On Jun 17, 9:34 am, Wrexsoul <d2387...@bsnow.net> wrote: > I'm shocked that this is missing from clojure.core: > > (defn accum [f init coll] > (loop [x init c coll] > (if (empty? c) > x > (recur (f x (first c)) (rest c))))) > > user=> (accum + 0 [1 2 3]) > 6 > user=> (accum + 0 [1 2 3 4 5]) > 15 > > This is one of the most basic, useful functions in functional > programming. :) > > Here's any triangular number: > > (defn tri [n] (accum + 0 (take n (iterate inc 1)))) > Maybe "apply" can be used user=> (apply + [1 2 3]) 6 user=> (apply + [1 2 3 4 5]) 15 user=> (defn tri [n] (accum + 0 (take n (iterate inc 1)))) #'user/tri user=> (tri 10) 55 user=> (def inf (iterate inc 1)) #'user/inf user=> (defn tri2 [n] (apply + (take n inf))) #'user/tri2 user=> (tri2 10) 55 user=> or "reduce"? user=> (reduce + 0 [1 2 3]) 6 user=> (reduce + 10 [1 2 3]) 16 user=> (reduce + 0 [1 2 3 4 5]) 15 Regards, Parth > Here's a lazy seq of them all: > > (def *tris* (for [i (iterate inc 1)] (tri i))) > > This, however, is more efficient (and demonstrates another case where > super-lazy-seq makes something very compact and readable): > > (defn accum-map [f init coll] > (super-lazy-seq [x init c coll] > (if (seq c) > (next-item x (f x (first c)) (rest c))))) > > (def *tris* (rest (accum-map + 0 (iterate inc 1)))) > > Notice how similar the accum-map code is to the accum code? With just > lazy-seq it would not be as clear. :) > > The "rest" in the replacement def for *tris* is because accum-map > returns the init value as the first value of the sequence, i.e., it's > equivalent to making a sequence of (tri 0), (tri 1), (tri 2), and so > on. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
