How to delete two minimum elements in a vector?

Question

Jiawei Weng 2019-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/440557-how-to-delete-two-minimum-elements-in-a-vector

回答： Steven Lord 2019-1-20

采纳的回答： madhan ravi

wonder how to delete two minimum elements in the vector and then calculate the mean value of new vector

the vector is like A=[ 70, 56 , 30, 10 , 83 , 78, 77, 90]

code should be flexibe enough to calculate for any vectors

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Walter Roberson 2019-1-20

Suppose A had three values that were all (say) 9. All 3 of them are the minimum. What do you want to do in that situation?

Image Analyst 2019-1-20

在 MATLAB Online 中打开

Your request is very imprecise.

What would you return for

v = [1 1 1 2 2 2 3]

??? Would you return

[1 2 2 2 3] (removing a pair of 1's), or
[3] (removing all the 1's and all the 2's?

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Answer 1

madhan ravi 2019-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/440557-how-to-delete-two-minimum-elements-in-a-vector#answer_357172

编辑：madhan ravi 2019-1-20

在 MATLAB Online 中打开

EDITED

n=2; % two smallest values
u=unique(A);
if numel(u)==1 || numel(u)==2
Result=[] 
else
Result=mean(A(~ismember(A,u(1:n))))
end

6 个评论
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Walter Roberson 2019-1-20

Typically when there are duplicates, people choose from one of several different strategies:

Use the count (two) as the number of unique values to remove, removing all copies of the minimum and all copies of the second lowest
Start counting the number of items removed so far. If the count (two) has not been reached yet, remove all copies of the lowest value (incrementing the counter by the number removed), and then loop back. For example the array [A B B B C D E F] would start at 0 removed, remove all 1 copy of A incrementing the removed count to 1, then not having reached the target would remove all 3 copies of B, incrementing the removed count to 4, and then having reached the target of 2 removals, would end
Start counting the number of items removed so far. If the count (two) has not been reached yet, count how many copies exist of the lowest value. If that number together with the number removed so far would exceed the target, then stop without further removal. So [A B B B C D E F] would become [B B B C D E F] and would stop early because the 3 B's would overflow the target of two removals,
Start counting the number of items removed so far. If the count (two) has not been reached yet, find all of the copies of the lowest, and remove at most (target minus count) of them. Which you remove is another decision, and it matters if you are preserving the order of things not being removed

madhan ravi 2019-1-20

Thank you sir Image Analyst edited the answer and sir Walter thank you for considering different situations.

Jiawei Weng 2019-1-20

thank you so much

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Answer 2

Erik Keever 2019-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/440557-how-to-delete-two-minimum-elements-in-a-vector#answer_357173

Hmm. Perhaps,

% generate some random junk

x = round(10*rand([24 1]));

% sort array

[~, idx] = sort(x);

% sort the indices of the (N-2) largest elements, cutting the 2 smallest out and select subset

xTrimmed = x(sort(idx(3:end)));

I've taken that you mean to remove precisely two elements which are lexcographically smallest. My code breaks the degeneracy among multiple smallest values by choosing the first ones to occur in the array, such that among [4 1 1 0 4 4 4], the 0 and the first 1 will be removed. For large arrays, avoiding my lazy second sort() by building the subsets explicitly would be faster (removing one of two N log N sort procedures).