Question

Suppose i want to store 3 lines in a file both in python and C++ .
I want to store it like this

aaa
bbb 
ccc ..

But i am giving ccc input first then bbb then aaa. How will I traverse the file from bottom to top and also store from bottom to top/?

Answer 1

It isn’t obvious from the title and question whether you want to store to a file, load from a file, or both, so I’ll cover both cases:

Reading

If it’s OK to load it all into memory at once (in Python):

list(reversed(list(open('foo.txt'))))

Otherwise, it gets a lot more difficult. Processing a file backwards requires that you read blocks of data a time from the end, scanning backwards through each block for newline marker, and stitching things back together at block boundaries.

Writing

If the data all fit in memory at once, put the numbers into a list (in Python):

open('foo.txt', 'w').writelines(reversed(data))

If data is an iterable, replace it with list(data).

If the data doesn’t fit in memory (e.g., you have some generator that spits out a ton of data), the problem will be much harder. The simplest solution that comes to mind is to just push the data into a sqlite database and then copy it into the file. Or you might just find it easier to use the data directly from sqlite.