@matrix printf() inside the while should probably be %c not %s, since it's a character, and not a string, that you're printing.
Also, why not use a "for" loop?
Did you make sure that "input_string" contains the data you want? It's C, so you need to make sure any previous input isn't handed down.
What I mean by that is: sometimes when you press enter to input text, the next input gets the '\n' as the input, because it was still in there from the last input. C is very annoying like that.