Clojure：序列回到向量 [英] Clojure: sequence back to vector

问题描述

如何在序列生成操作（如sort）后将序列转换回向量？在作为向量的序列上使用（vec ..）是昂贵的吗？

How can I cast a sequence back to vector after a sequence producing operation (like sort)? Does using (vec..) on a sequence that was a vector is costly?

一种可能性是创建一个新的矢量序列：

One (bad?) possibility is creating a new vector out of sequence:

(vec (sort [1 2 3 4 5 6]))

因为我需要随机访问（nth ..）到巨大的排序的向量 - 这是现在是排序后，可怕的O（n）随机访问时间大的序列

I am asking because I need random access (nth ..) to huge sorted vectors - which are now huge sequences after the sort, with horrible O(n) random access time

推荐答案

从我自己的测试（没有科学），你可以更好地直接在数组上工作，在你做大量的排序。但是如果你很少排序，并有很多随机访问做，虽然，使用向量可能是一个更好的选择，因为随机访问时间平均快40％以上，但排序性能是可怕的，由于将向量转换一个数组，然后回到一个向量。这是我的发现：

From my own tests (nothing scientific) you may be better with working directly on arrays in cases where you do lots of sorting. But if you sort rarely and have a lots of random access to do though, going with a vector may be a better choice as random access time is more than 40% faster on average, but the sorting performance is horrible due to converting the vector to an array and then back to a vector. Here's my findings:

(def foo (int-array (range 1000)))

(time
  (dotimes [_ 10000]
    (java.util.Arrays/sort foo)))

; Elapsed time: 652.185436 msecs

(time
  (dotimes [_ 10000]
    (nth foo (rand-int 1000))))

; Elapsed time: 7.900073 msecs

(def bar (vec (range 1000)))

(time
  (dotimes [_ 10000]
    (vec (sort bar))))

; Elapsed time: 2810.877103 msecs

(time
  (dotimes [_ 10000]
    (nth bar (rand-int 1000))))

; Elapsed time: 5.500802 msecs

PS：请注意，向量版本实际上并不存储任何地方，但这不应该改变结果，因为你会使用简单的绑定在一个循环的速度。

P.S.: Note that the vector version doesn't actually store the sorted vector anywhere, but that shouldn't change the result considerably as you would use simple bindings in a loop for speed.

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