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4.8: Nested If Then Else

  • Page ID
    10661
  • Overview

    Two-way selection structures may be nested inside other two-way selection structures, resulting in multi-way selection.

    Discussion

    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. To simplify the code structure, this is most often written as:

    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

    Key Terms

    multiway selection
    Using control structures to be able to select from more than two choices.
    nested control structures Placing one control structure inside of another.