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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.
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).
The syntax for the if then else control structure within the C++ programming language is:
if (expression)
{statement;
}else
{statement;
}
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:
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:
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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