Parameters

Parameters are the way program parts communicate with each other. One program part will call a subprogram to do some task. In order for the subprogram to perform the task some information may need to be given to it. Once the subprogram is done, it may need to give the calling program part the results of its task. Parameters have 3 communication methods:

In only - information is sent IN to the subprogram from the calling program part. The called subprogram gets a COPY of the value sent from the calling program part. This is called "pass by value". (Read only)
In out - information is both sent IN to the subprogram and comes OUT of the subprogram through the same parameter. For example, I may want to send in a value and have the subprogram change that value to something different. Both the calling program part and the subprogram have to have access to the same memory location so that the subprogram can change the value. This is called "pass by reference", or "pass by value- result". C does not have a pass by reference method (C++ does) but it can simulate the action by passing addresses by value. To allow both the calling program part and the called subprogram to have access to the same memory location, C uses the address of the parameter (pointers). The calling program part sends the address of the parameter whose memory needs to be shared to the subprogram. The subprogram receives an address. For example: get_int(&amount) would be used in the calling program part and void get_int(int *num) would be what the subprogram received. (Read-write)
Out only - information comes OUT of the subprogram into a designated memory location but no information comes in. In C, this is accomplished in the same way the in-out is accomplished;however, C and C++ do NOT enforce the idea that no information is passed in. (Write only)

Actual Parameters

Actual parameters are the values that are actually sent to the subprogram. Actual parameters are the way that the calling program part sends information to the called subprogram. In the case of procedures, they can also bring information back from the called subprogram. With functions, all information coming back from the called subprogram is sent back using a function return statement. In C, "return (value_of_type_return_type);".

Formal Parameters

Formal parameters are those defined in the subprogram. They must match the actual parameters in number, type, and order. If you send 3 integers, you must receive 3 integers. If you send an integer, a float, and a character, you must receive an integer, a float, and a character IN THAT ORDER.