Functions vs. Procedures

A procedural program is made up of the following parts:

A driver - usually main() in C
The driver lays out the logic of the full program and determines which action takes place first, second, etc. The driver does very little work, it usually calls on subprograms to do the actual work, very much like the hiring of subcontractors by a the building contractor who will oversee the entire project.
Subprograms - usually several in a program, rarely 0 or 1
- Subprograms do one job and that job should be decribed by the name of the subprogram. For example, the subprogram calc_area_rectangle() should calculate the area of a rectangle, nothing more or less.
- Subprograms come in two "flavors", functions and procedures.
  Functions adhere to the mathematical concept of a function and procedures are everything else.
- In general, try to use a function whenever it is appropriate. ONLY if it is not appropriate to do so, do you use a procedure.

PROCEDURES (void functions in C)

FUNCTIONS

General Purpose
Adds statements to a language
- input an integer
- get a real >= zero
- swap 2 integer values
- print a menu
Communication methods
- in only (pass by value)
- in out (pass an address in C)
- out only (pass an address in C)
Any results or information needed by the calling program part are returned through out only or in out parameters.
Any valid statement in the language including subprogram calls can be used in the body of a procedure.
The calling program part calls a procedure on a line by itself. A procedure is NOT part of an expression.

Restricted form of a procedure.
Adds operators to a language
- calculate the sum of 2 integers
- calculate the sine of an angle
- calculate the area of a rectangle
- determine if one integer is bigger than another integer
Communication type is in only (pass by value)
One result (no more than one, no less than one, exactly one) is returned to the calling program part through a special function return statement.
The data type of the returned value determines the type of the function.
Any valid statement in the language other than the following may be used in the body of the function. Calling another function is valid.
STATEMENTS THAT SHOULD NOT BE IN THE BODY OF A FUNCTION
- input
- output
- procedure calls
Functions are always called as part of another statement, typically as part of an assignment statement or output statement. Basically, any place you can use an expression like "a + b", you can use a function call such as "plus(a,b)"