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POTransposing arbitrary collection-of-collections in Scala
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  1. POTransposing arbitrary collection-of-collections in Scala
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    copied!<p>I have to often transpose a "rectangular" collection-of-collections in Scala, e.g.: a list of maps, a map of lists, a map of maps, a set of lists, a map of sets etc. Since collections can be uniformly viewed as a mapping from a specific domain to a co-domain (e.g.: a List[A]/Array[A] is a mapping from the Int domain to the A co-domain, Set[A]is a mapping from the A domain to the Boolean co-domain etc.), I'd like to write a clean, generic function to do a transpose operation (e.g.: turn a map of lists to the transposed list of maps). However, I'm having trouble because other than the () operator, Scala doesn't seem to have a unified API to view collections abstractly as mappings ?</p> <p>So I end up writing a separate transpose for each type of collection-of-collections as follows:</p> <pre><code>def transposeMapOfLists[A,B]( mapOfLists: Map[A,List[B]] ) : List[Map[A,B]] = { val k = ( mapOfLists keys ) toList val l = ( k map { mapOfLists(_) } ) transpose; l map { v =&gt; ( k zip v ) toMap } } def transposeListOfMaps[A,B]( listOfMaps: List[Map[A,B]]) : Map[A,List[B]] = { val k = ( listOfMaps(0) keys ) toList val l = ( listOfMaps map { m =&gt; k map { m(_) } } ) transpose; ( k zip l ) toMap } def transposeMapOfMaps[A,B,C]( mapOfMaps: Map[A,Map[B,C]] ) : Map[B,Map[A,C]] = { val k = ( mapOfMaps keys ) toList val listOfMaps = k map { mapOfMaps(_) } val mapOfLists = transposeListOfMaps( listOfMaps ) mapOfLists map { p =&gt; ( p._1, ( k zip p._2 ) toMap ) } } </code></pre> <p>Can someone help me unify these methods into one generic collection-of-collections transpose ? It will also help me (and I am sure others) learn some useful Scala features in the process.</p> <p>ps: I have ignored exception handling and have assumed the input collection-of-collections is rectangular, i.e., all of the inner collections' domain elements constitute the same set.</p>
 

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