Describe Selection – if and switch using C++.

if is decision making control and its general for is:

if (this condition is true)
               execute this statement;

The keyword if tells the compiler that what follows, is a decision control instruction. The condition following if is always enclosed in parenthesis. The statement(s) are executed if the condition is true.

cout << “\n Enter number:”;
cin >> no;
if (no < 10)
            cout<< “Entered number is less than 10”;

There is another variation to if stmt: if-else

if (no < 10)
            cout << “No is less than 10”;
else if (no == 10)
            cout << “No is equal to 10”;
else
            cout << “No greater than 10”;

Switch: It is used to make choice between multiple options. When a decision is to be made depending upon value of an expression and the programmer knows the possible values it is going to take.

The syntax is:

switch (integer expression)
{
             case constant1:
                          do this;
             case constant2:
                          do this;
             case constant3:
                          do this;
             default:
                          do this;
}

Consider following program code:
cout << “\n Enter number:”;
cin >> no;
switch (no)
{
              case 1:
                          cout << “I am in case 1”;
                          break;
              case 2:
                          cout << “I am in case 2”;
                          break;
              case 3:
                          cout << “I am in case 3”;
                          break;
              default:
                          cout << “I am lost!”;
}

If user enters no 1, 2 or 3, this code will print the appropriate message. For any other no, it will go to the default case and print “I am lost!”

The advantage of switch is that it leads to more structured program and the level of indentation is manageable, more so if there are multiple statements within each case of a switch. Thus switch statements are very useful in menu driven programs.