clojure - 排序映射对现有键返回 nil 值

Question

我尝试从排序映射中按键获取值，并按比较器返回的值nil。

(def tmap {1 {:v 1} 2 {:v 2} 3 {:v 3}})

(def tmap-sorted
  (apply sorted-map-by
         #(let [val-comp (- (compare
                             (get-in tmap [%1 :v])
                             (get-in tmap [%2 :v])))]
            (if (= val-comp 0)
              1
              val-comp))
         (flatten (vec tmap))))
; => {3 {:v 3} 2 {:v 2} 1 {:v 1}}

(get tmap-sorted 3)
;=> nil

预期:{:v 3}

实际:无

Answer 1

您正在创建自定义 Comparator与 compare正在 PersistentTreeMap 中使用(tmap-sorted 的类型)来查找值，但比较器永远不会返回 0，这意味着两个对象相等。

https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html

Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparator c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave "strangely." In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals.

如果您将比较器修改为 println 进行调试，您可以看到，当比较 3 与 3 时，您会得到 1 意味着它们不相等。

(def tmap {1 {:v 1} 2 {:v 2} 3 {:v 3}})
(def tmap-sorted (apply
                  sorted-map-by
                  #(let [val-comp
                         (- (compare
                             (get-in tmap [%1 :v])
                             (get-in tmap [%2 :v])))
                         ret (if (= val-comp 0)
                               1
                               val-comp)]
     (println "%1: " %1 " %2: " %2 " ret=" ret)
     ret)
                        (flatten (vec tmap))))


(get tmap-sorted 3)
;; %1:  3  %2:  2  ret= -1
;; %1:  3  %2:  3  ret= 1

(get tmap-sorted 1) 
;; %1:  1  %2:  2  ret= 1
;; %1:  1  %2:  1  ret= 1

因此，您需要修复您的 compare 函数以实现相等