--- title: foo - test layout: api --- <div class='api-docs-breadcrumbs'><a href="test/index">test</a> / <a href="test/foo">foo</a></div> # foo <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" markdown="1"> ``` kotlin fun main(args: Array<String>) { //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<Int, MutableList<String>> = words.groupByTo(mutableMapOf()) { it.length } // same content as in byLength map, but the map is mutable println("mutableByLength == byLength is ${mutableByLength == byLength}") // true //sampleEnd } ``` </div>