Compute differences bewteen vectors in a matrix
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Gianluca Ligresti
le 13 Déc 2018
Modifié(e) : Stephen23
le 13 Déc 2018
Hi all, I have a question for you:
I have a matrix of the following type, where the i-th vector is a row vector:
A = [v1;v2;v3;v4;...;vN]
I want to compute all the possible differences between vectors, except the self-one. I mean: I need a first iteration that returns me [v1-v2;v1-v3;v1-v4;....;v1-vN] but I DON'T want to consider v1-v1; then at the next iteration I want [v2-v1; v2-v3; v2-v4;...;v2-vN] and i DON'T want v2-v2.
I need to do this for all the rows of my matrix A, I prefer to obtain only one difference-matrix at time because then I have to do some processing over it. So can you suggest me an algorithm that exactly does this?
Thanks
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Stephen23
le 13 Déc 2018
Modifié(e) : Stephen23
le 13 Déc 2018
This is easy to achieve with basic indexing and an anonymous function:
>> A = randi(9,5,7)
A =
4 4 5 9 4 7 1
9 4 6 8 2 8 2
9 7 2 1 7 9 8
2 5 7 7 5 3 5
2 7 1 4 6 6 5
>> fun = @(k) A(k,:) - A([1:k-1,k+1:end],:);
>> fun(1)
ans =
-5 0 -1 1 2 -1 -1
-5 -3 3 8 -3 -2 -7
2 -1 -2 2 -1 4 -4
2 -3 4 5 -2 1 -4
>> fun(2)
ans =
5 0 1 -1 -2 1 1
0 -3 4 7 -5 -1 -6
7 -1 -1 1 -3 5 -3
7 -3 5 4 -4 2 -3
>> fun(3)
ans =
5 3 -3 -8 3 2 7
0 3 -4 -7 5 1 6
7 2 -5 -6 2 6 3
7 0 1 -3 1 3 3
>> fun(4)
ans =
-2 1 2 -2 1 -4 4
-7 1 1 -1 3 -5 3
-7 -2 5 6 -2 -6 -3
0 -2 6 3 -1 -3 0
>> fun(5)
ans =
-2 3 -4 -5 2 -1 4
-7 3 -5 -4 4 -2 3
-7 0 -1 3 -1 -3 -3
0 2 -6 -3 1 3 0
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KSSV
le 13 Déc 2018
N = 10 ;
v = rand(N,1) ;
iwant = zeros(N-1,N) ;
for i = 1:N
idx = setdiff(1:N,i) ;
iwant(:,i) = v(i)-v(idx) ;
end
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