416 PurelyFunctional.tv Newsletter

00 Index filter.md

Index filter

Clojure already has a function called filter. You pass it a predicate and it calls the predicate on each element. But what it doesn't have is a function called filter-index that calls a predicate on the indexes of the sequence to decide what to keep.

Examples

(filter-index even? "abcdefg") ;=> (\a \c \e \g) (keep even-indexed elements)
(filter-index prime? "abcdefghijk") ;=> (\c \d \f \h) (keep prime-number indexes)

Thanks to this site for the challenge idea where it is considered Hard in Python. The problem has been modified from the original.

Please submit your solutions as comments on this gist.

(defn index-tuples [coll]
  (loop [c 0
         rst coll
         acc []]
    (if (empty? rst)
      acc
      (recur (inc c)
             (drop 1 rst)
             (conj acc [c
                        (first rst)])))))

(defn filter-index [f coll]
  (->> coll
       index-tuples
       (filter (fn [[i _]] (f i)))
       (map second)))

let filterIndex pred seq = // (int -> bool) -> seq<'a> -> seq<'a>
    seq // seq<'a>
    |> Seq.zip [ 1 .. Seq.length seq ] // seq<int * 'a>
    |> Seq.where (fst >> pred) // seq<int * 'a>
    |> Seq.map snd // seq<'a>