The reduce-by approach (while cool) would not work for me because I
need to run multiple queries on the results.
- Dmitry
On Oct 18, 10:54 am, Alex Osborne <a...@meshy.org> wrote:
> 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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