10.3: Nested If Then Else
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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.