reduce & folds in Clojure

Bakery loaf orders folded with reduce, including a fold into a map.

;; reduce folds a two-argument function across a collection
(def loaves [4 9 2 7])
(println (reduce + loaves))

;; an init value seeds the accumulator and covers the empty case
(println (reduce + 100 loaves) (reduce + 0 []))

(def orders [{:item "rye" :loaves 4} {:item "sour" :loaves 9}
             {:item "rye" :loaves 2}])

;; reducing into a map: fnil supplies the 0 the first time a key lands
(def per-item
  (reduce (fn [acc {:keys [item loaves]}]
            (update acc item (fnil + 0) loaves))
          {} orders))

;; min and max fall out of reduce given the right two-argument fn
(println per-item (reduce min loaves) (reduce max loaves))

How it works

(reduce + loaves) folds with no init value, and (reduce + 0 []) shows what an init covers.
per-item reduces the order maps into a totals map, where (fnil + 0) supplies the zero on a key's first appearance.
(reduce min loaves) and (reduce max loaves) pull the extremes from the same fold.

Keywords and builtins used here

def
fn
fnil
max
min
println
reduce
update

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Type this snippet

Step 2 of 3 in Sequences, step 14 of 27 in Language basics.

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