Assign a set of values in a matrix depending on the index of it

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etudiant_is 2016-3-18

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回答： etudiant_is 2016-3-19

I have two matrices from which I compute the distances between Points in P1 and P2. In P1, I have only X and Y coordinates.

P1=[12 45
    34 7 ]

and in P2 , I have the index of the point (in the first column), X and Y coordinates. The index in the first column is not sorted and it is not complete. Like in the example, I don't have indices 1,4 and 5.

P2= [2 42 25
     6 37 57 
     3 55 16]

So after computing the distances bw the points of P1 and P2, I have the result in another matrix D. Now, I have another matrix Dall, where I have rows for all points (from P2 and those not also into it). I want to put the distances I got from D inside of Dall for the corresponding rows.

What I am doing with a for loop is

for i=1:size(P1,1)
    for j=1:size(P2,1)
         x=P2(i,1);
   Dall(x,j)  = D(i,j);  
    end   
end

Any more optimal way to do it?

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etudiant_is 2016-3-18

编辑：etudiant_is 2016-3-18

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If I do

Dall(P2(:,1),:)=4;

It works on the lines I want. But like this, it gives me an error.

Dall(P2(:,1),:)= D(:)

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Answer 1

etudiant_is 2016-3-19

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What worked for me is

Dall(P2(:,1),:)= D

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Answer 2

Image Analyst 2016-3-18

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You didn't really say what you're doing to get D. So what I did, using pdist2() in the Statistics and Machine Learning Toolbox, is

P1 = [12 45
      34 7 ]
P2 = [2 42 25
      6 37 57
      3 55 16]
distances = pdist2(P1, P2(:, 2:end))

I assume you did the same. So I got

distances =
      36.0555127546399    27.7308492477241     51.8652099195598
      19.6977156035922    50.0899191454728     22.8473193175917

The 2 rows correspond to the 2 points from P1, and the 3 columns are the distances to the corresponding 3 points in P2. But what I don't know is what you want Dall to look like.

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Image Analyst 2016-3-18

Sorry, but we can't figure out the "rule" for how you distribute the numbers from distances into Dall.

etudiant_is 2016-3-19

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it is based on the value I have in the first column of P2. In this example I have 2, 6 and 3. So the first distance will be in line 2, the second in line 6 and the third in line 3 of Dall. I already found how to do it with

Dall(P2(:,1),:)= D

Thanks

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