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An explanation of nesting and examples using nested if then else to demonstrate multiway selection.

Introduction to mulitway 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 actionelse 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 voteif age is less than 16 you can't driveelse you can driveelse you can voteif age is less than 21 you can't drinkelse 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 voteelse if age equal to 39you are middle aged elseif age equal to 65 you can consider retirementelse 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 voteelse if age equal to 39you are middle aged elseif 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 voteif age equal to 39 you are middle agedif 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; }

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The test expression is within the parentheses, but this is not a function call. The parentheses are part of the control structure. 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."; }

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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."; }}

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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.

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Source:  OpenStax, Programming fundamentals - a modular structured approach using c++. OpenStax CNX. Jan 10, 2013 Download for free at http://cnx.org/content/col10621/1.22
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