exception - How to build a vector via a call to reduce -

i'm trying figure why particular function isn't working expected. suspect error message has way i'm creating empty vector accumulator.

i have simple function returns sequence of 2-element vectors:

(defn zip-with-index   "returns sequence in each element of    form [i c] index of element , c    element @ index."    [coll]    (map-indexed (fn [i c] [i c]) coll)) 

that works fine. problem comes when try use in function

(defn indexes-satisfying   "returns vector containing indexes of coll satisfy    predicate p."   [p coll]   (defn accum-if-satisfies [acc zipped]     (let [idx (first zipped)           elem (second zipped)]       (if (p elem)          (conj acc idx)         (acc))))   (reduce accum-if-satisfies (vector) (zip-with-index coll))) 

it compiles, when attempt use error:

user=> (indexes-satisfying (partial > 3) [1 3 5 7]) arityexception wrong number of args (0) passed to: persistentvector clojure.lang.afn.throwarity (afn.java:437) 

i can't figure out what's going wrong here. if there more 'clojure-like' way of doing i'm trying do, i'm interested in hearing also.

the problem on else clause of accum-if-satisfies, should acc not (acc).

you use filter , map instead of reduce. that:

(map #(first %)       (filter #(p (second %))              (zip-with-index coll))) 

you call map-indexed vector instead of (fn [i c] [i c]). whole code that:

(defn indexes-satisfying   [p coll]   (map #(first %)        (filter #(p (second %))                (map-indexed vector coll))))