How I can find the indices of 4 consecutive elements in the same row?

Question

Osama Hussein 2016-3-8

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评论： Image Analyst 2016-3-8

I have a big binary matrix, and I try to find the indices of certain elements. Let's say for example I have this matrix

SA = [ 0 0 0 0 0 1 1 0 0 0 1 0 1 0 1 1 0 0 1 1; 
       1 0 0 0 1 1 1 0 0 0 1 0 1 1 1 0 1 0 0 1;
       0 1 0 0 0 0 1 0 1 1 1 0 1 0 1 0 0 1 1 1];

each 4 consecutive elements is considered together, so how I can find the indices of 1 0 1 0? I just need the indices for the first element, and assume no repetition for the same consecutive 4 elements!

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Answer 1

KSSV 2016-3-8

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https://ww2.mathworks.cn/matlabcentral/answers/272021-how-i-can-find-the-indices-of-4-consecutive-elements-in-the-same-row#answer_212734

在 MATLAB Online 中打开

clc; clear all 
SA = [ 0 0 0 0 0 1 1 0 0 0 1 0 1 0 1 1 0 0 1 1; 
     1 0 0 0 1 1 1 0 0 0 1 0 1 1 1 0 1 0 0 1;
     0 1 0 0 0 0 1 0 1 1 1 0 1 0 1 0 0 1 1 1];   % Your matrix
[m,n] = size(SA) ;                                 % Dimensions of your matrix
B = [1 0 1 0] ;                                    % Matrix to compare 
myidx = [] ;                              % Initialize your indices needed                                         
% Loop for each row and column 
for i = 1:m 
    for j = 1:n-length(B)
        if SA(i,j) == B(1)
            if SA(i,j+1)==B(2) && SA(i,j+2) == B(3) && SA(i,j+3) == B(4) 
                myidx = [myidx ; [i,j]] ;
            end
        end
     end
 end

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Osama Hussein 2016-3-8

Thank you, The code works, I just need to choose the indices which begins at 1 or 5 or 9 ... since each 4 consecutive elements are together. I think I can do this, Thank you very much :)

Stephen23 2016-3-8

See Image Analyst's answer for a much faster and neater solution.

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Answer 2

Image Analyst 2016-3-8

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https://ww2.mathworks.cn/matlabcentral/answers/272021-how-i-can-find-the-indices-of-4-consecutive-elements-in-the-same-row#answer_212736

在 MATLAB Online 中打开

I offer a much simpler solution:

for row = 1 : size(SA, 1)   
  columns{row} = strfind(SA(row,:), [1,0,1,0])
end

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Stephen23 2016-3-8

编辑：Stephen23 2016-3-8

在 MATLAB Online 中打开

Columns is simply a cell array of the column indices. You could even do it on one line using cellfun:

col = cellfun(@(v)strfind(v,[1,0,1,0]),num2cell(SA,2),'Uni',0);

The answer you accepted has two nested loops, thirteen lines of code, and multiple temporary variables. MATLAB code does not need to be so complicated to perform trivial tasks like this!

Image Analyst 2016-3-8

在 MATLAB Online 中打开

Thanks Stephen. Osama, look at the output of it:

columns = 
    [11]    [15]    [1x2 double]

So columns{1} tells where 1 0 1 0 shows up in row #1. You can see that that happens at column #11 in row #1. So that one is correct.

columns{2} tells where 1 0 1 0 shows up in row #2. You can see that that happens at column #15 in row #2. So that one is also correct.

columns{3} is a 2 element array which is [11, 13]. It tells where 1 0 1 0 shows up in row #3. You can see that that happens both at column #11 and at column #13 in row #3. So that one is also correct.

So why do you think it may be giving you the wrong answer? What columns do you think the pattern should show up in? What do you think the right answer should be?

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