[plt-scheme] code speed

From: Robby Findler (robby.findler at gmail.com)
Date: Thu Jan 25 13:10:40 EST 2007

Yes, its likely to be the extra allocation.

If you can figure out when it is safe to kill the old vector (and
that's fairly often) you'll probably see a speedup there too.


On 1/25/07, Joshua Zucker <joshua.zucker at gmail.com> wrote:
> In an inner loop of something I'm working on, I had initially written
> the following code:
>     (build-vector (vector-length stack)
>                   (lambda (x) (cond [(<= x n) (vector-ref stack (- n x))]
>                                     [else (vector-ref stack x)]))))
> to reverse the first n items of a vector.
> I can make it slightly faster (I think) if I do it with mutation, but
> I have to be careful because sometimes I really want a copy of the
> vector, not a mutation ...
> But then I saw that there were built-in functions for vector copying
> and so on.  So I tried
> (vector-append (vector-reverse-copy stack 0 (add1 n)) (vector-copy
> stack (add1 n))))
> and was very surprised to find that this was MUCH slower.
> Maybe it's the allocation/deallocation of memory for the two vectors
> that get appended together to make the final one?  Or too much
> redundant copying going on?
> Any suggestions are much appreciated.
> --Joshua Zucker
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