL' in previous_line:
        print current_line

          previous_line = current_line

Notice also that your call to readlines() was unnecessary.  You can 
iterate through a text file directly with a for loop.  That won't matter 
for a small file, but if the file is huge, this saves memory, plus it 
could save a pause a the beginning while the whole file is read by 
readlines().  I also added a close(), for obvious reasons.


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