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Subject: Re: iterating over a file with two pointers
From: Travis Griggs <travisgriggs@gmail.com>
In-Reply-To: <e30d9950-7b29-43ed-b85b-455a8d0e9fee@googlegroups.com>
Date: Wed, 18 Sep 2013 09:18:54 -0700
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On Sep 18, 2013, at 5:07 AM, nikhil Pandey <nikhilpandey90@gmail.com> =
wrote:

> On Wednesday, September 18, 2013 4:51:51 PM UTC+5:30, Chris Angelico =
wrote:
>> On Wed, Sep 18, 2013 at 9:12 PM, nikhil Pandey =
<nikhilpandey90@gmail.com> wrote:
>>=20
>>> hi,
>>=20
>>> I want to iterate over the lines of a file and when i find certain =
lines, i need another loop starting from the next of that "CERTAIN" line =
till a few (say 20) lines later.
>>=20
>>> so, basically i need two pointers to lines (one for outer loop(for =
each line in file)) and one for inner loop. How can i do that in python?
>>=20
>>> please help. I am stuck up on this.
>>=20
>>=20
>>=20
>> After the inner loop finishes, do you want to go back to where the
>>=20
>> outer loop left off, or should the outer loop continue from the point
>>=20
>> where the inner loop stopped? In other words, do you want to locate
>>=20
>> overlapping sections, or not? Both are possible, but the solutions
>>=20
>> will look somewhat different.
>>=20
>>=20
>>=20
>> ChrisA
>=20
> Hi Chris,
> After the inner loop finishes, I want to go back to the next line from =
where the outer loop was left i.e the lines of the inner loop will be =
traversed again in the outer loop.
> 1>>I iterate over lines of the file
> 2>> when i find a match in a certain line, i start another loop till =
some condition is met in the subsequent lines
> 3>> then i come back to where i left and repeat 1(ideally i want to =
delete that line in inner loop where that condition is met, but even if =
it is not deleted, its OK)


Just curious, do you really need two loops and file handles? Without =
better details about what you're really doing, but as you've provided =
more detail, it seems to me that just iterating the lines of the file, =
and using a latch boolean to indicate when you should do additional =
processing on lines might be easier. I modified Chris's example input to =
look like:

alpha
*beta
gamma+
delta
epsilon
zeta
*eta
kappa
tau
pi+
omicron

And then shot it with the following:

#!/usr/bin/env python3
with open("samplein.txt") as file:
    reversing =3D False
    for line in (raw.strip() for raw in file):
        if reversing:
            print('____', line[::-1], '____')
            reversing =3D not line.endswith('+')
        else:
            print(line)
            reversing =3D line.startswith('*')

Which begins reversing lines as its working through them, until a =
different condition is met.

Travis Griggs

	=09=