Functions

Today we will discuss a very important C++ Phenomenon ie Functions

Introduction:

Functions basically work on the principle of "Divide and conquer" ie: 

• Construct a program from smaller pieces or components.
• Each piece more manageable than the original program

these smaller pieces or components help in Reusability of code, instead of writing same code again and again, Such  Small, independent piece of code/module  is a ‘Function’ in C++

–Programmers use ‘new’ or ‘prepackaged’ modules

•New: programmer-defined functions

•Prepackaged: from the standard library

For using a function, one should only know how to call it….That is how to make a ‘function call’.

Functions and Terminology:

Function Prototype:

A function prototype contains;

  return_data_type     function_name(parameter_data_types);

  e.g       int square(int);

it is not required if ‘definition’ appears before the function is called

No Parameter names are required in a function prototype , all you need is parameter type.

Function Definition:

Format for a function definition is given below:

 return-value-type  function-name( parameter-list ){
   declarations and statements }

– Parameter list

• Comma separated list of parameters

– Data type needed for each parameter

• If no parameter, use void or leave blank

– Return-value-type

• Data type of result returned (use void if nothing returned)

• Function Prototype and function header must match

– Function prototype

– Tells compiler argument type and return type of function

– int square( int );

• Function takes an int and returns an int

– return keyword

– Returns data, and control goes to calling function

• If no data to return, use return;

– If no return statement is written, control returns to calling function when reaches right brace of called function

– Functions cannot be defined inside other functions

 Function Call:

this is basically calling/invoking a function, for example:
              –square(x);

•Function name and information (arguments) it needs (argument x)

•After finished, passes back result to calling function

given below are two examples to show the creation and usage of programmer defined functions.

Math Library Functions:

Perform common mathematical calculations

– Include the header file <cmath>

•      Functions is called by writing

– functionName (argument);

or

– functionName(argument1, argument2,…);

•     for Example:

  cout << sqrt( 900.0 );

– sqrt (square root) function The preceding statement would print 30.0

– All functions in math library return a double

– Function arguments can be

– Constants

– sqrt( 4 );

– Variables

– sqrt( x );

– Expressions

– sqrt( sqrt( x ) ) ;

– sqrt( 3 - 6x );

given below is a table of some math library functions:

Detailed Discussion of Function Prototypes:

•          Function prototype contains

–        Function name

–        Parameters (data type)

–        Return type (void if returns nothing)

–        Only needed if function definition after function call

•          Prototype must match function definition header

–        Function prototype

                        double maximum( double, double, double );

–        Function Header

                        double maximum( double x, double y, double z )

                        {

                         …

                        }

•          A function call that does not match the function prototype is a syntax error

•          Function signature:

–        Part of prototype with name and parameters

•          double maximum( double, double, double );

–        Force arguments to be of proper type ie Argument Coercion

•          Converting int (4) to double (4.0)

      cout << sqrt(4)

–        Conversion rules

•          Arguments usually converted automatically

•          Changing from double to int can truncate data

–        3.4 to 3