计算列表在数学中所需的不同值 [英] Count how many different values a list takes in Mathematica

问题描述

我想获取在列表中找到的不同值的数量。

I would like to get the number of different values found in a List.

例如：

a = {1,2,3,4,5} 的输出为5，而 b = { 1,1,1,2,2} 。

推荐答案

命令还给出所需的结果：

Just for amusement, all the following commands also give the desired result:

Length@Gather@l

Length@Union@l

Length@Tally@l

Count[BinCounts@l, Except@0]

Count[BinLists@l, Except@{}]

Length@Split@Sort@l

Length@GatherBy[l, # &]

Length@Split@SortBy[l, # &]

当然还有更多。

编辑

这是一个小小的时间实验（不严重）

Here is a little timing experiment (not serious)

l = RandomInteger[{1, 10^2}, 10^7];
t2[x_] := {Timing[x], ToString[HoldForm@x]};
SetAttributes[t2, HoldAll]
Grid[Reverse /@
  {t2[Length@DeleteDuplicates[l]],
   t2[Length@Tally[l]],
   t2[Length@Gather[l]],
   t2[Count[BinCounts[l], Except@0]],
   t2[Length@Union[l]],
   t2[Length@Split@Sort@l],
   t2[Count[BinLists[l], Except@0]]},
 Frame -> All]

BTW：注意 BinLists [] code> BinCounts []

BTW: Note the difference between BinLists[ ] and BinCounts[ ]

编辑

DeleteDuplicates vs Tally

t = Timing;
ListLinePlot@Transpose@
  Table[l = RandomInteger[{1, 10^i}, 10^7];
   {Log@First@t@Length@DeleteDuplicates@l,
    Log@First@t@Length@Tally@l},
   {i, Range[7]}]

当心！

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