Hey Lee,
(vec ...) is NOT the same as (into [] ...) in this case.
Whenever you use a reducing function, like r/map, r/filter, r/mapcat, and
so on, you are not, in fact, performing any computations on the collection
to which you apply it. These functions simply wrap the collection with a
kind of delayed operation that will later be triggered by applying reduce
to it.
So, if I have a collection called samples, and I run (r/map inc samples),
I just get a reducible object back. Same thing with a chain of these
functions as in (r/filter odd? (r/map inc samples)). I just get back a
reducible object (which itself is wrapping another reducible object). When
you run reduce on this whole thing, each of the delayed operations are
combined together into one giant reduce function, as if you had written
something like this:
(reduce + (r/filter odd? (r/map inc samples)))
=> (reduce (fn [sum sample] (let [sample' (inc sample)] (if (odd?
sample') (+ sample' sum) sum))) 0 samples)
This is the reason that you need to use (into [] ...) rather than (vec
...). Running vec on a reducible object will just throw a RuntimeException.
When you use into, you will be applying reduce (since that's how it is
implemented), and as long as you use into with a vector, map, or set, it
will be run using transients for efficiency.
Uses transients:
(into [] (r/map inc (range 10)))
(into {} (r/map #(vector % 0) (range 10)))
(into #{}| (r/map inc (range 10)))
Doesn't use transients (but still works). Note that the list will have
been reversed because of the nature of list consing:
(into () (r/map inc (range 10)))
Alright, hopefully that's enough from me for now. Good luck with your
program.
~Gary
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