Clojure Matrix Representation

Question

What is a good representation for matrices in Clojure? I\'m interested in dealing with dense matrices of floating point numbers. The \"list of lists\" representation springs

Answer 1

I'm no expert, but here's my opinion anyway :)

list-of-lists is probably the most natural Clojure idiom for representing matrices. This structure also lends itself nicely to map/reduce kinds of operations. Clojure is also pretty efficient at handling sequences - probably better than most alternatives.

I can't swear to this, but I think I've seen Clojure working 3 or all 4 of my CPUs hard on programs I wrote that were functional in style but didn't make any attempt to be parallel. I suspect the compiler is finding some opportunities for parallel processing on its own.

I think the sequence types created by Clojure will work as Lists in Java, or at least be Iterable. That's probably good enough for what you want, though you may run into problems if you try to treat those structures as modifiable in Java.

Lists are best accessed sequentially. If you're planning to jump around a lot in the matrix, a vector-of-vectors may suit you a little better, performance-wise. I suspect that beats using the nth function.

As a former C programmer, I briefly considered that you could implement your matrix as a one-dimensional structure (i.e. a straight sequence or better a vector), and do your own index computations to find the right element. You could use the partition function to step through it... well, that could be made to work but I suspect there are very good reasons not to.

Answer 2

Rich Hickey’s Clojure is a JVM-based Lisp that represents PersistentVector (not a PersistentList) with a 32-way tree.

If you would like to write your own matrix Type i would use PersistentVector otherwise the best choice is to use Parallel Colt with Incanter.