How to find specific elements in a matrix

Question

Kendal 2022-11-8

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1846573-how-to-find-specific-elements-in-a-matrix

回答： Walter Roberson 2022-11-8

Good Day,

I have a matrix: R55 = [2,4,6,9,11;7,8,9,10,6;1,2,6,4,5;,6,9,8,5,6;1,6,5,7,5];

I would like to find the specific locations of all the 6's (rows and colums) using a for loop and using logical matrices rather than using just the "find" function.

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Answer 1

Voss 2022-11-8

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https://ww2.mathworks.cn/matlabcentral/answers/1846573-how-to-find-specific-elements-in-a-matrix#answer_1094528

在 MATLAB Online 中打开

R55 = [2,4,6,9,11;7,8,9,10,6;1,2,6,4,5;,6,9,8,5,6;1,6,5,7,5]
R55 = 5×5
     2     4     6     9    11
     7     8     9    10     6
     1     2     6     4     5
     6     9     8     5     6
     1     6     5     7     5
val = 6;

Using a for loop:

rows = [];
columns = [];
m = size(R55,1);
for ii = 1:numel(R55)
    if R55(ii) == val
        rows(end+1) = mod(ii-1,m)+1;
        columns(end+1) = ceil(ii/m);
    end
end
rows, columns
rows = 1×6
     4     5     1     3     2     4
columns = 1×6
     1     2     3     3     5     5

Using two nested for loops:

rows = [];
columns = [];
[m,n] = size(R55);
for jj = 1:n
    for ii = 1:m
        if R55(ii,jj) == val
            rows(end+1) = ii;
            columns(end+1) = jj;
        end
    end
end
rows, columns
rows = 1×6
     4     5     1     3     2     4
columns = 1×6
     1     2     3     3     5     5

Using a logical matrix, without find:

is_val = R55 == val
is_val = 5×5 logical array
   0   0   1   0   0
   0   0   0   0   1
   0   0   1   0   0
   1   0   0   0   1
   0   1   0   0   0
idx = 1:numel(R55);
[rows,columns] = ind2sub(size(R55),idx(is_val))
rows = 1×6
     4     5     1     3     2     4
columns = 1×6
     1     2     3     3     5     5

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Answer 2

Walter Roberson 2022-11-8

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https://ww2.mathworks.cn/matlabcentral/answers/1846573-how-to-find-specific-elements-in-a-matrix#answer_1094533

This kind of task is usually easier to write with nested for loops over size() of the array. If you are required to use a single for loop then loop over the linear indices and ind2sub to translate to row and column