How to use For loop or any other loop to rearrange elements in a matrix using Matlab?

Question

M.S. Khan 2020-11-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/666358-how-to-use-for-loop-or-any-other-loop-to-rearrange-elements-in-a-matrix-using-matlab

编辑： Walter Roberson 2020-11-29

Boundary_closed_1s_3s.txt

using the attache file for matrix. i am using for loop but i am making mistake some where.

My objective in the columns or rows should contains elements eithr 1 or 3 to generate a boundary of the sphere while the remaining elements should be zero.

For example, if in a row: 3 3 0 0 ... 1, i want first element or last element should be either 1 or 3 but other elments inside of the row or column should be zero. So it should be like: 3 0 0 0 .. 1

Any guidance please. Thanks in advance.

matrix = load('Boundary_closed_1s_3s.txt')
[m, n] = size(matrix)
For i = 1:m
    for j = 1:n
        if matrix(i,j)==3
        else
            matrix(i,j)==0
        end
    end 
end
    

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M.S. Khan 2020-11-28

Then outcome should be like: 0 0 3 0 0 0 0 0 3 0

Regards for reply.

Walter Roberson 2020-11-28

Is the rule that you should alternate taking the first and last of each group? I see places in the file where there are a number of different groups on the same row.

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Answer 1

Ameer Hamza 2020-11-28

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https://ww2.mathworks.cn/matlabcentral/answers/666358-how-to-use-for-loop-or-any-other-loop-to-rearrange-elements-in-a-matrix-using-matlab#answer_558348

编辑：Ameer Hamza 2020-11-28

在 MATLAB Online 中打开

Try this

M = readmatrix('Boundary_closed_1s_3s.txt');
for i = 1:size(M,1)
    idx1 = find(M(i,:), 1, 'first');
    idx2 = find(M(i,:), 1, 'last');
    M(i, idx1+1:idx2-1) = 0;
end

imshow() of matrix before

after

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M.S. Khan 2020-11-28

i have marked the elements which i dont want because these elements dont represent the boundary of the sphere. i want to repalce them either with 1 or 0. Thanks for all cooperation.

M.S. Khan 2020-11-28

Dr. Ameer, i am trying to utilize your code of find function to introduce 0 between first '1' and last '1', so automatically 3 will be replaced with 0 but its not coming i dont know why.

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Answer 2

Walter Roberson 2020-11-28

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https://ww2.mathworks.cn/matlabcentral/answers/666358-how-to-use-for-loop-or-any-other-loop-to-rearrange-elements-in-a-matrix-using-matlab#answer_558728

在 MATLAB Online 中打开

The following code is untested.

M = readmatrix('Boundary_closed_1s_3s.txt');
position_of_first = sum(cumprod(~M, 2),2) + 1;
position_of_last = size(M,2) - sum(cumprod(fliplr(~M),2),2);
rows_with_data = find(position_of_last ~= 0);
newM = sparse([rows_with_data, rows_with_data], [position_of_first(rows_with_data), position_of_last(rows_with_data)], 1);

When you examine the results, I suspect you will want to redefine the problem. For example on row 4, that stretch of 1's will be replaced by just the first 1 and the last 1, but that will leave an empty upper boundary for rows 13 and 14.

I would suggest to you that you would be better of taking logical(M) and then using bwskel() https://www.mathworks.com/help/images/ref/bwskel.html or similar morphological operations.

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M.S. Khan 2020-11-29

Thanks Walter for your support. Problem is still there. can we use a for loop to locate the 3s which are coming in the next row or colmun to replace with o. i am thinking how to apply that approach.