Matlab Find() Function

Hello, I have a matrix -

M = [−1 −1 0 0 ; 1 0 −1 0 ; 0 1 1 −1; 0 0 0 1 ; 1 −1 0 0 ]

Basically I have to go along the first row and find an element value of "-1", then look along the columns which the value "-1" was found and look for value "1". I have then to sum the row index of the column which the value "1" was found. (Loop of sorts) until I've reached the end.

Hope that makes sense and thanks

[Yahia Gebril]

Also forgot to add, this code should be applicable for any matrix not just the one given

[Image Analyst]

Is there always a 1 directly below the "first -1 after a 1"?

Did you try a simple for loop?

[Yahia Gebril]

Yeah so u go from -1 to 1 then to -1. This is what i have so far, what do you think? Also how would the For loop work?

Answer 1

[Image Analyst]

Do you mean like this:

M = [-1 -1 0 0 ; 1 0 -1 0 ; 0 1 1 -1; 0 0 0 1 ; 1 -1 0 0 ]
[rows, columns] = size(M)
output = zeros(rows, 2); % List of starting column and ending column for every row.
col1 = find(M(1, :) == -1, 1, 'first');
output(1, 1) = col1;
for row = 2 : rows
    col2 = col1 - 1 + find(M(row, col1 : end) == -1, 1, 'first');
    if isempty(col2)
        % No more results found.  Reached last column of array.
        break;
    end
    % Save results
    output(row, 1) = col1;
    output(row, 2) = col2;
    col1 = col2;
end
output

Not sure why you're wanting the last 1 in row 4 since there is no -1 that follows it.

[Yahia Gebril]

That looks pretty solid, however for the output, I have to keep a sum of the row position that the number "1" is found. For example, in the matrix given previously, it would be D=1+3+4=8. But the answer given is great man, thank you so much for your time.

.

[Image Analyst]

You can use cumsum() or sum() at the end to sum up all the individual indexes.