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with a calculation or action. If any of the given values is not truthy, the
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<span class="version label label-default" data-toggle="tooltip" data-placement="bottom" title="">1.7.2</span>
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<h1>Check for required values</h1>
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<small class="dont-index">Source: <a href="https://github.com/rstudio/shiny/blob/HEAD/R/utils.R" class="external-link"><code>R/utils.R</code></a></small>
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<div class="hidden name"><code>req.Rd</code></div>
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</div>
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<div class="ref-description">
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<p>Ensure that values are available (<a href="isTruthy.html">"truthy"</a>) before proceeding
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with a calculation or action. If any of the given values is not truthy, the
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operation is stopped by raising a "silent" exception (not logged by Shiny,
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nor displayed in the Shiny app's UI).</p>
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</div>
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<div id="ref-usage">
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<div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">req</span><span class="op">(</span><span class="va">...</span>, cancelOutput <span class="op">=</span> <span class="cn">FALSE</span><span class="op">)</span></span></code></pre></div>
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</div>
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<div id="arguments">
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<h2>Arguments</h2>
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<dl><dt>...</dt>
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<dd><p>Values to check for truthiness.</p></dd>
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<dt>cancelOutput</dt>
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<dd><p>If <code>TRUE</code> and an output is being evaluated, stop
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processing as usual but instead of clearing the output, leave it in
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whatever state it happens to be in.</p></dd>
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</dl></div>
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<div id="value">
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<h2>Value</h2>
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<p>The first value that was passed in.</p>
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</div>
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<div id="details">
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<h2>Details</h2>
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<p>The <code>req</code> function was designed to be used in one of two ways. The first
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is to call it like a statement (ignoring its return value) before attempting
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operations using the required values:</p>
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<p></p><div class="sourceCode"><pre><code><span id="cb1-1"><a href="#cb1-1"></a>rv <-<span class="st"> </span><span class="kw">reactiveValues</span>(<span class="dt">state =</span> <span class="ot">FALSE</span>)</span>
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<span id="cb1-2"><a href="#cb1-2"></a>r <-<span class="st"> </span><span class="kw">reactive</span>({</span>
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<span id="cb1-3"><a href="#cb1-3"></a> <span class="kw">req</span>(input<span class="op">$</span>a, input<span class="op">$</span>b, rv<span class="op">$</span>state)</span>
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<span id="cb1-4"><a href="#cb1-4"></a> <span class="co"># Code that uses input$a, input$b, and/or rv$state...</span></span>
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<span id="cb1-5"><a href="#cb1-5"></a>})</span></code></pre><p></p></div>
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<p>In this example, if <code>r()</code> is called and any of <code>input$a</code>,
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<code>input$b</code>, and <code>rv$state</code> are <code>NULL</code>, <code>FALSE</code>, <code>""</code>,
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etc., then the <code>req</code> call will trigger an error that propagates all the
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way up to whatever render block or observer is executing.</p>
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<p>The second is to use it to wrap an expression that must be truthy:</p>
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<p></p><div class="sourceCode"><pre><code><span id="cb1-1"><a href="#cb1-1"></a>output<span class="op">$</span>plot <-<span class="st"> </span><span class="kw">renderPlot</span>({</span>
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<span id="cb1-2"><a href="#cb1-2"></a> <span class="cf">if</span> (<span class="kw">req</span>(input<span class="op">$</span>plotType) <span class="op">==</span><span class="st"> "histogram"</span>) {</span>
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<span id="cb1-3"><a href="#cb1-3"></a> <span class="kw">hist</span>(<span class="kw">dataset</span>())</span>
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<span id="cb1-4"><a href="#cb1-4"></a> } <span class="cf">else</span> <span class="cf">if</span> (input<span class="op">$</span>plotType <span class="op">==</span><span class="st"> "scatter"</span>) {</span>
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<span id="cb1-5"><a href="#cb1-5"></a> <span class="kw">qplot</span>(<span class="kw">dataset</span>(), <span class="kw">aes</span>(<span class="dt">x =</span> x, <span class="dt">y =</span> y))</span>
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<span id="cb1-6"><a href="#cb1-6"></a> }</span>
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<span id="cb1-7"><a href="#cb1-7"></a>})</span></code></pre><p></p></div>
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<p>In this example, <code>req(input$plotType)</code> first checks that
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<code>input$plotType</code> is truthy, and if so, returns it. This is a convenient
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way to check for a value "inline" with its first use.</p>
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</div>
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<div id="using-req-false-">
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<h2>Using <code>req(FALSE)</code></h2>
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<p>You can use <code>req(FALSE)</code> (i.e. no condition) if you've already performed
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all the checks you needed to by that point and just want to stop the reactive
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chain now. There is no advantange to this, except perhaps ease of readibility
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if you have a complicated condition to check for (or perhaps if you'd like to
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divide your condition into nested <code>if</code> statements).</p>
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</div>
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<div id="using-canceloutput-true">
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<h2>Using <code>cancelOutput = TRUE</code></h2>
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<p>When <code>req(..., cancelOutput = TRUE)</code> is used, the "silent" exception is
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also raised, but it is treated slightly differently if one or more outputs are
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currently being evaluated. In those cases, the reactive chain does not proceed
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or update, but the output(s) are left is whatever state they happen to be in
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(whatever was their last valid state).</p>
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<p>Note that this is always going to be the case if
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this is used inside an output context (e.g. <code>output$txt <- ...</code>). It may
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or may not be the case if it is used inside a non-output context (e.g.
