function_v1.ppt

2
Function
 A program segment that carries out some
specific, well-defined task
 Example
 A function to add two numbers
 A function to find the largest of n numbers
 A function will carry out its intended task
whenever it is called or invoked
 Can be called multiple times

3
Why Functions?
 Allows one to develop a program in a
modular fashion
 Divide-and-conquer approach
 Construct a program from small pieces or
components
 Use existing functions as building blocks for
new programs
 Abstraction: hide internal details (library
functions)

4
 Every C program consists of one or more
functions
 One of these functions must be called main
 Execution of the program always begins by
carrying out the instructions in main
 Functions call other functions as instructions

5
Function Control Flow
void print_banner ()
{
printf(“************n”);
}
int main ()
{
. . .
print_banner ();
. . .
print_banner ();
}
int main ()
{
print_banner ();
print_banner ();
}
print_banner {
}
print_banner {
}

6
 Calling function (caller) may pass
information to the called function (callee)
as parameters/arguments
For example, the numbers to add
 The callee may return a single value to
the caller
Some functions may not return anything

7
void main()
{ float cent, fahr;
scanf(“%f”,&cent);
fahr = cent2fahr(cent);
printf(“%fC = %fFn”,
cent, fahr);
}
float cent2fahr(float data)
{
float result;
result = data*9/5 + 32;
return result;
}
Calling/Invoking the cent2fahr function
Calling function (Caller)
Called function (Callee) parameter
Returning value
Parameter passed

8
Defining a Function
 A function definition has two parts:
 The first line, called header
 The body of the function
return-value-type function-name ( parameter-list )
{
declarations and statements
}

9
 The first line contains the return-value-type,
the function name, and optionally a set of
comma-separated arguments enclosed in
parentheses
 Each argument has an associated type
declaration
 The arguments are called formal
arguments or formal parameters
 The body of the function is actually a block of
statement that defines the action to be taken
by the function

10
Parameter passing
 When the function is executed, the value
of the actual parameter is copied to the
formal parameter
int main ()
{ . . .
double circum;
. . .
area1 = area(circum);
. . .
}
double area (double r)
{
return (3.14*r*r);
}
parameter passing

11
int gcd (int A, int B)
{
int temp;
while ((B % A) != 0) {
temp = B % A;
B = A;
A = temp;
}
return (A);
}
BODY
Return-value type Formal parameters
Value returned
Example of function definition

12
Return value
 A function can return a value
 Using return statement
 Like all values in C, a function return value has a type
 The return value can be assigned to a variable in the
caller
int x, y, z;
scanf(“%d%d”, &x, &y);
z = gcd(x,y);
printf(“GCD of %d and %d is %dn”, x, y, z);

13
Function Not Returning Any Value
 Example: A function which prints if a number is
divisible by 7 or not
void div7 (int n)
{
if ((n % 7) == 0)
printf (“%d is divisible by 7”, n);
else
printf (“%d is not divisible by 7”, n);
return;
}
Optional
Return type is void

14
return statement
 In a value-returning function (return type is not void), return
does two distinct things
 specify the value returned by the execution of the
function
 terminate that execution of the callee and transfer control
back to the caller
 A function can only return one value
 The value can be any expression matching the return
type
 but it might contain more than one return statement.
 In a void function
 return is optional at the end of the function body.
 return may also be used to terminate execution of the
function explicitly.
 No return value should appear following return.

15
void compute_and_print_itax ()
{
float income;
scanf (“%f”, &income);
if (income < 50000) {
printf (“Income tax = Niln”);
return;
}
if (income < 60000) {
printf (“Income tax = %fn”, 0.1*(income-50000));
return;
}
if (income < 150000) {
printf (“Income tax = %fn”, 0.2*(income-60000)+1000);
return ;
}
printf (“Income tax = %fn”, 0.3*(income-150000)+19000);
}
Terminate function
execution before
reaching the end

16
void main()
{ float cent, fahr;
cent, fahr);
}
{
float result;
return result;
}
Returning value
Parameter passed

17
How it runs
{
float result;
printf(“data = %fn”, data);
return result;
printf(“result = %fn”, result);
}
void main()
{ float cent, fahr;
printf(“Input is %fn”, cent);
printf(“%fC = %fFn”, cent, fahr);
}
$ ./a.out
32
Input is 32.000000
data = 32.000000
32.000000C = 89.599998F
$./a.out
-45.6
Input is -45.599998
data = -45.599998
-45.599998C = -50.079998F
$
Output

18
 Function prototypes are usually written at the
beginning of a program, ahead of any functions
(including main())
 Prototypes can specify parameter names or just
types (more common)
 Examples:
int gcd (int , int );
void div7 (int number);
 Note the semicolon at the end of the line.
 The parameter name, if specifed, can be
anything; but it is a good practice to use the
same names as in the function definition
Function Prototypes

