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POHow to use Java Collections.shuffle() on a Scala array?
 

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  1. POHow to use Java Collections.shuffle() on a Scala array?
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    <p>I have an array that I want to permutate randomly. In Java, there is a method Collections.shuffle() that can shuffle the elements of a List randomly. It can be used on an array too:</p> <pre><code>String[] array = new String[]{"a", "b", "c"}; // Shuffle the array; works because the list returned by Arrays.asList() is backed by the array Collections.shuffle(Arrays.asList(array)); </code></pre> <p>I tried using this on a Scala array, but the Scala interpreter responds with a lengthy answer:</p> <pre><code>scala&gt; val a = Array("a", "b", "c") a: Array[java.lang.String] = Array(a, b, c) scala&gt; java.util.Collections.shuffle(java.util.Arrays.asList(a)) &lt;console&gt;:6: warning: I'm seeing an array passed into a Java vararg. I assume that the elements of this array should be passed as individual arguments to the vararg. Therefore I follow the array with a `: _*', to mark it as a vararg argument. If that's not what you want, compile this file with option -Xno-varargs-conversion. java.util.Collections.shuffle(java.util.Arrays.asList(a)) ^ &lt;console&gt;:6: error: type mismatch; found : Array[java.lang.String] required: Seq[Array[java.lang.String]] java.util.Collections.shuffle(java.util.Arrays.asList(a)) ^ </code></pre> <p>What exactly is happening here? I don't want to compile my code with a special flag (-Xno-varargs-conversion), if that is the solution at all, just because of this.</p> <p>So, how do I use Java's Collections.shuffle() on a Scala array?</p> <p>I wrote my own shuffle method in Scala in the meantime:</p> <pre><code>// Fisher-Yates shuffle, see: http://en.wikipedia.org/wiki/Fisher–Yates_shuffle def shuffle[T](array: Array[T]): Array[T] = { val rnd = new java.util.Random for (n &lt;- Iterator.range(array.length - 1, 0, -1)) { val k = rnd.nextInt(n + 1) val t = array(k); array(k) = array(n); array(n) = t } return array } </code></pre> <p>It shuffles the array in place, and returns the array itself for convenience.</p>
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    1. CONote that shuffling in this way only works for arrays of reference types.
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    2. CO@starblue: Yes, the Java version works only for arrays of reference types. My own Scala shuffle() method also works with Scala arrays with primitives.
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