map-indexed

function

since v0.0-927

clojure.core/map-indexed

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Details:

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See Also:

• map
• keep-indexed

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Source docstring:

Returns a lazy sequence consisting of the result of applying f to 0
and the first item of coll, followed by applying f to 1 and the second
item in coll, etc, until coll is exhausted. Thus function f should
accept 2 arguments, index and item. Returns a stateful transducer when
no collection is provided.

(defn map-indexed
  ([f]
    (fn [rf]
      (let [i (volatile! -1)]
        (fn
          ([] (rf))
          ([result] (rf result))
          ([result input]
            (rf result (f (vswap! i inc) input)))))))
  ([f coll]
    (letfn [(mapi [idx coll]
              (lazy-seq
                (when-let [s (seq coll)]
                  (if (chunked-seq? s)
                    (let [c (chunk-first s)
                          size (count c)
                          b (chunk-buffer size)]
                      (dotimes [i size]
                        (chunk-append b (f (+ idx i) (-nth c i))))
                      (chunk-cons (chunk b) (mapi (+ idx size) (chunk-rest s))))
                    (cons (f idx (first s)) (mapi (inc idx) (rest s)))))))]
      (mapi 0 coll))))

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