Switch case

Case control ( Switch-case-default ) Structure :-
                                                                                        After studing about if-else statement we know that  if we want to execute set of instructions for some special condition  and some other set of instruction for some other condition , we use if-else statement . BUT if we have several different situations(cases) and also their corresponding statements , so in this situation it is difficult to use if-else structures because It make the program complex and is difficult to handle and the chances of mistake increase . It is also difficult to detect logical Error in this type of problem and also difficult to maintain the code . for this problem C provide us a structure called Case control structure or more correctly say switch-case-default structure . In this structure it is very easy to select one out of several alternatives . It not only provide more Flexibility to the program but also it is easy to handle from if-else statements .
Syntax :-
          switch( expression )
                  case constant1 :
                       statement-block1 ;
                       break ;
                  case constant2 :
                       statement-block2  ;
                       break ;
                  case constant3 :
                       statement-block-3  ;
                       break ;
                  default :
                       statement-block-default ;
In this 
switch  , case , default   -----> are keywords
expression                     -----> It must give integer or character value
                                                      < Float value is not allowed >
constant-n                      -----> It is the identification given to the choice .
                                                 It can be integer or character constant But float is not allowed.
                                                 We can use arithmetic expressions like
                                                  case 4 + 5 :  correct
                                                 But  use of variable is not allowed .ie. case a+b : Wrong 
break                              -----> is a keyword . It is use to end the switch structure and stop
                                                further execution of other "case" . It shift the control outside 
                                                the switch structure .
Now we try to see how it actually work .
The resulting value of expression is matched with every case constant .
If the value match any of the case constant so it start executing from its statement-block  and it goes executing statement-block Till it encounter break statement , due to this break statement the control is transferred outside the switch structure .
If No match is found , it start executing the default-statement-block .

The flow-chart shows clear diagrametical representation ,that How switch-case structure works.

switch case

One more thing is that we can place case-constants in any order ( ie. incrementing or decrementing ) or without order . Also the default can  be place on any position . ie.

          switch( expression )
            case 8 :
               statement-block-8 ;
               break ;
            case 5 :
               statement-block-5 ;
               break ;
            default :
               statementblock-default ;
               break ;
            case 11 :
               statement-block-11 ;
               break ;
            case 4 :
               statement-block-4 ;
               break ;

In character constant can be used in switch , In this the ASCII value of character is compair with each case value .

I want to tell you some points which must be remember during the use of switch-case-default . These points help you to increase your programming skill .
We can use any valid expression of character or integer type in switch . But Float type is Not allowed .
If we use character type in switch so the value of character expression , which is matched with case-value is ASCII value of character ie. case 65 is equal to case 'A' .            ( ASCII value of character 'A' is  65 )
It is not allowed to use variables in case-value (ie.  case a + b :  is not allowed ) .
We must need to use break statement after the end of each statement-block . we use break statement to stop further execution of other case's .
We can only use constant after case keyword ,variable are not allowed .
switch structure can also be used in Nerted form , But it is rarely used .
switch structure is mainly used for Menu based programs .
The value of two case-constants can not be equal . We can use both character and integer constant values in same switch but we should concentrate on the value choosen because it only match integer value . ie .
        switch( expression )
          case 65 :
              statement-block65 ;
              break ;
         case 'A' :
              statement-block-A ;

The ASCII value of character 'A' is equal to 65

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