EVOLUTION-MANAGER

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<table width="100%" summary="page for when {purrr}"><tr><td>when {purrr}</td><td style="text-align: right;">R Documentation</td></tr></table>

<h2>Match/validate a set of conditions for an object and continue with the action
associated with the first valid match.</h2>

<h3>Description</h3>

<p><code>when</code> is a flavour of pattern matching (or an if-else abstraction) in
which a value is matched against a sequence of condition-action sets. When a
valid match/condition is found the action is executed and the result of the
action is returned.
</p>

<h3>Usage</h3>

<h3>Arguments</h3>

<table summary="R argblock">
<tr valign="top"><td><code>.</code></td>
<td>
<p>the value to match against</p>
</td></tr>
<tr valign="top"><td><code>...</code></td>
<td>
<p>formulas; each containing a condition as LHS and an action as RHS.
named arguments will define additional values.</p>
</td></tr>
</table>

<h3>Value</h3>

<p>The value resulting from the action of the first valid
match/condition is returned. If no matches are found, and no default is
given, NULL will be returned.
</p>
<p>Validity of the conditions are tested with <code>isTRUE</code>, or equivalently
with <code>identical(condition, TRUE)</code>.
In other words conditions resulting in more than one logical will never
be valid. Note that the input value is always treated as a single object,
as opposed to the <code>ifelse</code> function.
</p>

<h3>Examples</h3>

<pre>
1:10 %&gt;%
  when(
    sum(.) &lt;=  50 ~ sum(.),
    sum(.) &lt;= 100 ~ sum(.)/2,
    ~ 0
  )

1:10 %&gt;%
  when(
    sum(.) &lt;=   x ~ sum(.),
    sum(.) &lt;= 2*x ~ sum(.)/2,
    ~ 0,
    x = 60
  )

iris %&gt;%
  subset(Sepal.Length &gt; 10) %&gt;%
  when(
    nrow(.) &gt; 0 ~ .,
    ~ iris %&gt;% head(10)
  )

iris %&gt;%
  head %&gt;%
  when(nrow(.) &lt; 10 ~ .,
       ~ stop("Expected fewer than 10 rows."))
</pre>

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