1. EKT120: Computer Programming
Week 5 – Functions (1)
UniMAP SemII-09/10
EKT120: Computer Programming

2. EKT120: Computer Programming
Outline
Why use functions?
Functions in C
Pre-defined functions
User-defined functions
Function prototypes
Function definitions
Function calls
What about number, order and type of parameter?
Functions that do not return a value
Functions that return a value
Miscellaneous about functions
Sample application
Scope and mechanics of passing values to functions
UniMAP SemII-09/10
EKT120: Computer Programming

3. EKT120: Computer Programming
Why use functions?
Let say you want to print one row of number 8 and one row of number 9
#include <stdio.h>
int main()
{ int i, j;
//print one row of number 8
for(i=1; i<=10; i++)
printf(“8");
printf("\n"); //go to new line
//print one row of number 9
for(j=1; j<=10; j++)
printf(“9“);
return 0; }
UniMAP SemII-09/10
EKT120: Computer Programming

4. EKT120: Computer Programming
Why use functions?
It seems that you are doing the same thing twice!!(i.e. printing two rows of numbers)
This is wasting time and not flexible!!
So, need to use function
UniMAP SemII-09/10
EKT120: Computer Programming

5. EKT120: Computer Programming
Why use functions?
#include <stdio.h>
void display(int); //function prototype
int main() {
display(8); //function call
display(9); //function call
return 0; }
void display(int value) //function definition
{ int i;
for(i=1; i<=10; i++)
printf("%d", value);
printf("\n"); //go to new line
UniMAP SemII-09/10
EKT120: Computer Programming

6. EKT120: Computer Programming
Functions in C
Functions can be created to execute small, frequently-used tasks
In C, there are predefined functions or sometimes called standard functions, and there are user-defined functions.
Predefined functions are already available functions that can be used, called library
The usage is like stdio.h, in which the library name must be #included at the top of the source code (preprocessor directive)
UniMAP SemII-09/10
EKT120: Computer Programming

7. Predefined Functions (Library)
Common libraries are stdio.h, math.h, string.h, and stdlib.h
stdio.h related functions: printf, scanf,etc
math.h related functions: sin, cos, exp, pow, sqrt, etc.
string.h related functions: strcmp, strcpy, strlen, etc.
stdlib.h related functions: abs, fabs
UniMAP SemII-09/10
EKT120: Computer Programming

8. Predefined Functions (Library)-example
#include <stdio.h>
#include <math.h>
#include <string.h>
void main() {
string name;
int vol1, vol2, n, R, kTemp, length;
strcpy(name, “Marina”);
vol2 = vol1 * exp(n * R * kTemp);
length = strlen(“Mahathir”); }
UniMAP SemII-09/10
EKT120: Computer Programming

9. User-Defined Functions
What do we need to define and make use of user-defined functions?
Function prototypes
Function definitions
Function calls
UniMAP SemII-09/10
EKT120: Computer Programming

10. EKT120: Computer Programming
Function Prototypes
Function prototype is a declaration; indicates the function exists
Should have function name, return type and parameter
Argument name is not compulsory in function header
Function prototype has the following form:
<return_type> <function_name> (arg_type arg_name, ...);
int sum (int num1,int num2);
int sum (int,int); //is also acceptable
semicolon
UniMAP SemII-09/10
EKT120: Computer Programming

11. EKT120: Computer Programming
Function Definitions
Function definition includes the body of a function
Function definition has the following form:
<return_type> <function_name> (arg_type arg_name, ...) {
… statements … }
int sum (int num1,int num2)
int add;
add = num1 + num2;
return(add);
Notice that argument name is used in the function body
Unlike function prototype, argument name in function definition must be included in function header
no semicolon
function header
UniMAP SemII-09/10
EKT120: Computer Programming

