Find first k largest elements?

Question

Rubel Ahmed 2021-8-13

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https://ww2.mathworks.cn/matlabcentral/answers/897387-find-first-k-largest-elements

评论： Matt J 2021-8-13

Hi all

I have an array say A = [1 5 7 4 8 10 14]. I want to find most largest, second most largest and 3rd most largest elements from A.

I have figured out that it can be done using maxk function which is abailable for update version of Matlab but I am using R2017a where maxk function is not availablle.

Anyway, I want to see the output like Ans = [14 10 8]. Please help.Thanks in advance.

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Answer 1

Matt J 2021-8-13

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https://ww2.mathworks.cn/matlabcentral/answers/897387-find-first-k-largest-elements#answer_766002

编辑：Matt J 2021-8-13

在 MATLAB Online 中打开

The efficient way would probably be to use a File Exchange MEX alternative to maxk() like this,

https://www.mathworks.com/matlabcentral/fileexchange/23576-min-max-selection?s_tid=srchtitle

but you could also do,

A = [1 5 7 4 8 10 14];
B=sort(A,'descend');
B=B(1:3)
B = 1×3
    14    10     8

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Rubel Ahmed 2021-8-13

@Matt J thanks but I have figured out this is also useful

[~, idx] = sort(A);

ans = A(sort(idx(end-2:end)));

Matt J 2021-8-13

在 MATLAB Online 中打开

I wouldn't recommend that.

A=rand(3e7,1);
tic;
 B=maxk(A,3);
toc
Elapsed time is 0.039257 seconds.
tic;
B=sort(A,'descend');
B=B(1:3);
toc;
Elapsed time is 1.968519 seconds.
tic;
[~,idx]=sort(A);
B=A(idx(end:end-2));
toc
Elapsed time is 2.338227 seconds.