Calculator Parser

I wanna know how to go about making a calculator Parser.
I want the user to be able to input problems of different lenghts, but i dont know how to about breaking the problem into the parts.

Im thinking maybe store the problem in a string/char[] and then check everyting one by one.

Any ideas on how to do that, or other ideas about a parser?

well, I'm sure people disagree with me because your not trying to make an expandable large scale app, but to do it right you should look into context-free grammars

For a quick one, buf everythying, first search for * and /, your operators with greater precedence, get the operands on each side and perform the calculation, then go through + and - and do the same thing