How to write a MATLAB code to compute a set of indices from an array?

Question

HAT 2021-12-6

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1604440-how-to-write-a-matlab-code-to-compute-a-set-of-indices-from-an-array

评论： HAT 2021-12-6

I would like to compute a set of indices T={j≠1,2| (1,j) and (j,2) belong to KV for j in {1,2,3,4} }, where

KV = {(4,1), (3,2), (1,3), (2,3), (2,4), (3,4)}. The answer is T = {3}. But I wanted to write a matlab code that gives the answer. To be more precise, form a given matrix B below,

B = [...
  0   0.6443   0;...
  1   2.4082   4.6555;...
  0.6443   1   3;...
4152   0   0   1];

KV represents the entries position whose entries are different from 0. Using MATLAB code, we can verify KV as follows.

clear
B = [...
    1    0   0.6443   0;...
    0    1   2.4082   4.6555;...
    0    0.6443   1   3;...
    0.4152   0   0   1];
n = max(size(B)); % max size 
missingEntries=6; %number of missing entries in B
knownValues = zeros(n*(n-1)-missingEntries,2); %PREALLOCATE
index = 1; % an index that verifies the position of i and j
for j = 1:n 
    for i = 1:n
        if B(i,j) ~= 0 && i~= j  
            knownValues(index,1) = i; % position of knownValues in the ith row
            knownValues(index,2) = j; % position of knownValues in the jth column
            index = index+1; % update index until (A(i,j) == 0) ends
        end
    end
end
knownValues;
KV = knownValues
KV = 6×2
     4     1
     3     2
     1     3
     2     3
     2     4
     3     4

But I got stuck to compute a set of indices T = { j≠1,2 such that both pairs (1,j) and (j,2) belong to KV for j in {1,2,3,4} }, where

KV = {(4,1), (3,2), (1,3), (2,3), (2,4), (3,4)} using MATLAB. The answer is T = {3}. But I wanted to write a matlab code that gives this answer.

I wonder if you could assist me.

Thanks in advance.

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Answer 1

Matt J 2021-12-6

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1604440-how-to-write-a-matlab-code-to-compute-a-set-of-indices-from-an-array#answer_848890

编辑：Matt J 2021-12-6

在 MATLAB Online 中打开

B = [...
    1    0   0.6443   0;...
    0    1   2.4082   4.6555;...
    0    0.6443   1   3;...
    0.4152   0   0   1];
n=size(B,1);
B(1:n+1:end)=0;
[i,j]=find(B);
T=setdiff( intersect(j(i==1),i(j==2)) ,[1,2]) 
T = 3