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POGeneralising a "next permutation" function
 

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    <p>Below is an implementation of a function that returns the lexographically next permutation. This is useful in one of the Euler problems. </p> <p>It's written to work on Strings (which I needed for that). However, it should work on any indexed sequence of comparable values. I've tried generalising it by changing the two occurrences of String to IndexedSeq[Char], but this gets an error:</p> <pre><code>euler-lib.scala:26: error: type mismatch; found : IndexedSeq[Char] required: String ((n.slice(pivot+1, successor):+ n(pivot)) + n.drop(successor+1)).reverse ^ </code></pre> <p>Why has the type inferencer inferred String there? I don't seem to have done any operation that requires a String?</p> <p>And can I make it more general still by having IndexedSeq["something-comparable"]? I've not been able to make this work.</p> <pre><code> // return the lexographically next permutation to the one passed as a parameter // pseudo-code from an article on StackOverflow def nextPermutation(n:String):String = { // 1. scan the array from right-to-left //1.1. if the current element is less than its right-hand neighbor, // call the current element the pivot, // and stop scanning // (We scan left-to-right and return the last such). val pivot = n.zip(n.tail).lastIndexWhere{ case (first, second) =&gt; first &lt; second } //1.2. if the left end is reached without finding a pivot, // reverse the array and return // (the permutation was the lexicographically last, so its time to start over) if (pivot &lt; 0) return n.reverse //2. scan the array from right-to-left again, // to find the rightmost element larger than the pivot // (call that one the successor) val successor = n.lastIndexWhere{_ &gt; n(pivot)} //3. swap the pivot and the successor, and //4. reverse the portion of the array to the right of where the pivot was found return (n.take(pivot) :+ n(successor)) + ((n.slice(pivot+1, successor):+ n(pivot)) + n.drop(successor+1)).reverse } </code></pre>
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    1. USThe Archetypal Paul
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    1. CONice method, by the way!
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      1. USDaniel C. Sobral
 

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