Writing to a text file, binary vs ascii

Question

So I am having the hardest time trying to understand this concept. I have a program that reads a text file, and writes it to another file and replaces the most common words with unsigned chars. But what I cannot for the life of me understand is how then do I determine the difference between the two.

If I write to the new file the original char I read in or an unsigned char value corresponding to 1-255, how then do I determine the difference when I go back in reverse to the original file contents?

Answer 1

When you write a file as binary, then a number such as "1253553" is written using 2 or 4 bytes (depending on the size of the int on the platform). So, in a binary file, you will see a sequence of 2 or 4 bytes representing that number. For chars, it should not make a difference as each char is represented on one byte.

Answer 2

Usually, you have to have some well known and obvious way to determine the format of your file.

One way to do this is to create your own file extension. You could naively expect that any file with that extension is in your compressed format, but it's actually quite likely other files out there have the same extension (e.g., ".dat" is probably a bad choice). So, you'll want to take further steps, like having the first few bytes of the file be something that is unlikely to be there in any other file (some "magic numbers"). Let's use two bytes, and let's simply choose 0xAB 0xCD as those two bytes.

So, when your program is presented with a file that has the proper extension, open it and read the first two bytes. If they're 0xAB and 0xCD, you can assume you're reading your special format.

This isn't a very strong way of accomplishing this task, but it is one way of doing it. You could get more extravagant if you like.

For more information, you might want to read the Wikipedia page on the subject. It's a start.