Create file of single instances of account number

Question

I have a list of account numbers, separated by line, generated by a log of transactions.

How do I consolidate this list into a file that will have only one occurrence of each account instead of many (where more than one transaction per account has been logged)?

Python is preferred but I can also use C.

Answer 1

with open(filename) as fin, open(newfilename, 'w') as fout:
    fout.writelines(set(fin))

Answer 2

I dont know what your log file looks like, but this should work nicely

in python:

file = open('filename.txt', r+)
accountNos = set(file)

file.truncate()
for x in accountNos:
    file.write(x)

file.close()

This takes each line out of the file, and stores them in a set. A set is a data structure that only stores unique elements and removes duplicates. In the second for loop you write the contents of that set back to the file.