Clojure中最高键的返回值

Question

I'm working with these two groups of key value pairs, which is being returned by another function. I'd like to write a function that will always find the highest key and return its corresponding value. In this example, I'd be returning 2 because 499 is the highest key. The data that I am working with is

({-99 0, 99 0} {-99 2, 499 2})

When I call

   (type ({-99 0, 99 0} {-99 2, 499 2}))

Within the function that is responsible for returning that data it, I get back

 (clojure.lang.PersistentTreeMap clojure.lang.PersistentTreeMap)

I hope that helps. Thanks!

Answer 1

This function will return the rightmost entry of a Clojure sorted map (the built-in implementation is called clojure.lang.PersistentTreeMap ) in logarithmic time :

(defn rightmost
  "Takes a Clojure sorted map sm and returns the entry at the greatest
  key (as determined by sm's comparator)."
  [sm]
  (first (rseq sm)))

Example:

(rightmost (sorted-map 1 1 2 2 3 3))
;= [3 3]

You can then fish out the value using the val function.

All the max-key / apply max -based solutions work in linear time instead. Needless to say, it's a huge difference.

If the other function could be convinced to return data.avl maps instead, you could access the element at any index in logarithmic time using nth :

;; works for data.avl sorted maps, not the built-ins
(nth (avl/sorted-map 1 1 2 2 3 3) 2)
;= [3 3]

Answer 2

(as-> (apply merge pair)
      merged
      (->> merged
           keys
           (apply max)
           merged))

Notice that when both maps have a "highest" key, the value of second one is returned.

Answer 3

This is a good use case for max-key (See this other SO question for a good example of its use), which I think has kind of a misleading name -- what it actually does is it takes a function and a collection, and it returns the item in the collection that has the highest result of applying the function to that item. You can use the function key , which returns the key of a key-value pair.

(Note that you need to concat your maps together so that you're working with a single collection of key-value pairs.)

(apply max-key key (concat {-99 0, 99 0} {-99 2, 499 2}))
;=> [499 2]

(second *1)
;=> 2

Answer 4

(defn val-for-max-key [maps]
  (->> (map (partial apply max-key key) maps)
       (apply max-key key)
       val))

EDIT: misunderstood desired return value

Answer 5

({-99 0, 99 0} {-99 2, 499 2}) is a lookup operation, where {-99 0, 99 0} is a dictionary and {-99 2, 499 2} is a key. Since the latter is not a key in the former, the expression will return nil.

When I evaluate (type ({-99 0, 99 0} {-99 2, 499 2})) , I get nil , because the type of nil is also nil .