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<h1 class="title"><span id="sec-usr" class="quarto-section-identifier"><span class="chapter-number">9</span> <span class="chapter-title">Handling Unknown Variables</span></span></h1>
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<p>Although it is possible to add <a href="custom-extensions.html#custom-variables">variables</a> to the parser, there may be times when you can’t anticipate the exact variable names that a user will enter. For example, a user could enter variables representing fiscal years in the format of <code>FY[year]</code>. From there, they would like to perform operation such as getting the range between them. In this situation, their expression may be something like this:</p>
<pre><code>ABS(FY1999 - FY2009)</code></pre>
<p>Here, one could expect the variables <code>FY1999</code> and <code>FY2009</code> to be treated as <code>1999</code> and <code>2009</code>, respectively. By subtracting them (and then taking the absolute value of the result), this should yield <code>10</code>. The problem is that you would need to set up custom variables prior for <code>FY1999</code> and <code>FY2009</code>. Even worse, to handle any additional years the user may enter, you would need to create custom variables for every year possible.</p>
<p>Rather than doing that, you can allow the parser to not recognize these variables as usual. At this point, it will fall back to a user-defined function that you provide to resolve it. This is called an “unknown symbol resolver” (USR), and this function will:</p>
<ul>
<li>Receive a <code>std::string_view</code> of the symbol (i.e., variable name) that the parser failed to recognize</li>
<li>Determine how to resolve the name
<ul>
<li>If it can resolve it, return a numeric value for the symbol</li>
<li>Otherwise, either return <code>te_parser::te_nan</code> (i.e., NaN) or throw an exception</li>
</ul></li>
</ul>
<p>When your function resolves a symbol, then its name and numeric value will be added to the parser. Any future evaluations will recognize this name and return the value you previously resolved it to.</p>
<div class="tipsection">
<p>To change the value for a variable that was resolved previously, use <code>te_parser::set_constant()</code>.</p>
</div>
<p>This function is set up in the parser by passing it to <code>te_parser::set_unknown_symbol_resolver()</code> and can take one of the following signatures:</p>
<div class="minipage">
<pre><code> te_type callback(std::string_view);
te_type callback(std::string_view, std::string&);</code></pre>
</div>
<p>The first version will accept the unknown symbol and either return a resolved value or <code>te_parser::te_nan</code>. The second version is the same, except that it also accepts a string reference to write a custom message to. (This message can later be retrieved by calling <code>te_parser::get_last_error_message()</code>.)</p>
<div class="notesection">
<p>If your USR throws a <code>std::runtime_exception</code> with an error message in it, then that message will also be available through <code>te_parser::get_last_error_message()</code>.</p>
</div>
<div class="minipage">
<p><code>te_parser::set_unknown_symbol_resolver()</code> can accept either a function pointer or a lambda. Here is a simple example using a function:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp code-with-copy"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="dt">te_type</span> ResolveResolutionSymbols<span class="op">(</span><span class="bu">std::</span>string_view<span class="op"> </span>str<span class="op">)</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a> <span class="co">// Note that this is case sensitive for brevity.</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> <span class="op">(</span>str <span class="op">==</span> <span class="st">"RES"</span> <span class="op">||</span> str <span class="op">==</span> <span class="st">"RESOLUTION"</span><span class="op">)</span> <span class="op">?</span></span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a> <span class="dv">96</span> <span class="op">:</span> te_parser<span class="op">::</span>te_nan<span class="op">;</span></span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
<div class="minipage">
<p>This can be connected as such:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp code-with-copy"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>te_parser tep<span class="op">;</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>set_unknown_symbol_resolver<span class="op">(</span>ResolveResolutionSymbols<span class="op">);</span></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="co">// Will resolve to 288, and "RESOLUTION" will be added as a</span></span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="co">// variable to the parser with a value of 96.</span></span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a><span class="co">// Also, beccause TinyExpr++ is case insensitive,</span></span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="co">// "resolution" will also be seen as 96 once "RESOLUTION"</span></span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a><span class="co">// was resolved.</span></span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>evaluate<span class="op">(</span><span class="st">"RESOLUTION * 3"</span><span class="op">);</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
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<div class="warningsection">
<p>Although <em>TinyExpr++</em> is case insensitive, it is your USR’s responsibility to process case-insensitively when resolving names. Once a name has been resolved, then the parser will recognize it case-insensitively in future evaluations.</p>
</div>
<div class="minipage">
