Matrix （Clojure 实现）

摘自《Clojure编程》

使用 Clojure 实现一个简单的矩阵。

(defprotocol Matrix
  "Protocol for working with 2d data structures."
  (lookup [matrix i j])
  (update [matrix i j value])
  (rows [matrix])
  (cols [matrix])
  (dims [matrix]))

(extend-protocol Matrix
  clojure.lang.IPersistentVector
  (lookup [vov i j]
    (get-in vov [i j]))
  (update [vov i j value]
    (assoc-in vov [i j] value))
  (row [vov]
    (seq vov))
  (cols [vov]
    (apply map vector vov))
  (dims [vov]
    [(count vov) (count (first vov))]))

(extend-protocol Matrix
  nil
  (lookup [x i j])
  (update [x i j value])
  (rows [x] [])
  (cols [x] [])
  (dims [x] [0 0]))

(defn vov
  "Create a vector of h w-item vectors."
  [h w]
  (vec (repeat h (vec (repeat w nil)))))