12. EKT120: Computer Programming
Function Calls
Consists of a function name followed by an argument expression list enclosed in parentheses
Function call has the following form:
<function_name> (exp, exp ...)
exp is an expression – can be variable or constant
result = sum(x,y);
UniMAP SemII-09/10
EKT120: Computer Programming

13. Example of function in program
//This program sums up two numbers
#include <stdio.h>
int sum(int, int); //function prototype
int main()
{ int x,y, result;
printf( “Enter x and y : ”);
scanf(“%d %d”, &x, &y);
result = sum(x,y); //function call
printf(“Sum is : %d”, result);
return 0; }
int sum(int num1, int num2)//function definition
{ int add;
add = num1+num2;
return(add);}
function header
EKT120: Computer Programming

14. What about number, order and type of parameter?
Number, order and type of parameters in the argument list of a function call and function definition MUST match.
If function prototype and definition have three parameters then the function call must have three parameters.
If the types are int, float and double in the prototype, the types in the function call should be int, float and double, respectively.
UniMAP SemII-09/10
EKT120: Computer Programming

15. What about number, order and type of parameter?(e.g1)
Note that there are two arguments for function prototype, function definition and function call; the first is int and the second is double. With these three we have met the number, order and type requirements.
UniMAP SemII-09/10
EKT120: Computer Programming

16. What about number, order and type of parameter?(e.g2)
int sum(int, int); //function prototype int sum(int num1, int num2) //function definition sum(x,y); //function call
Refer to program in slide 13
Number, order and type parameter are met because: there are two parameters, the parameters are listed in order i.e respectively and first parameter is int and second parameter is int.
UniMAP SemII-09/10
EKT120: Computer Programming

17. Functions that do not return a value
//This program sums up two numbers
#include <stdio.h>
void sum_print(int, int); //function prototype
void function1(); //function prototype
int main()
{ int x,y;
function1(); //function call
printf(“Enter x and y: ”);
scanf(“%d %d”, &x, &y);
sum_print(x,y); //function call
return 0; }
void sum_print(int num1, int num2) //function definition
{ int add;
add = num1+num2;
printf(“Sum is: %d”,add);
void function1()
{ printf(“Welcome to this program\n”); }
UniMAP SemII-09/10
EKT120: Computer Programming

18. Functions that return a value
//This program sums up two numbers
#include <stdio.h>
int sum(int,int); //function prototype
int main()
{ int x,y,result;
printf(“Enter x and y: ”);
scanf(“%d %d”, &x, &y);
result = sum(x,y); //function call
printf(“Sum is : %d”,result);
return 0; }
int sum(int num1, int num2) //function definition
{ int add;
add = num1+num2;
return(add);
UniMAP SemII-09/10
EKT120: Computer Programming

19. Miscellaneous about functions
Function call used as logical expression
int calc(int,int); //function prototype
int main(void)
{ int num1, num2;
scanf(“%d %d”,&num1,&num2);
if(calc(num1,num2)>100) //function call used as logical expression
printf(“result greater than 100”);
else
printf(“result less than 100”);
return 0; }
int calc(int n1,int n2)
{ int answer;
answer=n1+n2;
return(answer);
UniMAP SemII-09/10
EKT120: Computer Programming

20. Miscellaneous about functions
Function call used in printf statement
int calc(int,int); //function prototype
int main(void)
{ int num1,num2;
scanf(“%d %d”,&num1,&num2);
printf(“Jawapan : %d”,calc(num1, num2)); //function call returns a //value and puts in printf
return 0; }
int calc(int n1,int n2)
{ int answer;
answer=n1+n2;
return(answer);
UniMAP SemII-09/10
EKT120: Computer Programming

21. Miscellaneous about functions
Rules regarding naming convention for variables
num1 passes value to n1, num2 passes value to n2
Better use different variable names for parameters in main AND parameters in function definition
int calc(int,int); //prototype function
int main(void)
{ int num1,num2,result; //declare like this
scanf(“%d %d”,&num1,&num2);
result = calc(num1,num2); //function call
printf(“jawapan : %d“,result);
return 0; }
//function definition
int calc(int n1,int n2) //simply declare like this
{ int answer;
answer=n1+n2;
return(answer);
UniMAP SemII-09/10
EKT120: Computer Programming