19
int factorial (int m)
{
int i, temp=1;
for (i=1; i<=m; i++)
temp = temp * i;
return (temp);
}
int main()
{
int n;
for (n=1; n<=10; n++)
printf (“%d! = %d n”,
n, factorial (n) );
}
1! = 1
2! = 2
3! = 6 …….. upto 10!
Another Example
Output
int factorial (int m);
Function declaration
(prototype)
Function call
Function definition

20
 Called by specifying the function name and parameters
in an instruction in the calling function
 When a function is called from some other function, the
corresponding arguments in the function call are called
actual arguments or actual parameters
 The function call must include a matching actual
parameter for each formal parameter
 Position of an actual parameters in the parameter list
in the call must match the position of the
corresponding formal parameter in the function
definition
 The formal and actual arguments must match in their
data types
Calling a function

21
Example
void main ()
{
double x, y, z;
char op;
. . .
z = operate (x, y, op);
. . .
}
Actual parameters
double operate (double x, double y, char op)
{
switch (op) {
case ‘+’ : return x+y+0.5 ;
case ‘~’ : if (x>y)
return x-y + 0.5;
return y-x+0.5;
case ‘x’ : return x*y + 0.5;
default : return –1;
}
}
Formal parameters

22
void main()
{ float cent, fahr;
cent, fahr);
}
{
float result;
return result;
}
Returning value
Parameter passed
Local variables

23
Local variables
 A function can define its own local variables
 The locals have meaning only within the function
 Each execution of the function uses a new set of
locals
 Local variables cease to exist when the function
returns
 Parameters are also local

24
Local variables
/* Find the area of a circle with diameter d */
double circle_area (double d)
{
double radius, area;
radius = d/2.0;
area = 3.14*radius*radius;
return (area);
}
parameter
local
variables

25
Points to note
 The identifiers used as formal parameters are
“local”.
 Not recognized outside the function
 Names of formal and actual arguments may differ
 A value-returning function is called by including
it in an expression
 A function with return type T (≠ void) can be used
anywhere an expression of type T

26
 Returning control back to the caller
 If nothing returned
 return;
 or, until reaches the last right brace ending
the function body
 If something returned
 return expression;

27
Some more points
 A function cannot be defined within another
function
All function definitions must be disjoint
 Nested function calls are allowed
A calls B, B calls C, C calls D, etc.
The function called last will be the first to
return
 A function can also call itself, either directly or
in a cycle
A calls B, B calls C, C calls back A.
Called recursive call or recursion

28
Example: main calls ncr, ncr calls fact
int ncr (int n, int r);
int fact (int n);
void main()
{
int i, m, n, sum=0;
scanf (“%d %d”, &m, &n);
for (i=1; i<=m; i+=2)
sum = sum + ncr (n, i);
printf (“Result: %d n”,
sum);
}
int ncr (int n, int r)
{
return (fact(n) / fact(r) /
fact(n-r));
}
int fact (int n)
{
int i, temp=1;
for (i=1; i<=n; i++)
temp *= i;
return (temp);
}

29
Scope of a variable
 Part of the program from which the value of the variable
can be used (seen)
 Scope of a variable - Within the block in which the variable
is defined
 Block = group of statements enclosed within { }
 Local variable – scope is usually the function in which it is
defined
 So two local variables of two functions can have the
same name, but they are different variables
 Global variables – declared outside all functions (even
main)
 scope is entire program by default, but can be hidden in
a block if local variable of same name defined

30
Variable
Scope
Output:
A = 3
A = 1
#include <stdio.h>
int A = 1;
void main()
{
myProc();
printf ( "A = %dn", A);
}
void myProc()
{ int A = 2;
if ( A==2 )
{
A = 3;
printf ( "A = %dn", A);
}
}
Global variable
Hides the global A

31
Compute GCD of two numbers
int main() {
int A, B, temp;
scanf (“%d %d”, &A, &B);
if (A > B) {
temp = A; A = B; B = temp;
}
while ((B % A) != 0) {
temp = B % A;
B = A;
A = temp;
}
printf (“The GCD is %d”, A);
}
12 ) 45 ( 3
36
9 ) 12 ( 1
9
3 ) 9 ( 3
9
0
Initial: A=12, B=45
Iteration 1: temp=9, B=12,A=9
Iteration 2: temp=3, B=9, A=3
B % A = 0  GCD is 3
gcd.c

32
int x, y, z;
scanf(“%d%d”, &x, &y);
z = gcd(x,y);
printf(“GCD of %d and %d is %dn”, x, y, z);
Compute GCD of two numbers
(with function)

33
int gcd (int A, int B)
{
int temp;
while ((B % A) != 0) {
temp = B % A;
B = A;
A = temp;
}
return (A);
}
BODY
Return-value type Formal parameters
Value returned

function_v1.ppt

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