How can I index a vector based on the vector's values?

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Shahriyar Karim 2020-3-13

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https://ww2.mathworks.cn/matlabcentral/answers/510718-how-can-i-index-a-vector-based-on-the-vector-s-values

评论： Shahriyar Karim 2020-3-13

采纳的回答： BobH

在 MATLAB Online 中打开

Say I have a vector,

V = [2 5 4 1 3]

and I want to use the numbers of the vector itself to index. Starting from 1, I go to the first element in V; which = 2.

From there, I want to go to the 2nd element of V--5, then 3, then 4, and finally 1.

How would I go about extracting Vnew = [1 2 4 5 3]?

So far I've tried:

V(V) = [5 3 1 2 4]

I'm assuming it'll require some sort of loop but I'm completely stumped right now.

EDIT: Apologies for the confusion in the explanation above; I don't need the produced vector, but the order in which it's produced--if that makes any sense.

Starting at index 1 of V, I get 2. Then I use 2 as the index of V to get 5--use 5 to get 3--3 for 4---4 loops back to 1.

The part I need is the order in which I loop through. The first index will always be 1, but say the first element of V was 3. I'd then go to the THIRD element of V and Vnew for the third element would equal 2. Using this method, I need to output Vnew = [1 2 4 5 3].

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Shahriyar Karim 2020-3-13

That's an answer I got previously, but it isn't exactly what I'm looking for. I've tried to clarify a bit more in the original post

BobH 2020-3-13

编辑：BobH 2020-3-13

V = [2 5 4 1 3]

sequence followed is 1, then 2 5 3 4, then 1

2 5 3 4 1 are found in V in these positions 1 2 5 3 4

You consistently show a 4 in the third position of your desired output, and I can't see how you place a 4 there.

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Answer 1

BobH 2020-3-13

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https://ww2.mathworks.cn/matlabcentral/answers/510718-how-can-i-index-a-vector-based-on-the-vector-s-values#answer_420006

编辑：BobH 2020-3-13

在 MATLAB Online 中打开

V = [2 5 4 1 3];
R(1) = V(1);
for i = 2:length(V)
    R(end+1) = V( R(i-1) );
end
R  % new vector
R =
     2     5     3     4     1
% The sequence of indices followed within V
S = [1 R(1:length(V)-1)]
S =
     1     2     5     3     4

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Shahriyar Karim 2020-3-13

Okay this makes sense. I was overthinking it way too much--that's probably why I was confused over the indexing in your other comment; it makes sense now. Thank you!

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Answer 2

Fangjun Jiang 2020-3-13

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https://ww2.mathworks.cn/matlabcentral/answers/510718-how-can-i-index-a-vector-based-on-the-vector-s-values#answer_420005

在 MATLAB Online 中打开

I believe it is like this

V = [2 5 4 1 3];
newV=zeros(size(V));
index=1;
for k=1:numel(V)
    newV(k)=V(index)
    index=newV(k);
end

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Shahriyar Karim 2020-3-13

This is what I got before in a previous answer; but it isn't exactly what I'm looking for. I've edited the original post for a bit more clarification.