在Clojure中更改映射行为 [英] Changing map behaviour in Clojure

问题描述

我需要修改地图函数行为，以提供不具有最小集合大小但是具有最大值的映射，并为缺失的元素使用零。

I need to modify map function behavior to provide mapping not with minimum collection size but with maximum and use zero for missing elements.

标准行为：

(map + [1 2 3] [4 5 6 7 8]) => [5 7 9]

需要的行为：

(map + [1 2 3] [4 5 6 7 8]) => [5 7 9 7 8]

我写了函数来做到这一点，但似乎不太可扩展varargs。

I wrote function to do this, but it seems not very extensible with varargs.

(defn map-ext [f coll1 coll2]
  (let [mx (max (count coll1) (count coll2))]
    (map f
     (concat coll1 (repeat (- mx (count coll1)) 0))
     (concat coll2 (repeat (- mx (count coll2)) 0)))))

有更好的方法吗？

推荐答案

另一个惰性变体，可用于任意数量的输入序列：

Another lazy variant, usable with an arbitrary number of input sequences:

(defn map-ext [f ext & seqs]
  (lazy-seq
   (if (some seq seqs)
     (cons (apply f (map #(if (seq %) (first %) ext) seqs))
           (apply map-ext f ext (map rest seqs)))
     ())))

用法：

user> (map-ext + 0 [1 2 3] [4 5 6 7 8])
(5 7 9 7 8)

user> (map-ext + 0 [1 2 3] [4 5 6 7 8] [3 4])
(8 11 9 7 8)

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