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<h1>foo</h1>
<a name="$foo()"></a>
<div class="signature"><code><span class="keyword">fun </span><span class="identifier">foo</span><span class="symbol">(</span><span class="symbol">)</span><span class="symbol">: </span><span class="identifier">Unit</span></code></div>
<div class="sample"><pre><code class="lang-kotlin">

fun main(args: Array&lt;String&gt;) {
//sampleStart
val words = listOf("a", "abc", "ab", "def", "abcd")
val byLength = words.groupBy { it.length }

println(byLength.keys) // [1, 3, 2, 4]
println(byLength.values) // [[a], [abc, def], [ab], [abcd]]

val mutableByLength: MutableMap&lt;Int, MutableList&lt;String&gt;&gt; = words.groupByTo(mutableMapOf()) { it.length }
// same content as in byLength map, but the map is mutable
println("mutableByLength == byLength is ${mutableByLength == byLength}") // true
//sampleEnd
}</code></pre></div>