clojure.core.reducers/reduce with core.matrix.operators/+

[cej]

Hello,

  I am trying to figure out how to use clojure.core.reducers/reduce with core.matrix.operators/+.

Here is a little test example that I am thinking about:

(require '[clojure.core.reducers :as r])
(:refer-clojure :exclude [* - + == /])
(use 'clojure.core.matrix)
(use 'clojure.core.matrix.operators)

(def a [[1 2 3 4][1 -2 3 4][1 2 -3 4]])

core.matrix works here as this applies abs onto each element of each of the subvectors

 (->> a
     (r/map abs)
     (into []))

 
 This gives me the result that I want, but it creates a vector of vectors beforehand

(apply + (->> a
     (r/map abs)
     (into [])))

 
Something like this would be better; if it worked.      

(->> a
     (r/map abs)
     (r/reduce  +))

Is there away to do this?

[Mike Anderson]

You should in general use "slices" if you want to get a sequence of the row vectors of a matrix. We can't guarantee that core.matrix arrays are reducible in general - it just happens to work if you use clojure vectors.

So you can do:

(->> (slices a)

        (r/map abs) 

        (r/reduce  +))

But note that you don't really even need reducers for this, since the following also works:

(->> (slices a)

        (map abs) 

        (reduce  +))

Also note that "abs" and "+" are the core.matrix versions here, i.e. they work on whole vectors at a time

[cej]

After playing with this today, I realized I was being stupid, this works:

(->>  a
     (r/map abs)
     (r/reduce  + [0 0 0 0]))