What's the best way to handle this sequence transformation in Clojure?

manpreet Best Answer 2 years ago

I'm new to Clojure, and I've been translating some data manipulation work I did recently as an aid to learning. I've got a function translation that works fine, and is shorter, but feels much less readable. Can anyone suggest a more readable and/or more idiomatic way to handle this?

In Python:

def createDifferenceVector(v,startWithZero=True):
   deltas = []
   for i in range(len(v)):
       if i == 0:
           if startWithZero:
               deltas.append(0.0)
           else:
               deltas.append(v[0])
       else:
           deltas.append(v[i] - v[i-1])
   return deltas

My attempt at a Clojure translation:

(defn create-diff-vector [v start-zero]
  (let [ext-v (if start-zero
                (cons (first v) v)
                (cons 0 v))]
    (for [i (range 1 (count ext-v))] 
      (- (nth ext-v i) (nth ext-v (- i 1))))))

It could be that it's less readable just because of my inexperience with Clojure, but in particular, the trick of prepending an element to the input vector feels to me like it obscures the intention. All the solutions I tried which didn't use the prepending trick were much longer and uglier.

Many sequence transformations are incredibly elegant in Clojure, but the ones that I find challenging so far are ones like this one, which a) lend themselves to manipulation by index rather than by element, and/or b) require special handling for certain elements.

Thanks for any suggestions.

manpreet 2 years ago

  input without the first element:   (41 59  26 53)
- input without the last element:    (31 41  59 26)
===================================================
  result:                            (10 18 -33 27)

(defn diff-vector [v start-zero?]
  (into [(if start-zero? 0 (first v))]
    (map - (rest v) v))))