EVOLUTION-MANAGER

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

<h2>Select One of a List of Alternatives</h2>

<h3>Description</h3>

<p><code>switch</code> evaluates <code>EXPR</code> and accordingly chooses one of the
further arguments (in <code>...</code>).
</p>

<h3>Usage</h3>

<pre>
switch(EXPR, ...)
</pre>

<h3>Arguments</h3>

<table summary="R argblock">
<tr valign="top"><td><code>EXPR</code></td>
<td>
<p>an expression evaluating to a number or a character
string.</p>
</td></tr>
<tr valign="top"><td><code>...</code></td>
<td>
<p>the list of alternatives.  If it is intended that
<code>EXPR</code> has a character-string value these will be
named, perhaps except for one alternative to be used as a
&lsquo;default&rsquo; value.</p>
</td></tr>
</table>

<h3>Details</h3>

<p><code>switch</code> works in two distinct ways depending whether the first
argument evaluates to a character string or a number.
</p>
<p>If the value of <code>EXPR</code> is not a character string it is coerced to
integer.  Note that this also happens for <code><a href="factor.html">factor</a></code>s, with
a warning, as typically the character level is meant.  If the integer
is between 1 and <code>nargs()-1</code> then the corresponding element of
<code>...</code> is evaluated and the result returned: thus if the first
argument is <code>3</code> then the fourth argument is evaluated and
returned.
</p>
<p>If <code>EXPR</code> evaluates to a character string then that string is
matched (exactly) to the names of the elements in <code>...</code>.  If
there is a match then that element is evaluated unless it is missing,
in which case the next non-missing element is evaluated, so for
example <code>switch("cc", a = 1, cc =, cd =, d = 2)</code> evaluates to
<code>2</code>.  If there is more than one match, the first matching element
is used.  In the case of no match, if there is a unnamed element of
<code>...</code> its value is returned.  (If there is more than one such
argument an error is signaled.)
</p>
<p>The first argument is always taken to be <code>EXPR</code>: if it is named
its name must (partially) match.
</p>
<p>A warning is signaled if no alternatives are provides, as this is
usually a coding error.
</p>
<p>This is implemented as a <a href="Primitive.html">primitive</a> function that only evaluates
its first argument and one other if one is selected.
</p>

<h3>Value</h3>

<p>The value of one of the elements of <code>...</code>, or <code>NULL</code>,
invisibly (whenever no element is selected).
</p>
<p>The result has the visibility (see <code><a href="invisible.html">invisible</a></code>) of the
element evaluated.
</p>

<h3>Warning</h3>

<p>It is possible to write calls to <code>switch</code> that can be confusing
and may not work in the same way in earlier versions of <span style="font-family: Courier New, Courier; color: #666666;"><b>R</b></span>.  For
compatibility (and clarity), always have <code>EXPR</code> as the first
argument, naming it if partial matching is a possibility.  For the
character-string form, have a single unnamed argument as the default
after the named values.
</p>

<h3>References</h3>

<p>Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
<em>The New S Language</em>.
Wadsworth &amp; Brooks/Cole.
</p>

<h3>Examples</h3>

<pre>
require(stats)
centre &lt;- function(x, type) {
  switch(type,
         mean = mean(x),
         median = median(x),
         trimmed = mean(x, trim = .1))
}
x &lt;- rcauchy(10)
centre(x, "mean")
centre(x, "median")
centre(x, "trimmed")

ccc &lt;- c("b","QQ","a","A","bb")
# note: cat() produces no output for NULL
for(ch in ccc)
    cat(ch,":", switch(EXPR = ch, a = 1, b = 2:3), "\n")
for(ch in ccc)
    cat(ch,":", switch(EXPR = ch, a =, A = 1, b = 2:3, "Otherwise: last"),"\n")

## switch(f, *) with a factor f
ff &lt;- gl(3,1, labels=LETTERS[3:1])
ff[1] # C
## so one might expect " is C" here, but
switch(ff[1], A = "I am A", B="Bb..", C=" is C")# -&gt; "I am A"
## so we give a warning

## Numeric EXPR does not allow a default value to be specified
## -- it is always NULL
for(i in c(-1:3, 9))  print(switch(i, 1, 2 , 3, 4))

## visibility
switch(1, invisible(pi), pi)
switch(2, invisible(pi), pi)
</pre>

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