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<head><title>Constants</title>
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<div class="MCBreadcrumbsBox_0"><span class="MCBreadcrumbsPrefix">You are here: </span><a class="MCBreadcrumbsLink" href="AlphaRef.htm">PBASIC Language Reference</a><span class="MCBreadcrumbsDivider"> > </span><span class="MCBreadcrumbs">Constants</span>
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</div>
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<h1>Constants</h1>
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<div class="ImagePlusCaption">
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<div class="Col2">
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<p>
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<img src="../../graphics/pgm_icon1.gif" border="0" alt="BS1 icon" title="BS1 icon" />
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<img src="../../graphics/pgm_icon2.gif" border="0" alt="BS2 icon" title="BS2 icon" />
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<img src="../../graphics/pgm_icon2e.gif" border="0" alt="BS2e icon" title="BS2e icon" />
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<img src="../../graphics/pgm_icon2p.gif" border="0" alt="BS2p icon" title="BS2p icon" />
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<img src="../../graphics/pgm_icon2pe.gif" border="0" alt="BS2pe icon" title="BS2pe icon" />
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<img src="../../graphics/pgm_icon2px.gif" border="0" alt="BS2px icon" title="BS2px icon" />
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</p>
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</div>
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<p style="text-align: right;"> </p>
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<p> </p>
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</div>
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<p class="clear"> </p>
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<p> </p>
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<p class="PlainText">Suppose you’re working on a program called "Three Cheers" that flashes LEDs,
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makes hooting sounds, and activates a motor that crashes cymbals together, all in
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sets of three. A portion of your PBASIC program might contain something like:</p>
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<p>
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<img align="absmiddle" src="../../graphics/stamp_all.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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FOR counter = 1 TO 3
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GOSUB Make_Cheers
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NEXT
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FOR counter = 1 TO 3
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GOSUB Blink_LEDs
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NEXT
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FOR counter = 1 TO 3
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GOSUB Crash_Cymbals
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NEXT
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</pre>
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<p class="PlainText">The numbers 1 and 3 in the code above are called constants. They are constants
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because, while the program is running, nothing can happen to change those numbers.
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This distinguishes constants from variables, which can change while the program
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is running.</p>
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<p class="PlainText">PBASIC allows you to use several numbering systems. By default, it assumes that
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numbers are in decimal (base 10), our everyday system of numbers. But you can also
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use binary and hexadecimal (hex) numbers by identifying them with prefixes. And
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PBASIC will automatically convert quoted text into the corresponding ASCII code(s).
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For example:</p>
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<table cellpadding="0" cellspacing="0" border="0">
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<tr>
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<td width="60">99</td>
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<td>decimal</td>
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</tr>
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<tr>
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<td>%1010</td>
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<td>binary</td>
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</tr>
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<tr>
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<td>$FE</td>
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<td>hex</td>
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</tr>
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<tr>
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<td>"A"</td>
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<td>ASCII code for A (65)</td>
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</tr>
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</table>
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<p> </p>
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<p class="PlainText">You can assign names to constants in a similar fashion to how variables are
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declared. On a BS1, it is identical to variable declarations and on the other
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BASIC Stamp modelss you use the <span class="keyword_in_text">CON</span> directive. Here is the syntax:</p>
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<p>
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<img align="absmiddle" src="../../graphics/bs1_inline.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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SYMBOL Name = ConstantValue
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</pre>
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<p>
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<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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Name CON ConstantValue
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</pre>
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<p class="PlainText">Once created, named constants may be used in place of the numbers they
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represent. For example:</p>
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<p>
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<img align="absmiddle" src="../../graphics/bs1_inline.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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SYMBOL Cheers = 3
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SYMBOL idx = B2
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Main:
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FOR idx = 1 TO Cheers
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GOSUB Make_Cheers
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NEXT
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</pre>
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<p>
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<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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Cheers CON 3
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idx VAR Nib
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Main:
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FOR idx = 1 TO Cheers
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GOSUB Make_Cheers
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NEXT
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</pre>
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<p class="PlainText">That code would work exactly the same as the previous <span class="keyword_in_text">FOR...NEXT</span> loops. The
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editor software would substitute the number 3 for the constant named Cheers
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throughout your program. Like variable names, labels and instructions, constant
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names are not case sensitive; CHEERS, and ChEErs would all be processed as
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identical to Cheers.</p>
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<p class="PlainText">Refer to the <a href="ElementsStyle.htm" target="" title="" alt="" class="MCXref_0">Elements of PBASIC Style</a> for
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suggested guidelines on naming constant values.</p>
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<p class="PlainText">Using named constants does not increase the amount of code downloaded to the
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BASIC Stamp, and it often improves the clarity of the program. Weeks after a
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program is written, you may not remember what a particular number was supposed to
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represent—using a name may jog your memory (or simplify the detective work needed
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to figure it out).</p>
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<p class="PlainText">Named constants also have another benefit. Suppose the "Three Cheers" program
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had to be upgraded to "Five Cheers." In the original example you would have to
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change all of the 3s to 5s. Search and replace would help, but you might
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accidentally change some 3s that weren't numbers of cheers, too. However, if you
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made smart use of a named constant, all you would have to do is change 3 to 5 in
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one place, the constant's declaration:</p>
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<p>
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<img align="absmiddle" src="../../graphics/bs1_inline.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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SYMBOL Cheers = 5
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</pre>
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<p>
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<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
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</p><pre class="BScode" xml:space="preserve">
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Cheers CON 5
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</pre>
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<p class="PlainText">Now, assuming that you used the constant cheers wherever your program needed
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'the number of cheers,' your upgrade would be done.</p>
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<p>
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<img align="absmiddle" src="../../graphics/bs2all_inline.gif" border="0" alt="" />
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</p>
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<p class="PlainText">On the BS2-family you can take this idea a step further by defining constants
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with expressions; groups of math and/or logic operations that the editor software
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solves (evaluates) at compile time (the time right after you start the download
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and before the BASIC Stamp starts running your program). For example, suppose the
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"Cheers" program also controls a pump to fill glasses with champagne. Perhaps the
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number of glasses to fill is always twice the number of cheers, minus 1 (another
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constant):</p><pre class="BScode" xml:space="preserve">
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Cheers CON 5 ' # of cheers
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Glasses CON Cheers * 2 - 1 ' # of glasses
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</pre>
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<p class="PlainText">As you can see, one constant can be defined in terms of another. That is, the
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number glasses depends on the number cheers.</p>
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<p MadCap:conditions="BSEconditions.BSEWebHelp (Primary)-INCLUDE"><a href="../../HomeTopics/HomePage.htm">Go to Welcome page</a>
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<div class="MasterFoot">
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<p style="text-align: right;"><span class="ContactInfoProjectName">BASIC Stamp Help</span> <![CDATA[ ]]><span class="ContactInfoVersion#">Version 2.5.4</span> <![CDATA[ ]]></p>
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<p style="text-align: right;">Copyright © <span class="ContactInfoCompanyName">Parallax Inc.</span></p>
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<p style="text-align: right;"><span class="SystemShortDate">8/8/2012</span>
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