MATLAB: How to select certain columns of a matrix only when the values in certain rows showing the the double of the value in a particular row of a column

bigger ascolumns selection

Hi everybody!
I have a Matrix with 5 rows and 2300 columns M(5,2300).
0.2	0.3	0.4	2     ...
3	0.5	1.9	2.5   ...
2	0.7	0.2	3     ...
1	1	1	1     ...
0.5	0.1	1.5	3.1   ...
looks like above. The numbers you see are ratios because all the values of a particular column were divided by the value in the 4th row (which is now 1). I intend to just keep the columns for future analysis when at least one value of the other rows in a column (row1:3 and row5 values) are >=2, meaning that they should be at least the double of the value in the 4th row. As an output from the columns above it should give me an index like [1,0,0,1].
Could you probably suggest a nice solution? I'm thankful for every help.

Best Answer

Let A be your 5*n matrix. 
% GEt ratio
R = A(1:3,:)./A(5,:) ; 
idx = R>=2 ;   % get indices where ratio is >= 2
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