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finding data from two different files.

Roy Smith roy at panix.com
Fri Oct 18 08:31:27 EDT 2013 
In article <mailman.1193.1382062311.18130.python-list at python.org>,
 "torque.india at gmail.com" <torque.india at gmail.com> wrote:

> Hi all,
> 
> I am new to python, just was looking for logic to understand to write code in 
> the below scenario.
> 
> I am having a file (filea) with multiple columns, and another file(fileb) 
> with again multiple columns, but say i want to use column2 of fileb as a 
> search expression to search for similar value in column3 of filea. and print 
> it with value of rows of filea.
> 
> filea:
> a 1 ab
> b 2 bc
> d 3 de
> e 4 ef
> .
> .
> .
> 
> fileb
> z ab 24
> y bc 85
> x ef 123
> w de 33 
> 
> Regards../ omps

Start by breaking this down into small tasks.  The first thing you need 
to be able to do is open filea, read it, and split each line up into 
columns.  You're going to want something along the lines of:

for line in open("filea"):
   col1, col2, col3 = line.split()

Play with that for a while and make sure you understand what's going on.  
There's the iteration over the lines of a file, the splitting of each 
line into a list of fields, and the unpacking of that list into three 
variables.  Each of those are very common operations that you'll be 
using often.

At some point, you're going to want to say, "I've got a line from fileb 
whose column 2 is 'ab'; what line from filea has 'ab' in column 3?"  
That call for a map.  In Python, it's called a dictionary.  As you read 
fileb, you'll want to build a map, something like:

map = {}
for line in open("filea"):
   col1, col2, col3 = line.split()
   map[col3] = line

Once you've done that, try:

>>> print map

and see what it gives you.  Then, read up on dictionaries

http://docs.python.org/2/tutorial/datastructures.html#dictionaries

and see if the hints I've given you are enough to get the rest of the 
way yourself.  If not, come back and ask more questions.

Oh, also, you didn't say what version of Python you're using.  My 
examples above assumed Python 2.  If you're using Python 3, some minor 
details may change, so let us know which you're using.
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