Clojure: reduce with three parameters

clojure

Solution

`partition` to the rescue.

(reduce (fn [i [a b]]
          (cond
            (and (even? a) (even? b)) (inc i)
            (and (odd? a) (odd? b))   (dec i)
            :else i))
        0 (partition 2 1 input))

Or a little more concise:

(reduce (fn [i pair]
          (condp every? pair
            even? (inc i)
            odd?  (dec i)
            i))
        0 (partition 2 1 input))

Problem

I often find myself in need of a kind of expanded `reduce` whenever I must process an item at time (like reduce), accumulate some kind of result (like reduce) but do the process based on the previous item of the sequence (unlike reduce). For example (a silly one), add 1 to the accumulator if both the current item and the previous one are even and subtract one of they're odd. This is just a stupid case, but I've run into this kind of problems often. I generally make a vector as accumulator, so that the first item is the real aggregation and the second one is the previous item. This is not very elegant and certainly verbose. Is there a core function to help in those cases? What is the most idiomatic way to deal with such problem? Thanks

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