I've been writing Clojure code today and have noticed the same pattern
show up multiple times, but can't find an elegant way to code it in
idiomatic Clojure. I feel like I'm missing an obvious solution...
anyone else see something I don't? Thanks in advance!
The problem boils down to the following minimal example: Suppose you
want to write a function "left-total" which takes a list of number and
returns pairs of said numbers, along with the total of numbers to the
left:
=> (left-total [3 5 10 1 2 7])
([3 0] [5 3] [10 8] [1 18] [2 19] [7 21])
I can think of several ways to write this function. Three acceptable
versions of this function are written below, but all of them are
hideous looking. Is there a better solution? (Note that any solution
should avoid thrashing the stack or performing more than the minimum
number of additions.)
(defn left-total [lst]
(loop [lst lst
acc nil
tot 0]
(if (seq lst)
(let [[cur & more] lst]
(recur more (cons [cur tot] acc) (+ tot cur)))
(reverse acc))))
(defn left-total [lst]
(reverse (first (reduce (fn [[acc tot] cur]
[(cons [cur tot] acc) (+ tot cur)])
[nil 0]
lst))))
(defn left-total [lst]
(let [tot (atom 0)]
(map (fn [cur]
(let [old-tot @tot]
(swap! tot (partial + cur))
[cur old-tot]))
lst)))
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