<p>The following is an example using a lambda and demonstrates the fiscal-year-variables scenario mentioned earlier:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp code-with-copy"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>te_parser tep<span class="op">;</span></span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="co">// Create a handler for undefined tokens that will recognize</span></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a><span class="co">// dynamic strings like "FY2004" or "FY1997" and convert them to 2004 and 1997.</span></span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>set_unknown_symbol_resolver<span class="op">(</span></span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a> <span class="co">// Handler should except a string (which will be the unrecognized token)</span></span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a> <span class="co">// and return a te_type.</span></span>
<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a> <span class="op">[](</span><span class="bu">std::</span>string_view<span class="op"> </span>str<span class="op">)</span> <span class="op">-></span> <span class="dt">te_type</span></span>
<span id="cb5-9"><a href="#cb5-9" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span></span>
<span id="cb5-10"><a href="#cb5-10" aria-hidden="true" tabindex="-1"></a> <span class="at">const</span> <span class="bu">std::</span>regex<span class="op"> </span>re<span class="op">{</span> <span class="st">"FY([0-9]</span><span class="sc">{4}</span><span class="st">)"</span><span class="op">,</span></span>
<span id="cb5-11"><a href="#cb5-11" aria-hidden="true" tabindex="-1"></a> <span class="bu">std::</span>regex_constants<span class="bu">::</span>icase<span class="op"> |</span> <span class="bu">std::</span>regex_constants<span class="bu">::</span>ECMAScript<span class="op"> };</span></span>
<span id="cb5-12"><a href="#cb5-12" aria-hidden="true" tabindex="-1"></a> <span class="bu">std::</span>smatch<span class="op"> </span>matches<span class="op">;</span></span>
<span id="cb5-13"><a href="#cb5-13" aria-hidden="true" tabindex="-1"></a> <span class="bu">std::</span>string<span class="op"> </span>var<span class="op">{</span> str <span class="op">};</span></span>
<span id="cb5-14"><a href="#cb5-14" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span><span class="bu">std::</span>regex_search<span class="op">(</span>var<span class="op">.</span>cbegin<span class="op">(),</span> var<span class="op">.</span>cend<span class="op">(),</span> matches<span class="op">,</span> re<span class="op">))</span></span>
<span id="cb5-15"><a href="#cb5-15" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span></span>
<span id="cb5-16"><a href="#cb5-16" aria-hidden="true" tabindex="-1"></a> <span class="co">// Unrecognized token is something like "FY1982," so extract "1982"</span></span>
<span id="cb5-17"><a href="#cb5-17" aria-hidden="true" tabindex="-1"></a> <span class="co">// from that and return 1982 as a number. At this point, the variable</span></span>
<span id="cb5-18"><a href="#cb5-18" aria-hidden="true" tabindex="-1"></a> <span class="co">// "FY1982" will be added to the parser and set to 1982. All future</span></span>
<span id="cb5-19"><a href="#cb5-19" aria-hidden="true" tabindex="-1"></a> <span class="co">// evaluations will see this as 1982 (unless set_constant() is called</span></span>
<span id="cb5-20"><a href="#cb5-20" aria-hidden="true" tabindex="-1"></a> <span class="co">// to change it).</span></span>
<span id="cb5-21"><a href="#cb5-21" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span>matches<span class="op">.</span>size<span class="op">()</span> <span class="op">></span> <span class="dv">1</span><span class="op">)</span></span>
<span id="cb5-22"><a href="#cb5-22" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span> <span class="cf">return</span> <span class="bu">std::</span>atol<span class="op">(</span>matches<span class="op">[</span><span class="dv">1</span><span class="op">].</span>str<span class="op">().</span>c_str<span class="op">());</span> <span class="op">}</span></span>
<span id="cb5-23"><a href="#cb5-23" aria-hidden="true" tabindex="-1"></a> <span class="cf">else</span></span>
<span id="cb5-24"><a href="#cb5-24" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span> <span class="cf">return</span> te_parser<span class="op">::</span>te_nan<span class="op">;</span> <span class="op">}</span></span>
<span id="cb5-25"><a href="#cb5-25" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span>
<span id="cb5-26"><a href="#cb5-26" aria-hidden="true" tabindex="-1"></a> <span class="co">// Can't resolve what this token is, so return NaN.</span></span>
<span id="cb5-27"><a href="#cb5-27" aria-hidden="true" tabindex="-1"></a> <span class="cf">else</span></span>
<span id="cb5-28"><a href="#cb5-28" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span> <span class="cf">return</span> te_parser<span class="op">::</span>te_nan<span class="op">;</span> <span class="op">}</span></span>
<span id="cb5-29"><a href="#cb5-29" aria-hidden="true" tabindex="-1"></a> <span class="op">});</span></span>
<span id="cb5-30"><a href="#cb5-30" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-31"><a href="#cb5-31" aria-hidden="true" tabindex="-1"></a><span class="co">// Calculate the range between to fiscal years (will be 10): </span></span>
<span id="cb5-32"><a href="#cb5-32" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>evaluate<span class="op">(</span><span class="st">"ABS(FY1999-FY2009)"</span><span class="op">)</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
<p>By default, the parser’s USR is a no-op and will not process anything. If you had provided a USR but then need to turn off this feature, then pass a no-op lambda (e.g., <code>[]{}</code>) or no-op object (<code>te_usr_noop{}</code>) to <code>set_unknown_symbol_resolver()</code>.</p>
