Question

来自Functional Java和一点Scala，我现在正逐步学习Clojure。目前，考虑到我有空闲时间，我正在阅读和练习计算机程序的结构和解释＆＃34;

我坚持以下练习，我知道如何解决问题，即完全删除let并将(take 2 ...直接放入map。

; Exercise 1.3
;; Define a procedure that takes three numbers as arguments 
;; and returns the sum of the squares of the two larger numbers.
(defn square-sum-largest-pair [a b c]
  "Sums the square of the largest two number of the three in input.
  Maybe a little overkill to use a list here, but it's just for fun."
  (let [[two-items] (take 2 (reverse (sort [a b c])))]
  (reduce + (map (fn [x] (m/expt x 2)) two-items))))

执行square-sum-largest-pair时会显示以下错误：

actual: java.lang.IllegalArgumentException: Don't know how to create ISeq from:     java.lang.Long
at clojure.lang.RT.seqFrom (RT.java:505)
    clojure.lang.RT.seq (RT.java:486)
    clojure.core$seq.invoke (core.clj:133)
    clojure.core$map$fn__4245.invoke (core.clj:2551)

为什么会这样？

Answer 1

问题是[]轮two-items。这会将two-items绑定到序列(take 2 (reverse (sort [a b c])))的第一个，忽略其余部分。试图使用这个 - 最大数字 - 作为后续map中的序列产生观察到的错误。

删除[]（为了简单起见与*平方），我们得到了

(defn square-sum-largest-pair [a b c]
  (let [two-items (take 2 (reverse (sort [a b c])))]
    (reduce + (map (fn [x] (* x x)) two-items))))

(square-sum-largest-pair 1 2 3)
13

Answer 2

因为你总是处理来自seq前面的相同固定数量的项目，你也可以通过使用解构为第一个元素创建绑定来解决这个问题：

(defn square-sum-largest-pair [a b c]
  (let [[x y] (reverse (sort [a b c]))]
    (+ (* x x) (* y y))))

Answer 3

不要将括号括起来两次。作为奖励，我对其进行了修改，因此您无需撤消已排序的列表：

(defn square-sum-largest-pair [a b c]
  "Sums the square of the largest two number of the three in input.
   Maybe a little overkill to use a list here, but it's just for fun."
  (let [two-items (take 2 (sort > [a b c]))]
    (reduce + (map (fn [x] (m/expt x 2)) two-items))))

让绑定序列作为输入映射，抛出异常

3 个答案: