Not like the previous solution here http://www.aygfsteel.com/chaocai/archive/2012/08/05/384844.html
The following solution not using the back tracing way is more concise and readable, but for the searching space becomes huger, the performance is much worser then the previous one.
(ns SICP.unit3)(defn conflictInCol? [s col]
(some #(= col %) s)
)
(defn conflictInDia? [s col]
(let [dia (count s)
n1 (fn [c] (Math/abs (- dia (.indexOf s c))))
n2 (fn [c] (Math/abs (- col c)))]
(some #(= (n1 %) (n2 %)) s)
)
)
(defn safe? [s col]
(not (or (conflictInCol? s col) (conflictInDia? s col)))
)
(defn next-level-queens [solutions-for-prev-level board-size current-level]
(let [solutions (atom [])]
(doseq [s solutions-for-prev-level]
(doseq [col (range 0 board-size)]
(if (safe? s col)
(reset! solutions (cons (conj s col) @solutions))
)
)
)
(if (< current-level (dec board-size))
(recur @solutions board-size (inc current-level))
(count @solutions)
)
)
)
(defn queens [board-size]
(next-level-queens (apply vector (map #(vector %) (range 0 board-size))) board-size 1)
)
Chao Cai (蔡超）
Sr. SDE
Amazon
The functions to support using XPath in Clojure.
Source Code
1 ;The code was implemented by caichao@amazon.com
2 ;You could use the code anyway, but should keep the comments
3 ;Created 2012.10
4 (ns clojure.ccsoft.xml
5 (:require [clojure.xml :as xml]))
6
7 (import '(java.io StringReader)
8 '(java.io ByteArrayInputStream))
9
10 (defn xml-structure [xml-txt]
11 [ (xml/parse (-> xml-txt
12 (.getBytes)
13 (ByteArrayInputStream.)
14 )
15 )]
16 )
17
18 (defn node [tag xmlStruct]
19
20 (first (filter #(= (:tag %) tag) (:content xmlStruct)))
21 )
22
23 (defn node [path xml-txt]
24 (loop [path path
25 xml-content (xml-structure xml-txt)
26 ]
27 (let [current-tag (first path) current-elem (first xml-content)]
28 (if (= (:tag current-elem ) current-tag)
29
30 (if (= (count path) 1)
31 current-elem
32 (recur (rest path) (:content current-elem ))
33 )
34 (if (> (count xml-content) 1)
35 (recur path (rest xml-content))
36 )
37 )
38 )
39 )
40 )
How to Use
(def cmd-example "<command>
<header>
<type>script</type>
<transaction_id>12345</transaction_id>
</header>
<body>
println 3+4;
</body>
</command>")
(node [:command :header :transaction_id] cmd-example)
The following program is about solving N-Queens problem (http://en.wikipedia.org/wiki/Eight_queens_puzzle) by Clojure. If you have the better solution in Clojure or Haskell, welcome to provide your solution.
(ns queens)
(defn conflictInRow? [queens newqueen]
(some #(= newqueen %) queens)
)
(defn conflictInDia? [queens newqueen]
(let [dia (count queens)
n1 (fn [queen] (Math/abs (- dia (.indexOf queens queen))))
n2 (fn [queen] (Math/abs (- newqueen queen)))]
(some #(= (n1 %) (n2 %)) queens)
)
)
(defn conflict? [queens newqueen]
(or (conflictInRow? queens newqueen) (conflictInDia? queens newqueen))
)
(def cnt (atom 0))
(defn put-queens [queens newqueen boardSize ]
(if (= (count queens) boardSize)
(do
(println queens)
(reset! cnt (inc @cnt))
)
(do
;(println queens)
(if (> newqueen boardSize)
(if (and (= (peek queens) boardSize) (= (count queens) 1))
(throw (Exception. (str "That's all " @cnt)))
(recur (pop queens) (inc (peek queens)) boardSize )
)
(if (conflict? queens newqueen)
(recur queens (inc newqueen) boardSize )
(do
(put-queens (conj queens newqueen) 1 boardSize )
(recur queens (inc newqueen) boardSize )
)
)
)
)
)
)
(defn queens [boardSize]
(put-queens [] 1 boardSize)
)
Chao Cai (蔡超）
Sr. Software Dev Engineer
Amazon.com