clojure 中的统一成本搜索

Question

此程序中似乎存在错误，如果有人可以提供帮助，我将不胜感激

(defn findmincostindex [frontier]
  (loop [i 0 n 0]
    (if (< i (count frontier))
      (if (< (:cost (get frontier i)) (:cost (get frontier n)))
        (recur (inc i) i)
        (recur (inc i) n))
      n)))


(defn uniformcostsearch [graph start end]
  ((fn [frontier explored]
    (if (empty? frontier)
      "Failure"
      (let [pathwithmincost (findmincostindex (into [] frontier))
      ;(let [pathwithmincost (findmincostindex frontier)
            path (:path (get frontier pathwithmincost))
            cost (:cost (get frontier pathwithmincost))
            node (peek path)
            childs (keys (graph node))]
        (if (= node end)
          path
          (recur 
            (concat (subvec frontier 0 pathwithmincost) (subvec frontier (inc pathwithmincost))
              (map (fn [c] {:path (conj path c) :cost (+ cost ((graph node) c))})
                (filter #(not (contains? explored %)) childs)))
            (conj explored node))))))
    [{:path [start] :cost 0}] #{}))



(def graph {
        "Oradea" {
            "Zerind" 71,
            "Sibiu" 151
        },
        "Zerind" {
            "Oradea" 71,
            "Arad" 75
        },
        "Arad" {
            "Zerind" 75,
            "Sibiu" 140,
            "Timisoara" 118
        },
        "Sibiu" {
            "Oradea" 151,
            "Arad" 140,
            "Fagaras" 99,
            "Rimnicu Vilcea" 80
        },
        "Fagaras" {
            "Sibiu" 99,
            "Bucharest" 211
        },
        "Rimnicu Vilcea" {
            "Sibiu" 80,
            "Pitesti" 97,
            "Craiova" 146
        },
        "Timisoara" {
            "Arad" 118,
            "Lugoj" 111
        },
        "Lugoj" {
            "Timisoara" 111,
            "Mehadia" 70
        },
        "Pitesti" {
            "Rimnicu Vilcea" 97,
            "Craiova" 138,
            "Bucharest" 101
        },
        "Mehadia" {
            "Lugoj" 70,
            "Drobeta" 75
        },
        "Drobeta" {
            "Mehadia" 75,
            "Craiova" 120
        },
        "Craiova" {
            "Drobeta" 120,
            "Rimnicu Vilcea" 146,
            "Pitesti" 138
        },
        "Bucharest" {
            "Pitesti" 101,
            "Fagaras" 211,
            "Giurgiu" 90,
            "Urziceni" 85
        },
        "Giurgiu" {
            "Bucharest" 90
        },
        "Urziceni" {
            "Bucharest" 85,
            "Vaslui" 142,
            "Hirsova" 98
        },
        "Hirsova" {
            "Urziceni" 98,
            "Eforie" 86
        },
        "Eforie" {
            "Hirsova" 86
        },
        "Vaslui" {
            "Iasi" 92,
            "Urziceni" 142
        },
        "Iasi" {
            "Neamt" 87,
            "Vaslui" 92
        },
        "Neamt" {
            "Iasi" 87
        }})

(println (uniformcostsearch graph "Neamt" "Iasi"))
(println (uniformcostsearch graph "Neamt" "Vaslui"))
(println (uniformcostsearch graph "Bucharest" "Arad"))

它应该输出这些行

['Neamt', 'Iasi']
['Neamt', 'Iasi', 'Vaslui']
['Bucharest', 'Pitesti', 'Rimnicu Vilcea', 'Sibiu', 'Arad']

但它却说：

clojure.lang.LazySeq 不能转换为 clojure.lang.IPersistentVector

当我使用时

(into [] frontier)

如果我单独使用前沿，它会说

java.lang.NullPointerException

Answer 1

异常发生在您的recur 中的第一个subvec。你会重复concat 的结果，这是一个惰性序列，你不能取一个惰性序列的子向量。一个快速的解决方法是将其包裹在 vec:

(vec (concat (subvec frontier 0 pathwithmincost) (subvec frontier (inc pathwithmincost))
              (map (fn [c] {:path (conj path c) :cost (+ cost ((graph node) c))})
                (remove explored childs))))

其他一些提示：

1. findmincostindex 本质上是对不太通用的 min-key 的重新实现，您可能可以通过使用它来使其更简洁。
2. 集合也是函数，如果它是集合的成员，则返回参数（真实值）。您可以使用它来改进(filter #(not (contains? explored %)) childs))) - 例如(remove explored childs)
3. 您的let 可以使用destructuring 缩短一点。

这是我的尝试：

(let [[idx {:keys [path cost]}] (apply min-key (comp :cost second) (map-indexed vector frontier))
      node (peek path)
      childs (keys (graph node))] 

这里的过程是

1. (map-indexed vector frontier) 使 frontier 成为索引和节点对。
2. min-key 查找具有最低 (:cost (second pair)) 值的对。
3. let 将名称 idx 绑定到该对的索引，并将 path 和 cost 绑定到节点的 :path 和 :cost 键。