1. Week 5 – Functions (1)
EKT120: Computer Programming

2. Outline Why use functions? Functions in C Function prototypes
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
EKT120: Computer Programming

3. 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 iLoop1, iLoop2;
//print one row of number 8
for(iLoop1=1; iLoop1<=10; iLoop1++)
printf(“8");
printf("\n"); //go to new line
//print one row of number 9
for(iLoop2=1; iLoop2<=10; iLoop2++)
printf(“9“);
return 0; }
EKT120: Computer Programming

4. 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
EKT120: Computer Programming

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

6. 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)
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
EKT120: Computer Programming

8. Predefined Functions (Library)-example
#include <stdio.h> #include <math.h> #include <string.h> void main() { string sName; int iVol1, iVol2, iN,iR, iKTemp, iLength; strcpy(sName, “Marina”); iVol2 = iVol1 * exp(iN * iR * iKTemp); iLength = strlen(“Mahathir”); }
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
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10. 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 fnSum (int iNum1,int iNum2);
int fnSum (int,int); //is also acceptable
semicolon
EKT120: Computer Programming

11. 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 fnSum (int iNum1,int iNum2)
int iAdd;
iAdd = iNum1 + iNum2;
return(iAdd);
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
EKT120: Computer Programming

12. Function Calls iResult = fnSum(x,y);
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
iResult = fnSum(x,y);
EKT120: Computer Programming

13. Example of function in program
//This program sums up two numbers
#include <stdio.h>
int fnSum(int, int); //function prototype
int main()
{ int iX, iY, iResult;
printf( “Enter x and y : ”);
scanf(“%d %d”, &iX, &iY);
iResult = fnSum(iX,iY); //function call
printf(“Sum is : %d”, iResult);
return 0; }
int fnSum(int iNum1, int iNum2)//function definition
{ int iAdd;
iAdd = iNum1+iNum2;
return(iAdd);}
Enter x and y : 1 2
Sum is : 3
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.
EKT120: Computer Programming

15. What about number, order and type of parameter?(e.g1)
prototype void fnFunction2(int iN, double dX);
function header void fnFunction2(int iN, double dX)
function call fnFunction2 (iM, dY);
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.
EKT120: Computer Programming

16. What about number, order and type of parameter?(e.g2)
int fnSum(int, int); //function prototype
int fnSum(int iNum1, int iNum2) //function definition
fnSum(iX,iY); //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.
EKT120: Computer Programming

17. Functions that do not return a value
//This program sums up two numbers #include <stdio.h> void fnSumPrint(int, int); //function prototype void fnFunction1(); //function prototype int main() { int iX,iY; fnFunction1(); //function call printf(“Enter x and y: ”); scanf(“%d %d”, &iX, &iY); fnSumPrint(iX,iY); //function call return 0; } void fnSumPrint(int iNum1, int iNum2) //function definition { int iAdd; iAdd = iNum1+iNum2; printf(“Sum is: %d”,iAdd); void fnFunction1() { printf(“Welcome to this program\n”); }
Welcome to this program
Enter x and y: 1 2
Sum is: 3
EKT120: Computer Programming

18. Functions that return a value
//This program sums up two numbers #include <stdio.h> int fnSum(int,int); //function prototype int main() { int iX,iY,iResult; printf(“Enter x and y: ”); scanf(“%d %d”, &iX, &iY); iResult = fnSum(iX,iY); //function call printf(“Sum is : %d”,iResult); return 0; } int fnSum(int iNum1, int iNum2) //function definition { int iAdd; iAdd = iNum1+iNum2; return(iAdd);
EKT120: Computer Programming

19. Miscellaneous about functions
Function call used as logical expression
int fnCalc(int,int); //function prototype
int main(void)
{ int iNum1, iNum2;
scanf(“%d %d”,&iNum1,&iNum2);
if(fnCalc(iNum1,iNum2)>100) //function call used as logical expression
printf(“result greater than 100”);
else
printf(“result less than 100”);
return 0; }
int fnCalc(int iN1,int iN2)
{ int iAnswer;
iAnswer=iN1+iN2;
return(iAnswer);
EKT120: Computer Programming

20. Miscellaneous about functions
Function call used in printf statement
int fnCalc(int,int); //function prototype
int main(void)
{ int iNum1,iNum2;
scanf(“%d %d”,&iNum1,&iNum2);
printf(“Sum = %d”,fnCalc(iNum1, iNum2)); //function call returns a //value and puts in printf
return 0; }
int fnCalc(int iN1,int iN2)
{ int iAnswer;
iAnswer=iN1+iN2;
return(iAnswer);
EKT120: Computer Programming

21. Miscellaneous about functions
Rules regarding naming convention for variables
iNum1 passes value to iN1, iNum2 passes value to iN2
Better use different variable names for parameters in main AND parameters in function definition
int fnCalc(int,int); //prototype function
int main(void)
{ int iNum1,iNum2,iResult; //declare like this
scanf(“%d %d”,&iNum1,&iNum2);
iResult = fnCalc(iNum1,iNum2); //function call
printf(“sum = %d“,iResult);
return 0; }
//function definition
int fnCalc(int iN1,int iN2) //simply declare like this
{ int iAnswer;
iAnswer=iN1+iN2;
return(iAnswer);
EKT120: Computer Programming

22. Sample application
Write a C program that calculates and prints addition and subtraction of numbers.
Your program should have functions:
fnAdd : adds two numbers
fnSubtract : subtracts two numbers
fnPrintResult : prints results from calculation
EKT120: Computer Programming

23. Sample application(cont)
#include <stdio.h> int fnAdd(int,int); int fnSubtract(int,int); void fnPrintResult(int); int main() { int iNum1,iNum2,iAnswer; char cOp; printf(“Enter two numbers and operator:”); scanf(“%d %d %c”, &iNum1,&iNum2,&cOp); switch(cOp) { case ‘+’ :iAnswer=fnAdd(iNum1,iNum2);break; case ‘-’ :iAnswer=fnSubtract(iNum1,iNum2);break; default: printf(“Invalid operator”); } fnPrintResult(iAnswer); return 0;
int fnAdd(int iX,int iY) { int iSum; iSum = iX+iY; return(iSum); } int fnSubtract(int iX,int iY) int iSub; iSub=iX-iY; return(iSub); void fnPrintResult(int iAns) printf(“Answer is %d”, iAns);
EKT120: Computer Programming

24. 1. Function prototype (3 parameters)‏
1 /* Fig. 5.4: fig05_04.c
2 Finding the maximum of three integers */
3 #include <stdio.h> 4
5 int fnMaximum(int, int, int); /* function prototype */ 6
7 int main()‏
8 {
9 int iA, iB, iC; 10
11 printf( "Enter three integers: " );
12 scanf( "%d %d %d", &iA, &iB, &iC );
13 printf( "Maximum is: %d\n", fnMaximum( iA, iB, iC ) ); 14
15 return 0;
16 } 17
18 /* Function maximum definition */
19 int fnMaximum(int iX, int iY, int iZ)‏
20 {
21 int iMax = iX; 22
23 if ( iY > iMax )‏
iMax = iY; 25
26 if ( iZ > iMax )‏
iMax = iZ; 28
29 return iMax;
30 }
1. Function prototype (3 parameters)‏
2. Function call
3. Function definition
Program Output
Enter three integers:
Maximum is: 85

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
EKT120: Computer Programming

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

27. Global Variable : Example
The output will be: 3 5
EKT120: Computer Programming

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

29. Local Variable : Example
The output will be: 3 5
EKT120: Computer Programming

30. End Week 5 – Functions (1)
Q & A!
EKT120: Computer Programming