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<h1>Constants</h1>
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<p class="PlainText">Suppose you’re working on a program called "Three Cheers" that flashes LEDs,
makes hooting sounds, and activates a motor that crashes cymbals together, all in
sets of three. A portion of your PBASIC program might contain something like:</p>
<p>
<img align="absmiddle" src="../../graphics/stamp_all.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
FOR counter = 1 TO 3
GOSUB Make_Cheers
NEXT
FOR counter = 1 TO 3
GOSUB Blink_LEDs
NEXT
FOR counter = 1 TO 3
GOSUB Crash_Cymbals
NEXT
</pre>
<p class="PlainText">The numbers 1 and 3 in the code above are called constants. They are constants
because, while the program is running, nothing can happen to change those numbers.
This distinguishes constants from variables, which can change while the program
is running.</p>
<p class="PlainText">PBASIC allows you to use several numbering systems. By default, it assumes that
numbers are in decimal (base 10), our everyday system of numbers. But you can also
use binary and hexadecimal (hex) numbers by identifying them with prefixes. And
PBASIC will automatically convert quoted text into the corresponding ASCII code(s).
For example:</p>
<table cellpadding="0" cellspacing="0" border="0">
<tr>
<td width="60">99</td>
<td>decimal</td>
</tr>
<tr>
<td>%1010</td>
<td>binary</td>
</tr>
<tr>
<td>$FE</td>
<td>hex</td>
</tr>
<tr>
<td>"A"</td>
<td>ASCII code for A (65)</td>
</tr>
</table>
<p> </p>
<p class="PlainText">You can assign names to constants in a similar fashion to how variables are
declared. On a BS1, it is identical to variable declarations and on the other
BASIC Stamp modelss you use the <span class="keyword_in_text">CON</span> directive. Here is the syntax:</p>
<p>
<img align="absmiddle" src="../../graphics/bs1_inline.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
SYMBOL Name = ConstantValue
</pre>
<p>
<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
Name CON ConstantValue
</pre>
<p class="PlainText">Once created, named constants may be used in place of the numbers they
represent. For example:</p>
<p>
<img align="absmiddle" src="../../graphics/bs1_inline.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
SYMBOL Cheers = 3
SYMBOL idx = B2
Main:
FOR idx = 1 TO Cheers
GOSUB Make_Cheers
NEXT
</pre>
<p>
<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
Cheers CON 3
idx VAR Nib
Main:
FOR idx = 1 TO Cheers
GOSUB Make_Cheers
NEXT
</pre>
<p class="PlainText">That code would work exactly the same as the previous <span class="keyword_in_text">FOR...NEXT</span> loops. The
editor software would substitute the number 3 for the constant named Cheers
throughout your program. Like variable names, labels and instructions, constant
names are not case sensitive; CHEERS, and ChEErs would all be processed as
identical to Cheers.</p>
<p class="PlainText">Refer to the <a href="ElementsStyle.htm" target="" title="" alt="" class="MCXref_0">Elements of PBASIC Style</a> for
suggested guidelines on naming constant values.</p>
<p class="PlainText">Using named constants does not increase the amount of code downloaded to the
BASIC Stamp, and it often improves the clarity of the program. Weeks after a
program is written, you may not remember what a particular number was supposed to
represent—using a name may jog your memory (or simplify the detective work needed
to figure it out).</p>
<p class="PlainText">Named constants also have another benefit. Suppose the "Three Cheers" program
had to be upgraded to "Five Cheers." In the original example you would have to
change all of the 3s to 5s. Search and replace would help, but you might
accidentally change some 3s that weren't numbers of cheers, too. However, if you
made smart use of a named constant, all you would have to do is change 3 to 5 in
one place, the constant's declaration:</p>
<p>
<img align="absmiddle" src="../../graphics/bs1_inline.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
SYMBOL Cheers = 5
</pre>
<p>
<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
</p><pre class="BScode" xml:space="preserve">
Cheers CON 5
</pre>
<p class="PlainText">Now, assuming that you used the constant cheers wherever your program needed
'the number of cheers,' your upgrade would be done.</p>
<p>
<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
</p>
<p class="PlainText">On the BS2-family you can take this idea a step further by defining constants
with expressions; groups of math and/or logic operations that the editor software
solves (evaluates) at compile time (the time right after you start the download
and before the BASIC Stamp starts running your program). For example, suppose the
"Cheers" program also controls a pump to fill glasses with champagne. Perhaps the
number of glasses to fill is always twice the number of cheers, minus 1 (another
constant):</p><pre class="BScode" xml:space="preserve">
Cheers CON 5 ' # of cheers
Glasses CON Cheers * 2 - 1 ' # of glasses
</pre>
<p class="PlainText">As you can see, one constant can be defined in terms of another. That is, the
number glasses depends on the number cheers.</p>
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