CLASS 15 - Functions in C
In Hour 14, "Scope and Storage Classes in C," you might have noticed that a function definition is always given first, before the function is called from a main() function. In fact, you can put a function definition anywhere you want, as long as you keep the function declaration at the first place before the function is called. You'll learn about many function features from the following topics covered in this lesson:
Function declarations
Prototyping
Values returned from functions
Arguments to functions
Structured programming
In addition, several C library functions and macros, such as time(), localtime(), asctime(), va_start(), va_arg(), and va_end() are introduced in this hour.
Learning Structured Programming
Now you've learned the basics of function declaration and definition. Before we go to the next hour, let's talk a little bit about structured programming in program design.
Structured programming is one of the best programming methodologies. Basically, there are two types of structured programming: top-down programming and bottom-up programming.
When you start to write a program to solve a problem, one way to do it is to work on the smallest pieces of the problem. First, you define and write functions for each of the pieces. After each function is written and tested, you begin to put them together to build a program that can solve the problem. This approach is normally called bottom-up programming.
On the other hand, to solve a problem, you can first work out an outline and start your programming at a higher level. For instance, you can work on the main() function at the beginning, and then move to the next lower level until the lowest-level functions are written. This type of approach is called top-down programming.
You'll find that it's useful to combine these two types of structured programming and use them alternately in order to solve a real problem.
Summary
- A function declaration alludes to a function that is defined elsewhere, and specifies what type of arguments and values are passed to and returned from the function as well.
- A function definition reserves the memory space and defines what the function does, as well as the number and type of arguments passed to the function.
- A function can be declared to return any data type, except an array or a function.
- The return statement used in a function definition returns a single value whose type must be matched with the one declared in the function declaration.
- A function call is an expression that can be used as a single statement or within other expressions or statements.
- The void data type is needed in the declaration of a function that takes no argument.
- To declare a function that takes a variable number of arguments, you have to specify at least the first argument, and use an ellipsis (...) to represent the rest of the arguments passed to the function.
- va_start(), va_arg(), and va_end(), all included in stdarg.h, are needed in processing a variable number of arguments passed to a function.
- time(), localtime(), and asctime() are three time functions provided by C. They can be used together to obtain a character string that contains information of local date and time based on the calendar time.
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