I am writing a calculator program in C. I need a function definition for a function called Subtract() that will allow me to calculate the difference of several numbers entered by the user also need one for division
Not that easy, I guess you need to parse a complex expression like a+b-c*d and evaluate, you need to learn about binary search tree, and about postorder traversal ,etc.
Not necessarily. If it's just a simple calculator program (as the original poster seems to be implying), then the program itself will likely be no more complex than a simple pocket calculator. Remember, a program only has to be as complex as the requirements state.