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<section id="introduction-to-prefix-expression-evaluator">
<h1>Introduction to prefix expression evaluator<a class="headerlink" href="#introduction-to-prefix-expression-evaluator" title="Permalink to this heading">¶</a></h1>
<p><strong>01-simple-expression.py</strong></p>
<p>The Expr object implements a tiny functional programming language
that can be used to write synthesis or signal processing algorithms.</p>
<p>The language uses prefix notation (also known as “Polish notation”)
to express audio algorithms. In prefix notation, Operators are written
before their operands, between braces. To add two number together, we
will write:</p>
<div class="highlight-scheme notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="nb">+</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="mi">2</span><span class="p">)</span><span class="w"></span>
</pre></div>
</div>
<p>If we want to multiply by 10 the result of this addition, then:</p>
<div class="highlight-scheme notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="nb">*</span><span class="w"> </span><span class="p">(</span><span class="nb">+</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="mi">2</span><span class="p">)</span><span class="w"> </span><span class="mi">10</span><span class="p">)</span><span class="w"></span>
</pre></div>
</div>
<p>And so on, it’s how we build our algorithm. To generate a sine wave, we do:</p>
<div class="highlight-scheme notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="nb">sin</span><span class="w"> </span><span class="p">(</span><span class="nb">*</span><span class="w"> </span><span class="nv">twopi</span><span class="w"> </span><span class="p">(</span><span class="nf">~</span><span class="w"> </span><span class="mi">440</span><span class="p">)))</span><span class="w"></span>
</pre></div>
</div>
<p>The <cite>~</cite> operator generates a ramp from 0 to 1 at a frequency given to its first
argument.</p>
<p>If the object generates only one channel output (the default), the last expression
in the script is the output signal. Otherwise, output signals must be created with
the <cite>out</cite> function (we will cover this topic in a later example).</p>
<p>For the detail of all available functions, see the prefix expression
<a class="reference external" href="http://ajaxsoundstudio.com/pyodoc/api/classes/expression.html">documentation</a>.</p>
<p>Here is a program that use the Expr object, with its own editor, to generate two
sine waves with frequency and gain control.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <a class="sphinx-codeautolink-a" href="../../api/alphabetical.html#module-pyo" title="pyo"><span class="nn">pyo</span></a> <span class="kn">import</span> <span class="o">*</span>
<span class="n">s</span> <span class="o">=</span> <a class="sphinx-codeautolink-a" href="../../api/classes/server.html#pyo.Server" title="pyo.lib.server.Server"><span class="n">Server</span></a><span class="p">()</span><span class="o">.</span><span class="n">boot</span><span class="p">()</span>
<span class="c1"># The expression to evaluate. It produces two sine waves, of</span>
<span class="c1"># frequency 500 and 600 Hz, with an amplitude of -12 dB.</span>
<span class="c1"># The ~ operator generates a ramp from 0 to 1 at a frequency</span>
<span class="c1"># given to its first argument.</span>
<span class="n">expression</span> <span class="o">=</span> <span class="s2">"""</span>
<span class="s2">// sum of two sine waves with controlled gain.</span>
<span class="s2">* (+ (sin (* twopi (~ 500)))</span>
<span class="s2"> (sin (* twopi (~ 600))))</span>
<span class="s2"> 0.25</span>
<span class="s2">"""</span>
<span class="c1"># We dont use any input with this expression (synthesis only),</span>
<span class="c1"># so we pass a signal filled with zeros.</span>
<a class="sphinx-codeautolink-a" href="../../api/classes/controls.html#pyo.Sig" title="pyo.lib._core.Sig"><span class="nb">input</span></a> <span class="o">=</span> <a class="sphinx-codeautolink-a" href="../../api/classes/controls.html#pyo.Sig" title="pyo.lib._core.Sig"><span class="n">Sig</span></a><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="c1"># Create the Expr object and show its expression editor. To</span>
<span class="c1"># re-evaluate the expression hit Ctrl+Enter when the editor</span>
<span class="c1"># has the focus.</span>
<a class="sphinx-codeautolink-a" href="../../api/classes/expression.html#pyo.Expr" title="pyo.lib.expression.Expr"><span class="n">expr</span></a> <span class="o">=</span> <a class="sphinx-codeautolink-a" href="../../api/classes/expression.html#pyo.Expr" title="pyo.lib.expression.Expr"><span class="n">Expr</span></a><span class="p">(</span><a class="sphinx-codeautolink-a" href="../../api/classes/controls.html#pyo.Sig" title="pyo.lib._core.Sig"><span class="nb">input</span></a><span class="p">,</span> <span class="n">expression</span><span class="p">)</span>
<span class="n">expr</span><span class="o">.</span><span class="n">editor</span><span class="p">()</span>
<span class="c1"># Shows the generated signal.</span>
<a class="sphinx-codeautolink-a" href="../../api/classes/analysis.html#pyo.Scope" title="pyo.lib.analysis.Scope"><span class="n">sc</span></a> <span class="o">=</span> <a class="sphinx-codeautolink-a" href="../../api/classes/analysis.html#pyo.Scope" title="pyo.lib.analysis.Scope"><span class="n">Scope</span></a><span class="p">(</span><a class="sphinx-codeautolink-a" href="../../api/classes/expression.html#pyo.Expr" title="pyo.lib.expression.Expr"><span class="n">expr</span></a><span class="p">)</span>
<span class="c1"># Converts the mono signal to stereo and sends it to the soundcard.</span>
<span class="n">pan</span> <span class="o">=</span> <a class="sphinx-codeautolink-a" href="../../api/classes/pan.html#pyo.Pan" title="pyo.lib.pan.Pan"><span class="n">Pan</span></a><span class="p">(</span><a class="sphinx-codeautolink-a" href="../../api/classes/expression.html#pyo.Expr" title="pyo.lib.expression.Expr"><span class="n">expr</span></a><span class="p">)</span><span class="o">.</span><span class="n">out</span><span class="p">()</span>
<span class="n">s</span><span class="o">.</span><span class="n">gui</span><span class="p">(</span><span class="nb">locals</span><span class="p">())</span>
</pre></div>
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