[Tutor] How to print the next line in python

[Rich Lovely]

2009/9/12 Dave Angel <davea at ieee.org>:
> 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.
>
>
> (untested)
>
> infile=open('somefile.txt','r')
>
> previous_line = ""
>
> for current_line in infile:
>   if 'CL' in previous_line:
>       print current_line
>
>         previous_line = current_line
> infile.close()
>
> 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.
>
> DaveA
>
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If we want to be really modern:

#Only works on python 2.5 or later.  On 2.6 (or higher...), the
following line is not needed.
from __future__ import with_statement

previous_line = ""

with open('somefile.txt','r') as infile:
    for current_line in infile:
        if 'CL' in previous_line:
            print current_line
        previous_line = current_line


-- 
Rich "Roadie Rich" Lovely

There are 10 types of people in the world: those who know binary,
those who do not, and those who are off by one.