Understanding Functions in C: Definition, Scope, and Usage

1. CSCI 130 Chapter 5 Functions

2. Functions • Functions are named uniquely • Performs a specific task • Is independent • should not interfere with other parts of program • May return a value

3. Function Example • #include <stdio.h> • int square(int, int); //function prototype • int main( void ) { • int a = 3; • int b = 4; • int c = 0; • c = square (a,b); //calling the function • printf("%d", c); • }

4. Function Example - cont’d • Function definition: • int square(int x, int y) { //function header • return (x*y); • } • Function name is: square • Function arguments are: 2 integers (x and y) • Function returns: 1 integer

5. How a function works • Is not executed unless called • Calling program sends in any arguments • information to perform the process • Control returns to statement following function call

6. Parts of a function • Prototype • function name • argument type(s) • return type • Definition • header • actual code

7. Local Variables • Variables declared within a function are local • only defined within the function • not accessible by other functions • Arguments are local as well • Memory used only for duration of function

8. Local Variables - Example 1 • (In main function) • radius = 3 • c = calculateArea(radius); • (Outside main function) • float calculateArea(int a) { • float pi = 3.14; //local variable • return pi * a * a; • }

9. Local variables - Example 2 • (In main function) • ... • int a = 3; • b = squareANumber(a); • printf(“%d ”, a); • printf(“%d”, b); • … • (Outside main function) • squareANumber (int a) { • a = a * a; • return a; • } • Output of this program is: 3 9

10. Local variables - Example 3 • (In main function) • radius = 3 • c = calculateArea(radius); • printf(“The value of pi used here is %f”, pi); //ERROR!!! • (Outside main function) • float calculateArea(int a) { • float pi = 3.14; //local variable • return pi * a * a; • }

11. Return values • Functions may have more than 1 return value, but only 1 is ever returned • bad structure, not suggested • Ex: • if (a > b) • return a; • else • return b;

12. Return types • Return type is declared in function header • Actual value being returned is in return statement • Ex: • int mult(int a, int b) { //return type is int • return (a * b); //returning a*b • }

13. No return type • void specifies that there is no return value • no return statement in function • Ex: • void writeErrorMessage() { • printf(“\nInvalid input”); • printf(“\nPlease check specifications”); • errorCount += 1; • }

14. Calling a function • Calling a function with no return • writeToFile(x); • Calling a function with a return value • x = square(3); • Calling a function as a parameter • c = sumTheValues(square(3), square(4)); • Multiple functions in an expression • b = square(3) + square(4);

15. Global Variables • Placed outside of all functions • Scope of variable is entire program • can be accessed by any function • memory is used for span of program