Clojurians Log v2

if you're only flattening 1 level, you can use mapcat:

user=> (flatten (map range (reverse (range 4))))
(0 1 2 0 1 0)
user=> (mapcat range (reverse (range 4)))
(0 1 2 0 1 0)

oh that's great thank you!

To support mapcat, beware of flatten: https://stuartsierra.com/2019/09/25/sequences-in-flatland

Also, your function will return a lazy seq (from map), and not a vector as the name implies.

Thank you!

what is better, apply vector or into []

mapv if it's the final call

In cases like that, if mapv won't do (like if your outer fn is mapcat), wrap it in vec.

Thanks, this is what I have now:

(defn vector-from-symmetric-matrix
  "Given a symmetric matrix, represent it as a vector"
  [X]
  (let [size (count X)
        row (mapcat range (reverse (range (inc size))))
        col (mapcat #(range % size) (range size))]
    (mapv (partial get-in X) (partition 2 (interleave row col)))))

that's great, no need for vec or into [] at that point

Thanks!

suppose I wanted to assert that X was square, should I use assert for that?

I haven't seen assert used much before

I just realized that "vector" is ambiguous in this context, so if "vector" in your fn name was meant in the mathematical sense (as it appears to be), returning a different kind of seq isn't necessarily "incorrect". Working with Clojure vectors tend to be simpler and more common, though.

You can use assert, or add it as a :pre-condition https://clojuredocs.org/clojure.core/defn

Or, for the Ultimate Power, you can do more thorough checks with https://clojure.org/guides/spec or https://github.com/metosin/malli

Now I have this:

(defn vector-from-symmetric-matrix
  "Given a symmetric matrix, represent it as a vector"
  [X]
  {:pre [(apply = (map count X))            ;; Do the rows all have the same length?
         (= (count X) (count (first X)))]}  ;; does the number of rows match the number of columns?
  (let [size (count X)
        row (mapcat range (reverse (range (inc size))))
        col (mapcat #(range % size) (range size))]
    (mapv (partial get-in X) (partition 2 (interleave row col)))))