# [Tutor] Re: Comparing lines in two files, writing result into a third

**pan@uchicago.edu
**
pan@uchicago.edu

*Wed Apr 23 15:05:02 2003*

>* I've tried various combinations of opening the files and 'if' nests, but I
*>* can't seem to get it right. Any hints on what structure to use. Thanks in
*>* advance.
*
Try this:
----------------------------------------------
f1=open('c:/py/digest/f1.txt', 'r')
f2=open('c:/py/digest/f2.txt', 'r')
f3=open('c:/py/digest/f3.txt', 'w')
a= [x.strip() for x in f1.readlines()] # a = ['1','3','4']
b= [x.strip() for x in f2.readlines()] # b = ['1','2','3','4','5']
c= [(((x in a) and (x+'*\n')) or (x+'\n')) for x in b]
f3.writelines(c)
f1.close()
f2.close()
f3.close()
----------------------------------------------
The main part is:
[(((x in a) and (x+'*\n')) or (x+'\n')) for x in b]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
in which (((x in a) and (x+'*\n')) or (x+'\n')) is equivilant to:
if x in a:
return x + '*\n'
else:
return x + '\n'
if let it be f(x), then it becomes:
c= [f(x) for x in b]
Note:
The example I show here doesn't take the following situation into
consideration:
f1: 1,3,4,6 <====
f2: 1,2,3,4,5
'6' is not in f2, so it is not included in the f3.
Since this situation is not described in your question, so I will
just leave it this way.
pan