22. EKT120: Computer Programming
Sample application
Write a C program that calculates and prints addition and subtraction of numbers.
Your program should have functions:
add : adds two numbers
subtract : subtracts two numbers
print_result : prints results from calculation
UniMAP SemII-09/10
EKT120: Computer Programming

23. Sample application(cont)
#include <stdio.h>
int add(int,int);
int subtract(int,int);
void print_result(int);
int main()
{ int num1,num2,answer;
char op;
printf(“Enter two numbers and operator:”);
scanf(“%d %d %c”, &num1,&num2,&op);
switch(op)
{ case ‘+’ :answer=add(num1,num2);break;
case ‘-’ :answer=subtract(num1,num2);break;
default: printf(“Invalid operator”);
exit(0); }
print_result(answer);
return 0;
int add(int x,int y) {
int sum;
sum = x+y;
return(sum); }
int subtract(int x,int y)
int sub;
sub=x-y;
return(sub);
void print_result(int ans)
printf(“Answer is %d”, ans);
UniMAP SemII-09/10
EKT120: Computer Programming

24. EKT120: Computer Programming
1 /* Fig. 5.4: fig05_04.c
2 Finding the maximum of three integers */
3 #include <stdio.h> 4
5 int maximum(int, int, int); /* function prototype */ 6
7 int main()‏
8 {
9 int a, b, c; 10
11 printf( "Enter three integers: " );
12 scanf( "%d %d %d", &a, &b, &c );
13 printf( "Maximum is: %d\n", maximum( a, b, c ) ); 14
15 return 0;
16 } 17
18 /* Function maximum definition */
19 int maximum(int x, int y, int z)‏
20 {
21 int max = x; 22
23 if ( y > max )‏
max = y; 25
26 if ( z > max )‏
max = z; 28
29 return max;
30 }
1. Function prototype (3 parameters)‏
2. Function call
3. Function definition
Program Output
Enter three integers:
Maximum is: 85
UniMAP SemII-09/10
EKT120: Computer Programming

25. Scope and Mechanics of Passing Values to Functions
Scope refers to the region in which a declaration is active
File scope is also called global variable
declared at the top of a source file
declarations not placed in any functions
can be used by any statements that are being executed in the system
Function scope is also called local variable
declared in a block { … }
scope is within its block – lifetime while the block is executed
UniMAP SemII-09/10
EKT120: Computer Programming

26. Global Variable : Example
#include <stdio.h>
int global = 3; //This is the global variable
void ChangeGlobal( );
int main(void)
{ printf("%d\n“, global); //Reference to global
//variable in a function
ChangeGlobal();
printf("%d\n", global);
return 0; }
void ChangeGlobal( )
{ global = 5; } //Reference to global
UniMAP SemII-09/10
EKT120: Computer Programming

27. Global Variable : Example
The output will be: 3 5
UniMAP SemII-09/10
EKT120: Computer Programming

28. Local Variable : Example
#include <stdio.h>
void ChangeLocal();
int main(void)
{ int local = 3; //This is a local variable
printf("%d\n", local); //Reference to local
//variable in a function
ChangeLocal();
printf("%d\n", local);
return 0; }
void ChangeLocal()
{ int local = 5; //This is another local variable
printf("%d\n", local); }
UniMAP SemII-09/10
EKT120: Computer Programming

29. Local Variable : Example
The output will be: 3 5
UniMAP SemII-09/10
EKT120: Computer Programming

30. EKT120: Computer Programming
End Week 5 – Functions (1)
Q & A!
UniMAP SemII-09/10
EKT120: Computer Programming