线性代数-在matlab中获得唯一元素的计数及其计数

Question

我有一个向量比方说

A = [1; 2; 3; 1; 2; 3; 1; 2; 3]

我怎样才能找到唯一的元素和它们的计数像这样?

1 = 3
2 = 3
3 = 3

我绝对可以循环遍历各个元素并使用一些简单的技术。我不想要这样的解决方案，因为我的实际数据将在数百万的范围内，所以我不能只循环遍历每个元素。

在matlab中最有效的方法是什么?

Answer 1

要想提高效率，很难比这更好了:

elements = unique(A);
counts = histc(A(:), elements);

Answer 2

不像这样的答案是正确的。但是它的效率(计算时间)实际上是可以提高的:

sA = sort([A(:); inf]); %// sort A
ind = diff(sA)~=0; %// index of last element of each run of equal values
elements = sA(ind); %// unique elements
counts = diff([0; find(ind)]); %// lengths of runs

基准测试

:

clear all
A = randi(100,1e6,1); %// Example data. Large column vector
%// Notlikethat's answer
tic
elements = unique(A);
counts = histc(A(:), elements);
toc
clear elements counts
%// This answer
tic
sA = sort([A(:); inf]);
ind = diff(sA)~=0;
elements = sA(ind);
counts = diff([0; find(ind)]);
toc
Elapsed time is 0.175594 seconds.
Elapsed time is 0.076032 seconds.