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111 lines
6.0 KiB
HTML
111 lines
6.0 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 - If </title>
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<link rel='stylesheet' href='arduino.css' type='text/css' />
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<meta name="verify-v1" content="TtxFIEJAB6zdJ509wLxjnapQzKAMNm9u0Wj4ho6wxIY=" />
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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> | <a class='wikilink' href='Changes.html'>Changes</a>
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</p>
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<p class='vspace'></p><h2>if (conditional) and ==, !=, <, > (comparison operators)</h2>
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<p><strong><code>if</code></strong>, which is used in conjunction with a comparison operator, tests whether a certain condition has been reached, such as an input being above a certain number. The format for an if test is:
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</p>
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<p class='vspace'></p><pre>if (someVariable > 50)
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{
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// do something here
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}
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</pre>
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<p class='vspace'></p><p>The program tests to see if someVariable is greater than 50. If it is, the program takes a particular action. Put another way, if the statement in parentheses is true, the statements inside the brackets are run. If not, the program skips over the code.
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</p>
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<p class='vspace'></p><p>The brackets may be omitted after an <em>if</em> statement. If this is done, the next line (defined by the semicolon) becomes the only conditional statement.
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</p><pre>
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if (x > 120) digitalWrite(LEDpin, HIGH);
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if (x > 120)
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digitalWrite(LEDpin, HIGH);
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if (x > 120){ digitalWrite(LEDpin, HIGH); }
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if (x > 120){
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digitalWrite(LEDpin1, HIGH);
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digitalWrite(LEDpin2, HIGH);
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} // all are correct
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</pre>
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<p class='vspace'></p><p>The statements being evaluated inside the parentheses require the use of one or more operators:
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</p>
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<p class='vspace'></p><h3>Comparison Operators:</h3>
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<pre> <strong>x == y</strong> (x is equal to y)
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<strong>x != y</strong> (x is not equal to y)
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<strong>x < y</strong> (x is less than y)
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<strong>x > y</strong> (x is greater than y)
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<strong>x <= y</strong> (x is less than or equal to y)
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<strong>x >= y</strong> (x is greater than or equal to y)
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</pre>
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<p class='vspace'></p><h4>Warning: </h4>
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<p>Beware of accidentally using the single equal sign (e.g.<code> if (x = 10)</code> ). The single equal sign is the assignment operator, and sets x to 10 (puts the value 10 into the variable x). Instead use the double equal sign (e.g.<code> if (x == 10) </code>), which is the comparison operator, and tests <em>whether</em> x is equal to 10 or not. The latter statement is only true if x equals 10, but the former statement will always be true.
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</p>
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<p class='vspace'></p><p>This is because C evaluates the statement <code> if (x=10) </code> as follows: 10 is assigned to x (remember that the single equal sign is the <a class='wikilink' href='Assignment.html'>assignment operator</a>), so x now contains 10. Then the 'if' conditional evaluates 10, which always evaluates to TRUE, since any non-zero number evaluates to TRUE. Consequently, <code>if (x = 10)</code> will always evaluate to TRUE, which is not the desired result when using an 'if' statement. Additionally, the variable x will be set to 10, which is also not a desired action.
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</p>
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<p class='vspace'></p><p><strong>if</strong> can also be part of a branching control structure using the <a class='wikilink' href='Else.html'>if...else</a>] construction.
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</p>
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<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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<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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</body>
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</html>
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