Week 5 – Functions (1)

1. Week 5 – Functions (1) PGT 106: Computer Programming

2. 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 PGT 106: 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“); printf("\n"); //go to new line return 0; } PGT 106: 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 PGT 106: 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 } PGT 106: 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) PGT 106: Computer Programming

7. Predefined Functions (Library) • Common libraries are stdio.h, math.h, string.h, and stdlib.h • stdio.hrelated 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 PGT 106: 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”); } PGT 106: 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 PGT 106: Computer Programming

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 hasthe following form: • <return_type> <function_name> (arg_type arg_name, ...); • int fnSum (int iNum1,int iNum2); • int fnSum (int,int); //is also acceptable semicolon PGT 106: Computer Programming

11. Function Definitions • Function definition includes the body of a function • Function definition hasthe 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 PGT 106: Computer Programming

12. Function Calls • Consists of a function name followed by an argument expression list enclosed in parentheses • Function call hasthe following form: • <function_name> (exp, exp ...) • exp is an expression– can be variable or constant • iResult = fnSum(x,y); PGT 106: Computer Programming

13. Example of function in program //This program sums up two numbers #include <stdio.h> intfnSum(int, int);//function prototype int main() { intiX, 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; } intfnSum(int iNum1, int iNum2)//function definition { intiAdd; iAdd = iNum1+iNum2; return(iAdd);} } function header PGT 106: 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. PGT 106: Computer Programming

15. What about number, order and type of parameter?(e.g1) prototype void fnFunction2(intiN, double dX); function header void fnFunction2(intiN, 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. PGT 106: 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. PGT 106: 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”); } PGT 106: 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); } PGT 106: 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); } PGT 106: 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 callreturns a //value and puts in printf return 0; } int fnCalc(int iN1,int iN2) { int iAnswer; iAnswer=iN1+iN2; return(iAnswer); } PGT 106: 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); } PGT 106: 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 PGT 106: Computer Programming

23. #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); } Sample application(cont) PGT 106: Computer Programming

24. 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 )‏ 24 iMax = iY; 25 26 if ( iZ > iMax )‏ 27 iMax = iZ; 28 29 return iMax; 30 } 1. Function prototype (3 parameters)‏ 2. Function call 3. Function definition Program Output Enter three integers: 22 85 17 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 PGT 106: 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 //variable in a function PGT 106: Computer Programming

27. Global Variable : Example The output will be: 3 5 PGT 106: 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); } PGT 106: Computer Programming

29. Local Variable : Example The output will be: 3 5 3 PGT 106: Computer Programming

30. End Week 5 – Functions (1) Q & A! PGT 106: Computer Programming