A function is a block of code which only runs when it is called. You can pass data, known as parameters, into a function. Functions are used to perform certain actions, and they are important for reusing code: Define the code once, and use it many times.

1. RITHIKA. R. S,
I M.Sc. Bioinformatics,
Sri Krishna Arts and Science College, Coimbatore

2. Introduction…
 A function is a group of statements that together perform a
task.
 It can be executed from as many different parts in a program
as required, it can also return a value to calling program.
 A C program consists of one or more functions.
 Function is a subprogram that helps to reduce the
complexity of a program

3. Example…
int add (int x, int y)
{
int z;
z=x+y;
return z;
}

4. Why functions?
 Writing functions avoids rewriting the same code
over and over.
 Using function it becomes easier to write a program
and keep track of what they are doing.

5. Types of function in C…
Library functions
 Functions such as printf(), scanf() and sqrt() are present in c
library and they are predefined functions.
User defined functions
 A user can create their own functions for performing any specific
task of program. This is said to be user defined functions.

6. Function definition
 Function definition is also known as function implementation
and includes the following elements:
 Function name
 Function type (return type)
 List of parameters
 Local variable declaration
 Function statements and
 A return statement

7. return_type function_name (parameter list )
{
local variable declaration;
executable statement1;
executable statement2;
…..
…..
return statement;
}
The general format of a function definition is:

8. Function definition elements…
 Function header: This consists of 3 parts namely; Function type,
Function name, Parameter list
 The function_type specifies the type of value that the function is
expected to return the program calling the function (such as int,
float, double).
 If function is not returning anything, then specify the return type
as void

9.  The function_name is any valid C identifier.
 The name should be appropriate to the task performed by the
function, such as sum, add, mul.
 Parameter list declares the variables that will receive the data.
 Parameters are also known as arguments.
 The parameters of the function are enclosed in the parenthesis,
the variables are separated by commas.
 There must be no semicolon after closing the parenthesis

10. Function declaration…
 Function declaration (function prototype) is declaring the
properties of a function.
 Consists of 4 parts
 Function type(return type)
 Function name
 Parameter list
 Terminating semicolon(;)
 The general format is:
function_type function_name(parameter list);

11.  For example, mul function can be declared as
int mul (int m, int n); /*function prototype*/
 Various acceptable forms for mul functions are:
 int mul (int, int);
 mul (int a, int b);
 mul (int, int);
 When no parameters and no return value, its prototype is
given as void display(void)

12. Prototype declaration…
 Function Prototype Declaration is a statement in which
programmer describes three information about a function:
 Symbol name of the function
 Return type of the function
 Arguments that will be taken as input
 A prototype declaration can be in 2 places in program
 (1)above all the functions and
 (2)inside a function definition.

13.  When a declaration is placed above all the functions, it is
referred to as global prototype.
 When a declaration is place in a function definition, it is
referred a local prototype

14. The return statement…
 A function may send a value back to the calling function
through the return statement.
 The called function can return only one value per call.
 The return statement can be written in any one of the forms
given
• return; or
• return (expression);

15.  The first form of plain return does not return any value. An example is
if (error)
return;
 The second form of return with expression returns a value. For example,
int mul (int x, int y)
{
int p;
p=x*y;
return(p);
}
This function returns the value of p, the product of x and y. the last 2
statements cane be combined as
return(x*y);

16.  A function may have more than one return statements.
if(x<=0)
return(0);
else
return(1);
 All functions return int type data by default. We can make a function to
return particular type of data by using type specifier in the function header.
 Functions that do computations using doubles or float, yet returns ints.
int product (void)
{
return(2.5*3.0)
}
will return the value 7, only integer part.