A question about sorting a vector

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Cantor Set 2019-9-7

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评论： Cantor Set 2019-9-8

Suppose I have a column vector V of length N and I map every element in V to an element y in Y so, we have a column vector Y of length N.

Suppose I want to sort Y in an ascending order and pick r elements from the vector V that corrosponds to the least r values in Y.

I thought of:

V; Y;
N; r;
[Ys idx]=sort(Y);
for i=1:r
    Best(i)=V(idx(Ys(i)))
end
Best(i)

So is this correct?

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the cyclist 2019-9-7

编辑：the cyclist 2019-9-7

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That code gives an error for me. I assume it does for you, too. So, how can it be right?

Is this homework? You are pretty close to a solution, but

idx(Ys(i))

is the wrong way to index, because Ys(i) is not a whole number that can be used in this way.

Cantor Set 2019-9-8

A part of a homework problem.

Thank you!

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Answer 1

Star Strider 2019-9-7

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It does not look correct to me, specifically because ‘Ys’ may not necessarily be an integer greater than 0, as required for subscript references in MATLAB, and also within the size limits of ‘V’. (We have no idea what ‘Y’ is, since you have not told us.)

This may give you the result you want, however you have not told us that, either. It nevertheless avoids the explicit loop:

V = randn(10,1);                            % Create Vector
Y = randn(10,1);                            % Create Vector
[Ys, idx]=sort(Y);
r = 5;                                      % Choose A Value
Best = V(idx(1:r))                          % Result