<p>Also, once a variable has been resolved by your USR, it will be added as a custom variable with the resolved value assigned to it. With subsequent evaluations, the previously unrecognized symbols you resolved will be remembered. This means that it will not to be resolved again and will result in the value that your USR returned from before.</p>
<p>If you prefer to not have resolved symbols added to the parser, then pass <code>false</code> to the second parameter to <code>set_unknown_symbol_resolver()</code>. This will force the same unknown symbols to be resolved again with every call to <code>compile(expression)</code> or <code>evaluate(expression)</code>. (The version of <code>evaluate()</code> which does not take any parameters will not force calling the USR again, see below.) This can be useful for when your USR’s symbol resolutions are dynamic and may change with each call.</p>
<div class="minipage">
<p>For example, say that an end user will enter variables that start with “STRESS,” but you are uncertain what the full name will be. Additionally, you want to increase the values of these variables with every evaluation. The following demonstrates this:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp code-with-copy"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>te_parser tep<span class="op">;</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>set_unknown_symbol_resolver<span class="op">(</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a> <span class="op">[](</span><span class="bu">std::</span>string_view<span class="op"> </span>str<span class="op">)</span> <span class="op">-></span> <span class="dt">te_type</span></span>
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span></span>
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a> <span class="at">static</span> <span class="dt">te_type</span> stressLevel<span class="op">{</span> <span class="dv">3</span> <span class="op">};</span></span>
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span><span class="bu">std::</span>strncmp<span class="op">(</span>str<span class="op">.</span>data<span class="op">(),</span> <span class="st">"STRESS"</span><span class="op">,</span> <span class="dv">6</span><span class="op">)</span> <span class="op">==</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb6-8"><a href="#cb6-8" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span> <span class="cf">return</span> stressLevel<span class="op">++;</span> <span class="op">}</span></span>
<span id="cb6-9"><a href="#cb6-9" aria-hidden="true" tabindex="-1"></a> <span class="cf">else</span></span>
<span id="cb6-10"><a href="#cb6-10" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span> <span class="cf">return</span> te_parser<span class="op">::</span>te_nan<span class="op">;</span> <span class="op">}</span></span>
<span id="cb6-11"><a href="#cb6-11" aria-hidden="true" tabindex="-1"></a> <span class="op">},</span></span>
<span id="cb6-12"><a href="#cb6-12" aria-hidden="true" tabindex="-1"></a> <span class="co">// purge resolved varialbes after each evaluation</span></span>
<span id="cb6-13"><a href="#cb6-13" aria-hidden="true" tabindex="-1"></a> <span class="kw">false</span><span class="op">);</span></span>
<span id="cb6-14"><a href="#cb6-14" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-15"><a href="#cb6-15" aria-hidden="true" tabindex="-1"></a><span class="co">// Initial resolution of STRESS_LEVEL will be 3.</span></span>
<span id="cb6-16"><a href="#cb6-16" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>evaluate<span class="op">(</span><span class="st">"STRESS_LEVEL"</span><span class="op">);</span></span>
<span id="cb6-17"><a href="#cb6-17" aria-hidden="true" tabindex="-1"></a><span class="co">// Because STRESS_LEVEL wasn't kept as a variable (with a value of 3)</span></span>
<span id="cb6-18"><a href="#cb6-18" aria-hidden="true" tabindex="-1"></a><span class="co">// in the parser, then subsequent evaluations will require</span></span>
<span id="cb6-19"><a href="#cb6-19" aria-hidden="true" tabindex="-1"></a><span class="co">// resolving it again:</span></span>
<span id="cb6-20"><a href="#cb6-20" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>evaluate<span class="op">(</span><span class="st">"STRESS_LEVEL"</span><span class="op">);</span> <span class="co">// Will be 4.</span></span>
<span id="cb6-21"><a href="#cb6-21" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>evaluate<span class="op">(</span><span class="st">"STRESS_LEVEL"</span><span class="op">);</span> <span class="co">// 5</span></span>
<span id="cb6-22"><a href="#cb6-22" aria-hidden="true" tabindex="-1"></a>tep<span class="op">.</span>evaluate<span class="op">(</span><span class="st">"STRESS_LEVEL"</span><span class="op">);</span> <span class="co">// 6</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
<p>As a final note, if you are not keeping resolved variables, your USR will only be called again if you call <code>evaluate</code> or <code>compile</code> with an expression. Because the original expression gets optimized, <code>evaluate()</code> it will not re-evaluate any variables. Calling <code>evaluate</code> with the original expression will force a re-compilation and in turn call the USR again.</p>
<div id="refs" class="references csl-bib-body hanging-indent" data-line-spacing="2" role="list" style="display: none">
<div id="ref-ExcelBible" class="csl-entry" role="listitem">
Alexander, Michael, and Dick Kusleika. <em>Microsoft Excel 365 Bible</em>. Wiley, 2022.
</div>
<div id="ref-TinyExpr" class="csl-entry" role="listitem">
Winkle, Lewis Van. <em>C Math Evaluation Library: TinyExpr</em>. 2016, <a href="https://codeplea.com/tinyexpr">https://codeplea.com/tinyexpr</a>.
</div>
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