Does Clojure map include level specification?

Question

In trying to write a mapping function that applies a function, i.e inc, dec, etc. to the elements of an input vector. The output is a vector with the function applied to each element and an indexing element.

Here is a sample of what I'm trying to input: Input 1: [+ [[1 2] [3 4 5]] 2]

Output1: [[2 2] [3 4]] [[5 4] [6 6] [7 8]]]

Input 2: [+ [1 2 3 4 5]]

Output 2: [[2] [4] [6] [8] [10]]

Symbolically: Input 2: [+ [a b c d e]]

Output 2: [[1+a] [2+b] [3+c] [4+d] [5+e]]

Input 3: [Plus, [[[[[1]]]]]]

Output 3: [[[[[[1+1]]]]] (would output 2 but I wrote out the operation)

Input 4: [Plus [[[[[1]]]]] 2]\

Output 4: [[1+[[[1]]]+[1 1]]]

Answer 1

There's clojure.core/map-indexed; it's similar but not exactly what you're looking for.

E.g.

(map-indexed vector '[a b c]) ;=> '([0 a] [1 b] [2 c])

This is awfully close to what I think you're going for:

(defn map-depth
  "Maps each element of s with an array of indices, as used by e.g. update-in,
   optionally at the given depth"
  ([f s depth path]
   (if (or (and depth (zero? depth))
           (not (sequential? s)))
     (f s path)
     (map-indexed (fn [i e]
                    (map-depth f e
                               (when depth (dec depth))
                               (conj path i)))
                  s)))
  ([f s depth]
   (map-depth f s depth []))
  ([f s]
   (map-depth f s nil [])))

E.g.

(map-depth vector '[a [b c] d])  ;=> '([a [0]] ([b [1 0]] [c [1 1]]) [d [2]])
(map-depth vector '[a b c] 0)             ;=> '[[a b c] []]
(map-depth vector '[[a b] [a b] [a b]] 1) ;=> '([[a b] [0]] [[a b] [1]] [[a b] [2]])

---

Are you coming from a Mathematica background, though?

It's important to remember also that Clojure's + operator doesn't play nicely with lists.

In Mathematica you can do

{1, 2, 3, 4} + 2 (* ->  {3 4 5 6} *)

But Clojure will complain; you have to work around it.

(+ [1 2 3 4] 2) ;=> ClassCastException
(map (partial + 2) [1 2 3 4]) ;=> (3 4 5 6)