Add map-nth.

Author: Raynes

src/useful/seq.clj

@@ -359,3 +359,13 @@ ineligible for garbage collection."
      (let [x (first coll)]
        (cons x (when-not (pred x)
                  (take-until pred (rest coll))))))))
+
+(defn map-nth
+  "Calls f on every nth element of coll. If start is passed, starts
+   at that element (counting from zero), otherwise starts with zero."
+  ([f nth coll] (map-nth f 0 nth coll))
+  ([f start nth coll]
+     (map #(% %2)
+          (concat (repeat start identity)
+                  (cycle (cons f (repeat (dec nth) identity))))
+          coll)))

test/useful/seq_test.clj

@@ -178,3 +178,8 @@
           s (sequeue 2 coll)]
       (is (= 1 (first s)))
       (is (thrown? Throwable (dorun s))))))
+
+(deftest test-map-nth
+  (is (= [2 2 4 4 6 6 8 8 10 10]
+         (map-nth inc 2 [1 2 3 4 5 6 7 8 9 10])))
+  (is (= ["" "x" "" "x"] (map-nth #(str % "x") 1 2 ["" "" "" ""]))))