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13.1: Nested If Then Else

  • Page ID
    10317
  • Introduction to Multiway Selection

    Nested Control Structures

    We are going to first introduce the concept of nested control structures. Nesting is a concept that places one item inside of another. Consider:

    if expression
      true action
    else
      false action 
    

     This is the basic form of the if then else control structure. Now consider:

    if age is less than 18
      you can't vote
      if age is less than 16
        you can't drive
      else
        you can drive
    else
      you can vote
      if age is less than 21
        you can't drink
      else
        you can drink 
    

    As you can see we simply included as part of the "true action" a statement and another if then else control structure. We did the same (nested another if then else) for the "false action". In our example we nested if then else control structures. Nesting could have an if then else within a while loop. Thus, the concept of nesting allows the mixing of the different categories of control structures.

    Multiway Selection

    One of the drawbacks of two way selection is that we can only consider two choices. But what do you do if you have more than two choices. Consider the following which has four choices:

    if age equal to 18
      you can now vote
    else
      if age equal to 39
        you are middle aged
      else
        if age equal to 65
          you can consider retirement
        else
          your age is unimportant 
    

    You get an appropriate message depending on the value of age. The last item is referred to as the default. If the age is not equal to 18, 39 or 65 you get the default message. In some situations there is no default action. Consider:

    if age equal to 18
      you can now vote
    else
      if age equal to 39
        you are middle aged
      else
        if age equal to 65
          you can consider retirement 
    

    The last if then else control structure has no "else". It’s implied "else do nothing". Without the default the multiway selection could be written as a series of "if then without the else" structures. Consider:

    if age equal to 18
      you can now vote
    if age equal to 39
      you are middle aged
    if age equal to 65
      you can consider retirement
    

    We have shown two ways to accomplish multiway selection. The choice of using nested if then else control structures or a series of if then control structures is decided on the existence of a default action (you must use nested if then else) or programmer preference if there is not a default action (you may use nested if then else or a series of if then control structures).

    If Then Else Syntax within C++

    The syntax for the if then else control structure within the C++ programming language is:

    C++ source code: Layout of an if then else
    if (expression)
      {
      statement;
      }
    else
      {
      statement;
      }  
    

    The test expression is within the parentheses, but this is not a function call. The parentheses are part of the controlstructure. Additionally, there is no semicolon after the parenthesis following the expression.

    C++ Example

    Multiway selection is often needed to cover all possibilities. Assume that the user has been prompted for the ages of two people with the answers stored in variables named age1 and age2. Consider:

    C++ Source Code:
    if(age1 > age2)
      {
      cout << "\n\nThe first person is older.";
      }
    else
      {
      cout << "\n\nThe second person is older.";
      } 
    

    What if the two persons are the same age? The program incorrectly says the second person is older. To solve this we must handle all three possibilities. Consider this mulitway selection example:

    C++ Source Code:
    if(age1 == age2)
      {
      cout << "\n\nThey are the same age.";
      }
    else
      {
      if(age1 > age2)
        {
        cout << "\n\nThe first person is older.";
        }
      else
        {
        cout << "\n\nThe second person is older.";
        }
      } 
    

    Definitions

    Nested Control Structures
    Placing one control structure inside of another.
    Multiway Selection
    Using control structures to be able to select from more than two choices.