====== 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
<code c++>

const float TAX_RATE = 0.0475;
...

tax = price * TAX_RATE;

</code>

Unacceptable Example:
<code c++>
tax = price * 0.0475;
</code>

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:
<code c++>
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 
}
</code>

Unacceptable Example:
<code c++>
const int MAX_ITEMS = 20;

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

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
    * <code c++>
const int TWENTY = 35;
</code>

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
<code c++>
const int FIRST_FIELD_WITDTH = 20;
const string FIRST_PROMPT = "Enter your Age";
...

cout << setw(FIRST_FIELD_WIDTH) << FIRST_PROMPT << endl;
</code>