Question

In MATLAB, is there a more concise way to handle discrete conditional indexing by column than using a for loop? Here’s my code:

x=[1 2 3;4 5 6;7 8 9];
w=[5 3 2];

q=zeros(3,1);
for i = 1:3
    q(i)=mean(x(x(:,i)>w(i),i));
end
q

My goal is to take the mean of the top x% of a set of values for each column. The above code works, but I’m just wondering if there is a more concise way to do it?

Answer 1

You mentioned that you were using the function PRCTILE, which would indicate that you have access to the Statistics Toolbox. This gives you yet another option for how you could solve your problem, using the function NANMEAN. In the following code, all the entries in x less than or equal to the threshold w for a column are set to NaN using BSXFUN, then the mean of each column is computed with NANMEAN:

x(bsxfun(@le,x,w)) = nan;
q = nanmean(x);