Constants

If a constant used in a program has a meaning, or it is possible that this constant may be changed in the future, the constant should be named. In general there should be no magic numbers or numbers that have a meaning but are not named, in a program.

Assume you are writing a program involving sales tax. The county tax rate is fixed at 4.75%. This rate will not change for the duration of the program, but in the future the tax rate may change, so this should be a named constant.

Acceptable Example

const float TAX_RATE = 0.0475;
...
 
tax = price * TAX_RATE;

Unacceptable Example:

tax = price * 0.0475;

At times the same constant may be given multiple names. Assume you are writing a program where the maximum number of items a person may purchase is 20. In addition, if the purchase price is over $20, the consumer will receive a discount. Each of these has a different meaning, so each should be declared as a named constant.

Acceptable Example:

const int MAX_ITEMS  = 20;
const int PRICE_BREAK = 20;
 
...
 
if (itemCount > MAX_ITEMS) {
 ...
}
 
if (price > PRICE_BREAK) {
    // the price was over $20, so give the customer a discount 
}

Unacceptable Example:

const int MAX_ITEMS = 20;
 
if (price > MAX_ITEMS) {
    // the price was over $20, so give the customer a discount
   ...
}

Note this is a very bad use. If the user decided to change MAX_ITEMS, a new bug would be introduced into the code.

In the last example, a programmer might be tempted to employ the following definition

const int TWENTY = 20;

This is unacceptable for multiple reasons.

TWENTY is no more informative than 20.
If the value of the constant changed, the programmer might be tempted to produce the following totally unacceptable code
```
const int TWENTY = 35;
```

Unless otherwise instructed, single use constants are usually not named unless they provide clarity to the program. Examples include a prompt used one time or an arbitrary field width.

The following is probably not necessairy

const int FIRST_FIELD_WITDTH = 20;
const string FIRST_PROMPT = "Enter your Age";
...
 
cout << setw(FIRST_FIELD_WIDTH) << FIRST_PROMPT << endl;