Decision Control Statements in C

DECISION CONTROL STATEMENTS

1. if Statement
2. if...else Statement
3. elseif Ladder
4. switch Statement
5. Conditional Operator

1.     if Statement - The if statement is a powerful decision making statement and is used to control the flow of execution of statements. The syntax and flowchart is shown below:

 

Syntax:     

            if (test expression)

{

    statement-block;

}

statement x;

 

·         The statement block may be a single statement or a group of statements.

·         The test expression may be a relational expression or logical expression.

·         If the test expression is true (i.e. it returns a nonzero value or 1) the statement-block will be executed, otherwise the statement block be skipped and execution will jump to the statement-x.

·         If condition is true both statement-block and statement-x are executed in sequence.

·         If there are only one statement in statement block then braces are not necessary otherwise these are necessary.

·         The indentation does not affect the logic of a program; it only enhance to increase the clarity of the program.

·         There can be any valid C statement inside if. It can be another if statement. It is called nesting of if.

 

2.     if … else Statement – With the if statement we can execute a group of statement when the condition is true but if condition is false we cannot execute a group of statements. It is possible with if… else statement. The syntax and flowchart is shown below:

Syntax:                                                                          

if (test expression)

{

     True block statement(s);

}

else

{

     False block statements(s);

}

Statement-x;

 

·         If the test expression is true then true-block stmt. will be executed, otherwise, false-block statement will be executed.

·         In either case, either true block or false block will be executed, not both.

·         Nesting of if-else statement – It is perfectly alright if we write an entire if – else construct withing, either the bosy of if statement or the body of   else statement. This is called ‘nesting’ of ifs.

 

3.     The else if Ladder - There is another way of putting ifs together when multipath decision are involved. A multipath decision is a chain of ifs in which the statement associated with each else is an if. It takes the following general form:

if (condition 1)

{   

statement block 1   

}

elseif (condition 2)

{   

statement block 2   

}

elseif (condition n)

{    

statement n;          

}

else

default-statement

 

statement-x

 

This construct is known as the else if ladder. The conditions are evaluated from the top (of the Ladder), downwards. As soon as a true condition is found, the statements associated with it are executed and the control is transferred to the statement-x (skipping the rest of the ladder).

 

When all the n conditions become false, then the final else containing the default-statement will be executed.

 

Example:

………..

If (code ==1)

    colour=”RED”;

else if (code==2)

    colour=”WHITE”;

else if (code==3)

    colour=”YELLOW”;

 else

    colour=”GREEN”;

     ……….

 

4.     switch Statement – 

 

The control statement which allows us to make a decision ffrom the number of choice is called a switch, or more correctly a switch-case-default, since these three keywords to together to make up the control statement. They most often appear as follows:

 

Syntax:                                                                            

switch (integer expression) 

{

     case 1:

                statements 1;

                break;

     case 2:

                statements2;

                break;

     ………….

     …………

     default:

          default-statement;

}

 

·         The integer expression following the keyword switch is any C expression that will yield an integer value. It could be and integer constant like 1, 2, 3 … or character constant ‘a’, ‘y’, ‘N’ or an expression that evaluate to an integer. Float value is not allowed in switch statement.

·         Which case statement will be satisfied by the value of switch that will be executed and after the execution the break statement exists the loop from the switch.

·         switch executes the case where the match is found and all the subsequent cases and the default as well.

·         There is no need of the default statement.

·         It is not necessary that we put the case statement in ascending order. We can kept them in any order.

·         We are also allowed to use char value in case and switch statements.

·         We can mix integer and character constant in different case of a switch.

·         Sometimes there may be not be any statement in some of the case in switch, but still they might turn out to be useful. This is shown below example.

·         Even if there are multiple statements to be executed in each case, there is no need to enclose those statements within a pair of braces. 

·         If we have no default case, then the program simply falls through the entire switch and continues with the next instruction (if any) that follows the control structure.

·         switch is the replacement for if because if after a better way of writing program as compare to if. There is no confusing more indentation as if in the switch.

·         There is one disadvantage that we cannot use any condition in case e.g. case I <= 20:

·         The break statement when used in a switch, takes the control outside the switch. However, use of continue will not take the control to the beginning of switch.

·         We can nest the switch statement.

·         If here is no break statement then all the statements given below of satisfied case will be executed simultaneously

 

5.     Conditional operator ( … ? ... : … )

Conditional operator takes three operands and consists of two symbols ? and : . Conditional operators are used for decision making in C. For example:

c=(c>0)?10:-10;

If c is greater than 0, value of c will be 10 but, if c is less than 0, value of c will be -10.