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PRISM Manual | ThePRISMLanguage / Commands
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<p><a class='wikilink' href='Main.html'>The PRISM Language</a> /
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</p><h1>Commands</h1>
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<p>The behaviour of each module is described by <em>commands</em>,
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comprising a <em>guard</em> and one or more <em>updates</em>.
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The first command of module <code>M1</code> in our <a class='wikilink' href='Example1.html'>example</a> is:
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<div class='vspace'></div>
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<div class='sourceblock ' id='sourceblock1'>
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<div class='sourceblocktext'><div class="prism"> [] <span class="prismident">x</span>=<span class="prismnum">0</span> -> <span class="prismnum">0.8</span>:(<span class="prismident">x</span>'=<span class="prismnum">0</span>) + <span class="prismnum">0.2</span>:(<span class="prismident">x</span>'=<span class="prismnum">1</span>); <br/>
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</div></div>
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<div class='sourceblocklink'><a href='http://www.prismmodelchecker.org/manual/ThePRISMLanguage/Commands?action=sourceblock&num=1' type='text/plain'>[$[Get Code]]</a></div>
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</div>
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<p class='vspace'>The guard <code>x=0</code> indicates that this describes the behaviour of the module when the variable <code>x</code> has value 0.
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The updates <code>(x'=0)</code> and <code>(x'=1)</code> and their associated probabilities state that the value of <code>x</code> will
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remain at 0 with probability 0.8 and change to 1 with probability 0.2.
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Note that the inclusion of updates in parentheses, e.g. <code>(x'=1)</code>, is essential.
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While older versions of PRISM did not report the absence of parentheses as an error, newer versions do.
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Note also that PRISM will complain if the probabilities on the right hand side of a command do not sum to one.
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</p>
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<p class='vspace'>The second command:
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</p>
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<div class='vspace'></div>
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<div class='sourceblock ' id='sourceblock2'>
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<div class='sourceblocktext'><div class="prism"> [] <span class="prismident">x</span>=<span class="prismnum">1</span> & <span class="prismident">y</span>!=<span class="prismnum">2</span> -> (<span class="prismident">x</span>'=<span class="prismnum">2</span>); <br/>
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</div></div>
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<div class='sourceblocklink'><a href='http://www.prismmodelchecker.org/manual/ThePRISMLanguage/Commands?action=sourceblock&num=2' type='text/plain'>[$[Get Code]]</a></div>
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</div>
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<p class='vspace'>illustrates that guards can contain constraints on any variable, not just the ones in that module,
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i.e. the behaviour of one module can depend on the state of another.
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Updates, however, can only specify values for variables belonging to the module.
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In general a module can <em>read</em> the variables of any other module, but only <em>write</em> to its own.
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When a command comprises a single update with probability 1, the <code>1.0:</code> can be omitted,
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as is done in the example above.
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</p>
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<p class='vspace'>If a module has more than one variable, updates describe the new value for each of them.
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For example, if it had two variables <code>x1</code> and <code>x2</code>, a possible command would be:
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</p>
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<div class='vspace'></div>
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<div class='sourceblock ' id='sourceblock3'>
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<div class='sourceblocktext'><div class="prism"> [] <span class="prismident">x1</span>=<span class="prismnum">0</span> & <span class="prismident">x2</span>><span class="prismnum">0</span> & <span class="prismident">x2</span><<span class="prismnum">10</span> -> <span class="prismnum">0.5</span>:(<span class="prismident">x1</span>'=<span class="prismnum">1</span>)&(<span class="prismident">x2</span>'=<span class="prismident">x2</span>+<span class="prismnum">1</span>) + <span class="prismnum">0.5</span>:(<span class="prismident">x1</span>'=<span class="prismnum">2</span>)&(<span class="prismident">x2</span>'=<span class="prismident">x2</span>-<span class="prismnum">1</span>); <br/>
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</div></div>
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<div class='sourceblocklink'><a href='http://www.prismmodelchecker.org/manual/ThePRISMLanguage/Commands?action=sourceblock&num=3' type='text/plain'>[$[Get Code]]</a></div>
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</div>
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<p class='vspace'>Notice that elements of the updates are concatenated with <code>&</code> and that each element must be bracketed individually.
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If an update does not give a new value for a local variable, it is assumed not to change.
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As a special case, the keyword <code><strong>true</strong></code> can be used to denote an update where no variable's value changes, i.e. the following are all equivalent:
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</p>
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<div class='vspace'></div>
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<div class='sourceblock ' id='sourceblock4'>
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<div class='sourceblocktext'><div class="prism">[] <span class="prismident">x1</span>><span class="prismnum">10</span> | <span class="prismident">x2</span>><span class="prismnum">10</span> -> (<span class="prismident">x1</span>'=<span class="prismident">x1</span>)&(<span class="prismident">x2</span>'=<span class="prismident">x2</span>);<br/>
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[] <span class="prismident">x1</span>><span class="prismnum">10</span> | <span class="prismident">x2</span>><span class="prismnum">10</span> -> (<span class="prismident">x1</span>'=<span class="prismident">x1</span>);<br/>
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[] <span class="prismident">x1</span>><span class="prismnum">10</span> | <span class="prismident">x2</span>><span class="prismnum">10</span> -> <span class="prismkeyword">true</span>;<br/>
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</div></div>
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<div class='sourceblocklink'><a href='http://www.prismmodelchecker.org/manual/ThePRISMLanguage/Commands?action=sourceblock&num=4' type='text/plain'>[$[Get Code]]</a></div>
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</div>
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<p class='vspace'>Finally, it is important to remember that the expressions on the right hand side of each update refer to the state of the model <em>before</em> the update occurs. So, for example, this command:
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</p>
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<div class='vspace'></div>
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<div class='sourceblock ' id='sourceblock5'>
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<div class='sourceblocktext'><div class="prism"> [] <span class="prismident">x1</span>=<span class="prismnum">0</span> & <span class="prismident">x2</span>=<span class="prismnum">1</span> -> (<span class="prismident">x1</span>'=<span class="prismnum">2</span>)&(<span class="prismident">x2</span>'=<span class="prismident">x1</span>) <br/>
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</div></div>
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<div class='sourceblocklink'><a href='http://www.prismmodelchecker.org/manual/ThePRISMLanguage/Commands?action=sourceblock&num=5' type='text/plain'>[$[Get Code]]</a></div>
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</div>
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<p class='vspace'>updates variable <code>x2</code> to 0, not 2.
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<h3><a class='wikilink' href='../Main/Main.html'>PRISM Manual</a></h3>
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<p><strong><a class='wikilink' href='Main.html'>The PRISM Language</a></strong>
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</p><ul><li><a class='wikilink' href='Main.html'>Introduction</a>
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</li><li><a class='wikilink' href='Example1.html'>Example 1</a>
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