Decision making Statements

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.

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 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.

#include <iostream>
using namespace std;
int main ()
{
// local variable declaration:
int a = 10;
// check the boolean condition
if( a < 20 )
{
// if condition is true then print the following
cout << “a is less than 20;” << endl;
}
cout << “value of a is : ” << a << endl;
return 0;
}

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.

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

if(boolean_expression)
{
// statement(s) will execute if the boolean expression is true
}
else
{
// statement(s) will execute if 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.

Example

#include <iostream>
using namespace std;
int main ()
{
// local variable declaration:
int a = 100;
// check the boolean condition
if( a < 20 )
{
// if condition is true then print the following
cout << “a is less than 20;” << endl;
}
else
{
// if condition is false then print the following
cout << “a is not less than 20;” << endl;
}
cout << “value of a is : ” << a << endl;
return 0;
}

When the above code is compiled and executed, it produces the following result:
a is not less than 20;
value of a is : 100

Switch Statement
A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case.

Syntax   The syntax for a switch statement in C++ is as follows:

switch(expression)
{
case constant-expression :
statement(s);
break; //optional
case constant-expression :
statement(s);
break; //optional
// you can have any number of case statements.
default : //Optional
statement(s);
}

The following rules apply to a switch statement:
 The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type.
 You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.
 The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal.
 When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.
 When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.
 Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached.
 A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in
the default case.