Alex Osborne wrote:
> If the three output lists themselves are too large, I'd just explicitly
> sum your units with reduce:
>
> (reduce
> (fn [counts data]
> (let [type (record-type data)]
> (assoc counts type (+ (units data)
> (get counts type 0)))))
> {} (get-idata))
>
> => {:sales 1233, :upgrades 17, :demos 42, nil 30}
Actually come to think of it, this sort of thing is common enough that
you could pull out a 'reduce-by' function like this:
(defn reduce-by [grouper f val coll]
(reduce
(fn [m x]
(let [group (grouper x)]
(assoc m group (f (get m group val) x))))
(sorted-map) coll))
Then group-by could be easily defined in terms of it:
(defn group-by [f coll]
(reduce-by f conj [] coll))
And it makes your unit summing example:
(reduce-by record-type
(fn [count data] (+ count (units data)))
0 (get-idata))
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