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90 lines
4.6 KiB
HTML
90 lines
4.6 KiB
HTML
15 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html>
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<head>
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<title>Arduino - Const </title>
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<link rel='stylesheet' href='arduino.css' type='text/css' />
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</head>
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<body>
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<div id="page">
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<!--PageHeaderFmt-->
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<div id="pageheader">
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<div class="title"><a href="http://www.arduino.cc"/>Arduino</a></div>
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<div class="search">
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<!-- SiteSearch Google -->
<FORM method=GET action="http://www.google.com/search">
<input type=hidden name=ie value=UTF-8>
<input type=hidden name=oe value=UTF-8>
<INPUT TYPE=text name=q size=25 maxlength=255 value="">
<INPUT type=submit name=btnG VALUE="search">
<input type=hidden name=domains value="http://www.arduino.cc/">
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<input type=hidden name=sitesearch value="http://www.arduino.cc/">
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</FORM>
<!-- SiteSearch Google -->
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</div>
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</div>
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<!--/PageHeaderFmt-->
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<!--PageLeftFmt-->
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<div id="pagenav" style="text-align: right">
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<div style="float: left;">
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<p><a class='wikilink' href='http://arduino.cc/en/Main/Buy'>Buy</a>
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<a class='wikilink' href='http://arduino.cc/en/Main/Software'>Download</a>
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<a class='wikilink' href='Guide_index.html'>Getting Started</a>
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<a class='wikilink' href='http://arduino.cc/en/Tutorial/HomePage'>Learning</a>
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<a class='wikilink' href='index.html'>Reference</a>
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<a class='wikilink' href='http://arduino.cc/en/Main/Hardware'>Hardware</a>
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<a class='wikilink' href='FAQ.html'>FAQ</a>
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</p>
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<p class='vspace'></p>
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</div>
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<a class="urllink" href="http://www.arduino.cc/blog/" rel="nofollow">Blog »</a> |
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<a class="urllink" href="http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl" rel="nofollow">Forum »</a> |
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<a class="urllink" href="http://www.arduino.cc/playground/" rel="nofollow">Playground »</a>
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</div>
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<!--/PageLeftFmt-->
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<div id="pagetext">
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<!--PageText-->
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<div id='wikitext'>
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<p><strong>Reference</strong> <a class='wikilink' href='index.html'>Language</a> (<a class='wikilink' href='Extended.html'>extended</a>) | <a class='wikilink' href='Libraries.html'>Libraries</a> | <a class='wikilink' href='Comparison.html'>Comparison</a>
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</p>
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<p class='vspace'></p><h2>const keyword</h2>
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<p>The <strong>const</strong> keyword stands for constant. It is a variable <em>qualifier</em> that modifies the behavior of the variable, making a variable "<em>read-only</em>". This means that the variable can be used just as any other variable of its type, but its value cannot be changed. You will get a compiler error if you try to assign a value to a <strong>const</strong> variable.
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</p>
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<p class='vspace'></p><p>Constants defined with the <em>const</em> keyword obey the rules of <em><a class='wikilink' href='Scope.html'>variable scoping</a></em> that govern other variables. This, and the pitfalls of using<em>#define</em>, makes the <em>const</em> keyword a superior method for defining constants and is preferred over using <em><a class='wikilink' href='Define.html'>#define</a></em>.
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</p>
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<p class='vspace'></p><h4>Example</h4>
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<pre>
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const float pi = 3.14;
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float x;
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// ....
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x = pi * 2; // it's fine to use const's in math
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pi = 7; // illegal - you can't write to (modify) a constant
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</pre><h4><strong>#define</strong> or <strong>const</strong></h4>
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<p>You can use either <strong>const</strong> or <strong>#define</strong> for creating numeric or string constants. For <a class='wikilink' href='Array.html'>arrays</a>, you will need to use <strong>const</strong>. In general <em>const</em> is preferred over <em>#define</em> for defining constants.
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</p>
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<p class='vspace'></p><p>See also:
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</p><ul><li><a class='wikilink' href='Define.html'>#define</a>
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</li><li><a class='wikilink' href='Volatile.html'>volatile</a>
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</li></ul><p class='vspace'></p><p><a class='wikilink' href='index.html'>Reference Home</a>
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</p>
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<p class='vspace'></p><p><em>Corrections, suggestions, and new documentation should be posted to the <a class='urllink' href='http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?board=swbugs' rel='nofollow'>Forum</a>.</em>
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</p>
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<p class='vspace'></p><p>The text of the Arduino reference is licensed under a
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<a class='urllink' href='http://creativecommons.org/licenses/by-sa/3.0/' rel='nofollow'>Creative Commons Attribution-ShareAlike 3.0 License</a>. Code samples in the reference are released into the public domain.
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</p>
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</div>
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</div>
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<!--PageFooterFmt-->
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<div id="pagefooter">
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<a href='#'>Edit Page</a> | <a href='#'>Page History</a> | <a href='#' target='_blank'>Printable View</a> | <a href='http://arduino.cc/en/Site/AllRecentChanges'>All Recent Site Changes</a>
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</div>
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<!--/PageFooterFmt-->
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</div>
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</body>
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</html>
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