Smaller, simpler, subcomponent of program Provides abstraction

Transcription

1 C Function Overview

2 Function Smaller, simpler, subcomponent of program Provides abstraction» hide low-level details» give high-level structure to program, easier to understand overall program flow» enables separable, independent development C functions» zero or multiple arguments passed in» single result returned (optional)» return value is always a particular type In other languages, called procedures, subroutines,

3 Example of High-Level Structure main() { SetupBoard(); /* place pieces on board */ DetermineSides(); /* choose player1/2 */ /* Play game */ do { Player1Turn(); Player2Turn(); } while (NoOutcomeYet()); } Structure of program is evident, even without knowing implementation

4 Functions in C Declaration (also called prototype) int Max(int x, int y); type of return value name of function types of all arguments Function call -- used in expression a = x * Max(f, g+2); evaluate arguments 2, execute function 3. use return value in expression

5 Function Definition State type, name, types of arguments must match function declaration give name to each argument (doesn't have to match declaration) int Max(int x, int y) { } if (x >= y) return x; else return y; gives control back to calling function and returns value - 4 -

6 Why Declaration? Since function definition also includes return and argument types, why is declaration needed? Use might be seen before definition. Compiler needs to know return and arg types and number of arguments. Definition might be in a different file, written by a different programmer.» include a "header" file with function declarations only» compile separately, link together to make executable - 5 -

7 Example double ValueInDollars(double amount, double rate); main() {... dollars = ValueInDollars(francs, DOLLARS_PER_FRANC); printf("%f francs equals %f dollars.\n", francs, dollars);... } declaration function call (invocation) double ValueInDollars(double amount, double rate) { return amount * rate; } definition - 6 -

8 Top-Down Design Top-down design is the process of taking a large problem and breaking it into a handful of smaller problems. Each of those smaller problems may be broken into even smaller problems, and so on. Although there are other approaches to designing programs, top-down design is the primary technique used by all programmers

9 Principles of Top-Down Design: Size Don t let any function get too large. I like to stay under 25 lines of code; we ll allow no more than 35 lines

10 Principles of Top-Down Design: Use Programmer-Defined Functions As you are designing a program, you should identify smaller parts and propose your own programmer-defined function to solve each smaller part. Give thought to each new programmer-defined function, looking for opportunities to make a function that will be useful in many settings. When you are proposing such a function:» Immediately write a prototype & documentation» Temporarily put a stub for the function definition» Keep prototypes and definitions in alphabetical order» Go ahead and use the function in your program» Come back later to change the stub to a real implementation - 9 -

11 Principles of Top-Down Design: Incremental Development and Testing Never write more than 30 to 40 lines without stopping, compiling, and testing as much as possible. When you are satisfied with a test that you ve carried out, make a copy of your program with a new name. These copies (every 30 to 40 lines) are called your restore points

12 Programming Style Some items from our style requirements o o o o Use short functions Use meaningful names with an underscore as part of the name when there are several words stuck together The names of a function must be a noun or noun phrase that describe the function s return value. If a function has no return value (a void function) then its name must be a verb or verb phrase that describes the job it does