Use conditions on a matrix and manipulate the results

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Hello,
I have a matrix:
a = [1 1 1 1; 0.99 1 1 1; 0.98 0.99 1 1; 0.97 0.99 0.99 0.99]
I would like to obtain the following result:
b = [0 0 0 0; 2 0 0 0; 0 3 0 0; 0 0 4 4]
It means that at the first time when 1 changes to 0.99 look up which row is it and write it in the 'b' matrix at the same place. If this condition is not true, write zero.
Thank you!

Accepted Answer

Star Strider
Star Strider on 29 Sep 2014
This works:
a = [1 1 1 1; 0.99 1 1 1; 0.98 0.99 1 1; 0.97 0.99 0.99 0.99];
q1 = a(1:end-1,:) - a(2:end,:); % Subtract Rows From Offset Rows
q2 = a(2:end,:)==0.99; % Find Offset Rows = 0.99
q3 = [zeros(1,size(a,2)); q1 & q2]; % Create Logical Matrix (Cond 1 & Cond 2)
b = bsxfun(@times, q3, [1:size(a,1)]'); % Use ‘bsxfun’ To Number The Row Entries

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