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copied!<p>The reason <em>that</em> particular code is slow is because it's working on primitives but it's using generic operations, so the primitives have to be boxed. (This could be improved if <code>List</code> and its ancestors were specialized.) This will probably slow things down by a factor of 5 or so.</p> <p>Also, algorithmically, those operations are somewhat expensive, because you make a whole list, and then make a whole new list throwing a few components of the intermediate list away. If you did it in one swoop, then you'd be better off. You could do something like:</p> <pre><code>list collect (case e if (e*2>10) => e*2) </code></pre> <p>but what if the calculation <code>e*2</code> is really expensive? Then you could</p> <pre><code>(List[Int]() /: list)((ls,e) => { val x = e*2; if (x>10) x :: ls else ls } </code></pre> <p>except that this would appear backwards. (You could reverse it if need be, but that requires creating a new list, which again isn't ideal algorithmically.)</p> <p>Of course, you have the same sort of algorithmic problems in Java if you're using a singly linked list--your new list will end up backwards, or you have to create it twice, first in reverse and then forwards, or you have to build it with (non-tail) recursion (which is easy in Scala, but inadvisable for this sort of thing in either language since you'll exhaust the stack), or you have to create a mutable list and then pretend afterwards that it's not mutable. (Which, incidentally, you can do in Scala also--see <code>mutable.LinkedList</code>.)</p>

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