Simple File Manipulation in a Console Application with C++

The goal of this tutorial is to show you how to make a C++ console application that will read data from a file called "input.txt" that's formatted like this (with each new entry on its own line):

1234567
1234567
1234567

and output it to a file called "output.txt" that is formatted like this (standard CSV (Comma Separated Values) format, except with 25 columns per row maximum):

1234567,1234567,1234567,...(25 times)
1234567,1234567,1234567,...(25 times)
1234567,1234567,1234567,...(18 times)