c introduction to functions

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c    introduction to functions Powered By Docstoc
					Function
In C , C++
What is function?
 The functions are like subtasks. They receive some
  information, do some process and provide a result.

 We have a main() [ function ] in every C program
Writing a function
 When we write a function. It must start with a name,
  parentheses and sourrounding braces just like a main().

 Function are very important in code resuse.
Categories of functions
 There are two categories of functions

 1. Function that return a value
 2. Function that do not return a value
Function that return a value
 Suppose we have a function that calculates the square of an
  integer such that function will return the square of the integer
Function that do not return a value
 Suppose we have a function that display some information
  on the screen so this function is not return a value to the
  calling program
              Structure of function
 The declaration syntax of a function is as follows

  return-value-type function-name(argument-list)
       {

             declarations and statements

              }
The first line is the function header and the declaration
and statement part is the body of a function
Structure of a function
A.   return-value_type:
B.   Function-name:
C.   argument-list:
D.   Declarations and Statements:
A. return-value-type
 Frist line of the function

 Function may or may not return a value, if a function a value
  that must be of a valid data-type(unique I mean only one
  data-type) may be int,float,char,etc
 Int data-type only return integer value not float, char or string
  etc.
Return-value-type(continue)
 How can we return some value from a function?

 The keyword is return which is used to return some value
  from the function
 It does two things, returns some value to the calling program
  and also exits from the function.
Do not Return-value-type
 There may be some functions which do not return any value.
  For such functions, the return_value_type is void . ”void” is a
  keyword is used in c , c++ language
Function Name
 The same rules of variable naming conventions are applied
  to functions name. Function name should be self-explanatory
  like square, squareRoot, circleArea etc.
Argument-list
 argument-list: Argument list contains the information which
  we pass to the function. Some function does not need any
  information to perform the task. In this case, the argument list
  for such functions will be empty. Arguments to a function are
  of valid data type like int number, double radius etc.
Decleration and statements
 This is the body of the function. It consists of declarations
  and statements
 The task of the function is performed in the body of the
  function
Example
//This function calculates the square of a number and returns it

int square(int number)
{
        int result = 0;
        result = number*number;
        return result;
}
Function Calling mechanism
 How a program can use a function? It is very simple. The
  calling program just need to write the function name and
  provide its arguments (without data types). It is important to
  note that while calling a function we don’t write the return
  value data-type or the data-type of arguments

 See the example…
Example
// This program calculates the square of a given number
# include <iostream.h>
          main()
         {
          int number, result;
          result=0;
          number=0;
          // Getting the input from the user
          cout << “ Please enter the number to calculate the
          square ”;
          cin>>number;
          // Calling the function
          square(int number);
          result=square(number);
          cout << “ The square of “ << number << “ is “ << result;

        }
Declaration and definition of a function
  Declaration and definition are two different things. Declaration is
   the prototype of the function, that includes the return type, name
   and argument list to the function and definition is the actual
   function code. Declaration of a function is also known as signature
   of a function.
  As we declare a variable like int x; before using it in our program,
   similarly we neto declare function before using it. Declaration and
   definition of a function can be combined together if we write the
   complete function before the calling functions. Then we don’t need
   to declare it explicitly. If we have written all of our functions in a
   different file and we call these functions from main( ) which is
   written in a different file. In this case , the main( ) will not be
   compiled unless it knows about the functions declaration.

				
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Description: Lerning method of how to write a function in c and the main structure of function
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