1. Functions
Chapter 5
CS12 - Computer Programming 1
Chapter 5

2. Objectives Understand the concept of functions in C
Learn and include function prototype and function definition in C program
Know what is global/external and local variables, when to include them in C program
Appreciate the purpose of parameter passing and how it works in C program
Learn and understand how to incorporate functions in program for validation check
Chapter 5
CS12 - Computer Programming 1

3. Why functions are used in C?
to avoid writing the same instructions repeatedly
make it easier to organize programs and keep track of what they were doing
each subroutine (function) could be written and checked out more or less independently
the variables in each subroutine were protected from inadvertent tampering by other subroutines
Chapter 5
CS12 - Computer Programming 1

4. The Structure of Functions
There are three program elements involved in using a function:
The function definition
The function itself is referred to as function definition
The call to the function
The function is called by main() by using its name including the parenthesis that following the name
The function prototype
A function is declared in a similar way at the beginning of a program before it is called
Chapter 5
CS12 - Computer Programming 1

5. Chapter 5
CS12 - Computer Programming 1

6. The Function Definition
A function definition has following syntax:
<type> <function name>(<parameter list>){
<local declarations>
<sequence of statements>
return <expression> }
A function returns a single result
One of the statements in the function body should have the form: return <expression>;
The value passed back by return should have the same type as the function.
Chapter 5
CS12 - Computer Programming 1

7. How to use or call a function?
A function call has the following syntax:
<function name>(<parameter list>)
There is a one-to-one correspondence between the parameters in a function call and the parameters in the function definition.
Chapter 5
CS12 - Computer Programming 1

8. Function Prototype
The function prototype has the following syntax: <type> <function name>(<type list>); The function prototype declares the interface, or input and output parameters of the function, leaving the implementation for the function definition.
Chapter 5
CS12 - Computer Programming 1

9. Local Variables
Functions which are known only within that function and other functions are not allowed to use them
Also known as automatic variables
The length of time a variable lasts is called lifetime.
Chapter 5
CS12 - Computer Programming 1

10. Functions that Return a Value
A function that uses no
arguments but returns a
value is a slightly more
complicated kind of
function:
a function that returns a
value.
Chapter 5
CS12 - Computer Programming 1

11. Using Arguments to Pass Data to a Function
The mechanism used to convey information to a function is the
argument
In the function
definition, a
variable name
could be placed
in the
parentheses
Chapter 5
CS12 - Computer Programming 1

12. Passing Multiple Arguments
We can pass as many arguments as we like to function
The value of the first
actual argument in the
calling program is
assigned to the first
formal argument in the
function, and the value
of the second actual
argument is assigned to
the second formal argument
Chapter 5
CS12 - Computer Programming 1

13. ‘Void’ type functions
A function returns a single result, but a void type function does not return anything (but does something, a sub-program)!
Example (void): Printing Cards
#include <stdio.h>
void printcard(int);
int main(){
int c1, c2, c3, c4, c5;
printcard(c1);
printcard(c2);
printcard(c3);
printcard(c4);
printcard(c5); }
void printcard(int cardnum){
if(cardnum==1)
Printf("A“);
else if(cardnum>=2 && cardnum<=10)
Printf(“%d”, cardnum);
else if(cardnum==11)
Printf("J“);
else if(cardnum==12)
Printf("Q“);
else if(cardnum==13)
Printf("K“); }
Chapter 5
CS12 - Computer Programming 1

14. Passing parameters by Value (&)
This is by default and desirable behavior!
An important fact to remember about parameter passing by value is that changes to the parameters inside the function body have no effect outside of the function.
Example:
int sum(int a, int b){
a = a + b;
return a; }
void main(){
int x, y, z;
x = 3; y = 5;
z = sum(x,y); }
Chapter 5
CS12 - Computer Programming 1

15. Passing parameters by reference
The value of the parameters passed to a function will also changed in the calling function
Example:
#include <stdio.h>
void Increment(int& Number){
Number = Number + 1;
Printf("The parameter Number: %d“, Number); }
void main(){
int I = 10;
Increment(I); // parameter is a variable
Printf("The variable I is: %d\n“, I );
Chapter 5
CS12 - Computer Programming 1

16. Passing parameters by referenc e
Example 2: int main ( ) { int a, b, x = 100; printf("Enter two numbers: "); scanf("%d%d", &a, &b); Add(a, b, x); printf("a, b, x contains %d, %d, %d\n", a, b, x); } void Add(int a, int b, int &x){ b = b - a; x = x + b;
Output
Enter two numbers: 6 8
a, b, x contains 6, 2, 102
a, b, x contains 6, 8, 102
CS12 - Computer Programming 1
Chapter 5

17. Programming Exercise
Print the sum and average of two numbers using Pass by value
Convert a value in inches to its equivalent value in Feet and inches using Pass by value
Chapter 5
CS12 - Computer Programming 1