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<code><a href="reactive.html">reactive()</a></code>, <code><a href="observe.html">observe()</a></code> or <code><a href="observeEvent.html">observeEvent()</a></code>)
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--- depending on whether or not there is an <code>output$...</code> that is triggered
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as a result of those calls. See the examples below for concrete scenarios.</p>
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</div>
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<div id="ref-examples">
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<h2>Examples</h2>
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<div class="sourceCode"><pre class="sourceCode r"><code><span class="r-in"><span><span class="co">## Only run examples in interactive R sessions</span></span></span>
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<span class="r-in"><span><span class="kw">if</span> <span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/interactive.html" class="external-link">interactive</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span> <span class="op">{</span></span></span>
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<span class="r-in"><span> <span class="va">ui</span> <span class="op"><-</span> <span class="fu"><a href="fluidPage.html">fluidPage</a></span><span class="op">(</span></span></span>
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<span class="r-in"><span> <span class="fu"><a href="textInput.html">textInput</a></span><span class="op">(</span><span class="st">'data'</span>, <span class="st">'Enter a dataset from the "datasets" package'</span>, <span class="st">'cars'</span><span class="op">)</span>,</span></span>
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<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/pkg/htmltools/man/builder.html" class="external-link">p</a></span><span class="op">(</span><span class="st">'(E.g. "cars", "mtcars", "pressure", "faithful")'</span><span class="op">)</span>, <span class="fu"><a href="https://rdrr.io/pkg/htmltools/man/builder.html" class="external-link">hr</a></span><span class="op">(</span><span class="op">)</span>,</span></span>
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<span class="r-in"><span> <span class="fu"><a href="renderTable.html">tableOutput</a></span><span class="op">(</span><span class="st">'tbl'</span><span class="op">)</span></span></span>
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<span class="r-in"><span> <span class="op">)</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span> <span class="va">server</span> <span class="op"><-</span> <span class="kw">function</span><span class="op">(</span><span class="va">input</span>, <span class="va">output</span><span class="op">)</span> <span class="op">{</span></span></span>
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<span class="r-in"><span> <span class="va">output</span><span class="op">$</span><span class="va">tbl</span> <span class="op"><-</span> <span class="fu"><a href="renderTable.html">renderTable</a></span><span class="op">(</span><span class="op">{</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span> <span class="co">## to require that the user types something, use: `req(input$data)`</span></span></span>
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<span class="r-in"><span> <span class="co">## but better: require that input$data is valid and leave the last</span></span></span>
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<span class="r-in"><span> <span class="co">## valid table up</span></span></span>
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<span class="r-in"><span> <span class="fu">req</span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/exists.html" class="external-link">exists</a></span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">data</span>, <span class="st">"package:datasets"</span>, inherits <span class="op">=</span> <span class="cn">FALSE</span><span class="op">)</span>,</span></span>
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<span class="r-in"><span> cancelOutput <span class="op">=</span> <span class="cn">TRUE</span><span class="op">)</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span> <span class="fu"><a href="https://rdrr.io/r/utils/head.html" class="external-link">head</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/get.html" class="external-link">get</a></span><span class="op">(</span><span class="va">input</span><span class="op">$</span><span class="va">data</span>, <span class="st">"package:datasets"</span>, inherits <span class="op">=</span> <span class="cn">FALSE</span><span class="op">)</span><span class="op">)</span></span></span>
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<span class="r-in"><span> <span class="op">}</span><span class="op">)</span></span></span>
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<span class="r-in"><span> <span class="op">}</span></span></span>
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<span class="r-in"><span></span></span>
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<span class="r-in"><span> <span class="fu"><a href="shinyApp.html">shinyApp</a></span><span class="op">(</span><span class="va">ui</span>, <span class="va">server</span><span class="op">)</span></span></span>
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<span class="r-in"><span><span class="op">}</span></span></span>
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</code></pre></div>
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