Question

I have a vector of points in 2D space such as:

[[0 0] [1 1] [2 2]]

And need to compute the distance between some point and each point in the vector to determine which point in the vector is closest to the point I’m querying. I need to know both the distance and the closest point, so I want a vector of hashes like this:

[{:point [0 0] :dist 2.7} {:point [1 1] :dist 3.5} {:point [2 2] :dist 4.3}]

How can I transform the former into the latter? I tried something like this:

(defn closest-point [point all-points]
    (map #({:point % :dist (distance point %)}) all-points))

But I get an error:

(user=> ArityException Wrong number of args (0) passed to: PersistentArrayMap  clojure.lang.AFn.throwArity (AFn.java:437)

Answer 1

This is more readable if you use a for comprehension instead of a map. Also, to actually select the nearest point, you can just use min-key:

(defn closest-point [point all-points]
  (apply min-key :dist
         (for [p all-points]
           {:point p :dist (distance point p)})))