clojure.core/reductions

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Type: function Added: Clojure 1.2 Examples: 4 Runnable: 2

([f coll] [f init coll])

Returns a lazy seq of the intermediate values of the reduction (as per reduce) of coll by f, starting with init.

Examples

by gstamp on 2010-08-09 ▶

(reductions + [1 1 1 1]) (1 2 3 4) (reductions + [1 2 3]) (1 3 6) ;; This is just like reduce except that the calculation is collected during the reduce. (assert (= (reduce + [1 2 3]) (last (reductions + [1 2 3])))) nil

by Dimagog on 2015-02-04 . May have evaluation errors. ▶

(reductions conj [] '(1 2 3)) ([] [1] [1 2] [1 2 3])

by pavanred on 2015-03-27 ▶

(reductions + [1 2 3 4 5]) (1 3 6 10 15) ;;defining the function to perform the same reductions (reductions (fn [sum num] (+ sum num)) [1 2 3 4 5]) ;;(1 3 6 10 15) ;;reductions using a init value 100 (reductions (fn [sum num] (+ sum num)) 100 [1 2 3 4 5]) ;;(100 101 103 106 110 115) ;;defining a function for the same reductions (defn add [sum num] #_=>(+ sum num)) ;;#'user/add (reductions add [1 2 3 4 5]) ;;(1 3 6 10 15)

by matrix10657 on 2017-03-06 . May have evaluation errors. ▶

;; useful for performing lazy calculations which rely on ;; previous calculations ;; e.g. Taking an infinite list of posts with some height and ;; adding an offset to each, which is the sum of all previous ;; heights (def posts (repeat {:height 50})) #'user/posts (take 3 posts) ({:height 50} {:height 50} {:height 50}) (def posts-with-offsets #_=> (map #(assoc %1 :offset %2) #_=> posts #_=> (reductions + 0 (map :height posts)))) #'user/posts-with-offsets (take 3 posts-with-offsets) ({:height 50, :offset 0} {:height 50, :offset 50} {:height 50, :offset 100})

Note by betegelse

I think the description above would be clearer if it stated that the result of reductions is equivalent to the sequence of results of applying reduce to (take n coll), with n starting with 1, that is, equivalent to

(defn my-reduce [op coll] (for [n (range (count coll))] (reduce op (take (inc n) coll)))

Note by jayzawrotny

In other libraries and languages this function may be referred to as "scan".

See also

• clojure.core/reduce (22)
• clojure.core/reduced (3)

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