roster https://groups.google.com/forum/#!topic/clojure/bouoMeogZyU

roster.clj

(let [day->index    (into {} (map-indexed (fn [k v] [v k]) "MTWRFSN"))
      ;; So M (Monday) maps to 0, S (Saturday) maps to 5
      initial-state (into [] (repeat 7 0))]
  ;; Create an empty vector of seven zeroes
  (reduce (fn [x i] (assoc x i 1)) initial-state (map day->index "MWF")))
;; Map each day to an index.
;; (day->index \M) is 0
;; so (map day->index "MWF)
;; is (0 2 4)
;; But we want [1 0 1 0 1 0 0]
;; So start with the seven zeroes, and add 1s for each date