Functions

FUNCTIONS

• Functions in C is to decompose a program into its component functions means smaller part to ease solving the problem
• A C program is made up of one or more functions, exactly one of which must be named main
• The execution of the C program begins and terminates with main.
• The main function can call other functions to do specific jobs
• Functions is an independent module and each functions solves part of the problem

Benefit of Functions

• Can test the components separately
• Can change one function without changing or affecting the other functions
• Provide a way to reuse code that is required in more than one place.

Declared and Define Functions

• Like any other object in C, a function must be declared and defined
• In order to use a function, there are three steps to be done:

1. Function declaration
2. Functions definition (function body)
3. Functions called

1. Function Declaration

• A function declaration consist of three parts
  Return Type----Function Name----------(Parameter List);
  int--------------addnum------------------(int num1, int num2);
• The return type of the function can be any of the basic data type (void, int, float, char)
• The name of the function is a valid C identifier.
• The parameter list are the place holder for the arguments that the function expects
• The function declaration must end with a semicolon
• The function declaration normally declare as global declaration in order to be used in the whole program

2. Function Definition

• A function definition is made up of two parts
  • Function header
  • Function body
• A function header is same as the function declaration just that there is no semicolon at the end of the function header
• A function body is a compound statement which enclosed between open and closed braces
• The function body contains local variable and declarations and statements and terminated by a return statement
• A return statement return a value to the main program and exit the function
• Example:

int addnum (int num1, int num2)
{
int sum;
sum = num1 + num2;
return sum;
}

3. Function call

• A function call is used to call or invoke the function to be used in the C program
• A function call is an post fix expression
• Example:

void main (void)
{
int a, b, result;
scanf(“%d%d”, &a, &b)l;
result = addnum (a, b);
printf(“The sum is: %d”, result);
return;
}

The name of the function is used in three ways:

1. For function declaration
2. In a function call
3. For function definition

There are two ways to call a function in C:

Call By Value

• Every argument to a function is an expression, which has a value
• C passes argument to an invoked function by making a copy of the expression value, storing it in a temporary cell
• Only the copy of the value is passed to the function argument
• The original data in the calling function are unchanged. As only the copy of the values are passed to the function.
• Example:

#include
int change (int x);
int main (void){
int a;
printf(“Enter a value: “);
scanf(“%d”, &a);
printf(“Value before call function: %d\n”, a);
change(a);
printf(“Value after call function: %d\n”, a);
return 0;
}

int change (int x){
x = 10;
return 0;
}

Output:
Enter a value: 5
Value before call function: 5
Value after call function: 5

Call by Reference

• It links the variable identifiers in the calling function to their corresponding parameters in the called function
• When the called function changes a value in a variable, then it actually changes the variables in the calling function
• This is done by passing the an address to the called function
  
  • & - the address operator
  • * - the indirection operator
• Example
  

#include
int change (int *x);
int main (void){
int a;
printf(“Enter a value: “);
scanf(“%d”, &a);
printf(“Value before call function: %d\n”, a);
change(&a);
printf(“Value after call function: %d\n”, a);
return 0;
}

int change (int *x){
*x = 10;
return 0;
}

Output:
Enter a value: 5
Value before call function: 5
Value after call function: 10

Standard Library Functions

• To perform various data manipulations functions will build in functions in the header files
• The functions called abs, fabs, and labs return the absolute value of a number
• An absolute number value is the positive rendering of the value regardless of its sign
• Example:

int abs (int num)
abs (-5) = 5
double fabs (double num)
fabs(-5.6) = 5.6

• The ceil function return the smallest integer value greater than or equal to the number
• Example:

ceil (1.2) = 2
ceil (-2.85) =-2

• The floor function returns he largest integral value that is equal to or less than a number
• Example:

floor (1.2) = 1
floor (-2.85) = -3

• The pow function returns the value of the x raised to the power y – that is xy
• Example:

pow (3.0, 4.0) = 81.0
pow (3.4, 2.3) =16.687893

• The sqrt function returns the non-negative square root of number
• Example:

sqrt (36.0) = 6.0

• The rand function returns a pseudorandom integer between 0 and RAND_MAX, which is defined in the standard library as the largest number that rand can generate
• Example:

rand( )

• Generalizing the algorithm for generating a random number between ranges is

  • rand ( ) % ((max + 1) – min) + min