MATLAB Answers


return row index of values greater than 0 to a 3 dimensional array

Asked by Charles on 21 Apr 2019 at 11:34
Latest activity Answered by Andrei Bobrov
on 21 Apr 2019 at 19:51
I have a Matrix A, of m x n dimensions.
I wish to go through each row from left to right and return the index of values greater than 0. I have tried the folowing which returns the column index for the first row which has values. for example row 26.
I have tried the following but I am not getting the outputmatrix with all entries. I fear the entries are being overwritten
A =[2,-4,-0.5,0.34;0.01,4,-0.5,0.34;-10,4,-0.2,0.6;-10,4,-0.2,0.6;-19,15,-0.7,0.6];
% Now we have input matrix A we want to return the col index of each row element greater than 0.
% inputMatrix= A
for i = 26:size(A(2:end,:),1)
for ij = 1:size(A(2:end,:),2)
[rows, columns] = find(inputMatrix > 0)
A =
2 -4 -0.5 0.34
0.01 4 -0.5 0.34
-10 4 -0.2 0.6
-10 4 -0.2 0.6
-19 15 -0.7 0.6
However I wish to go through each row, and store these column indices in a 3 dimensional array where each page represents the set of column indices from each row of A
Example output is a 3 dimensional array of column indices. - Matrix E
1, 4 (page 1)
1, 2, 4 (page 2)
2, 4 (page 3
2, 4 (page 4)
2,4 (page 5)
I then want to use these column indices to return the value from the corresponding rows of a matrix C and matrix D. Matrix C and D are the same dimentions as A
I then want to multiply C values by D values and return a 3 dimension array of results. - Matrix F
The results will be of the same dimension as Matrix E
Any help appreciated.


Your page 2 output is longer than the other outputs. How do you want to handle varying lengths of output ?
i have updated the code somewhat. I hope its clearer.
Yes i understaqdn the page 2 is longer. Note sure what the answet is. Perhaps I can fill with zeros to make all the same length, but not sure how to do this
i also want to store the output in the output matrix and i wonder if i have the last line of the code
correct,s o that is store all indices and not overtirwe anything

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2 Answers

Answer by Image Analyst
on 21 Apr 2019 at 17:33
 Accepted Answer

Try this:
A = [2,-4,-0.5,0.34;0.01,4,-0.5,0.34;-10,4,-0.2,0.6;-10,4,-0.2,0.6;-19,15,-0.7,0.6];
[rows, columns] = find(A > 0)


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Explain which of those (row, column) pairs do not represent a non-zero value.
for example A(2,1) is zero when rows(2) and columns(2), which give a (row, column) pair of (2, 1) say that it's non-zero (in fact for this location, A(2,1) is 0.01 which is in fact non-zero)
Are you sure you're thinking in terms of (x,y) and not (row, column)? Matrices are not indexed (x,y) as you probably (or should) know.
In your example up top, explain how you get each element of E.
Hi Thanks for this. Your correct, but what I am seeking as output is merely the column index for elements > 0. in each row vector
Thus I am expecting is
1, 4
2, 4
2, 4
2, 4
i now know that in row 1 i have columns 1 and 4,
in row two, i have columns 1, 2, and 4 etc etc
I guress your output if correct afterall. I need to savre these answer in the outout matrix. how do I do that?
Output E, does represent the column indices you have prodcued with your answer, so your correct. i would like to fit the row and columns into the output matrix, so i I can use it to index into another matrix. How do i pllace tyhe indcies into the output matrix?

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Answer by Andrei Bobrov
on 21 Apr 2019 at 19:51

[ii,jj] = find(A > 0);
out = accumarray(ii,jj,[],@(x){sort(x)'});


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