3. DECISION-MAKING STATEMENTS 

Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.
 Following is the general from of a typical decision making structure found in most of the programming languages: 

C++ programming language provides following types of decision making statements.

Statement Description     if statement An ‘if’ statement consists of a boolean expression  followed by one or more statements.   if...else statement An ‘if’ statement can be followed by an optional  ‘else’ statement, which executes when the boolean  expression is false.   switch statement A ‘switch’ statement allows a variable to be tested 



 C++
   for equality against a list of values.   nested if statements You can use one ‘if’ or ‘else if’ statement inside  another ‘if’ or ‘else if’ statement(s).   nested switch statements You can use one ‘switch’ statement inside another  ‘switch’ statement(s). 


If Statement
 An if statement consists of a boolean expression followed by one or more statements.

Syntax
 The syntax of an if statement in C++ is:


if(boolean_expression)

{

// statement(s) will execute if the boolean expression is true

}

If the boolean expression evaluates to true, then the block of code inside the if statement will be executed. If boolean expression evaluates to false, then the first set of code after the end of the if statement (after the closing curly brace) will be executed.

Flow Diagram 

 When the above code is compiled and executed, it produces the following result:


a is less than 20; 

value of a is : 10 

if…else Statement
 An if statement can be followed by an optional else statement, which executes when the boolean expression is false.

If the boolean expression evaluates to true, then the if block of code will be executed, otherwise else block of code will be executed.

When the above code is compiled and executed, it produces the following result:


a is not less than 20; 

value of a is : 100 

if...else if...else Statement
 An if statement can be followed by an optional else if...else statement, which is very usefull to test various conditions using single if...else if statement.
 When using if , else if , else statements there are few points to keep in mind.
 • An if can have zero or one else's and it must come after any else if's. 
 • An if can have zero to many else if's and they must come before the else. 
 • Once an else if succeeds, none of he remaining else if's or else's will be tested.