Question

I have a question regarding the fastest way to compute the RMSE between a single vector and an array of vectors. Specifically, I have a vector A representing an point and would like to find the index in a list B of points that A is closest to. Right now I am using:

    tempmat = bsxfun(@minus,A,B);
    tempmat1 = sqrt(sum(tempmat.^2,2);
    index = find(tempmat1 == min(tempmat1));

this takes about 0.058 seconds to calculate the index. Is there a faster way in MATLAB of doing this? I performing this calculations literally millions of times.

Many thanks for reading,
Joe

Answer 1

tempmat = bsxfun(@minus,A,B);
tmpmat1 = sum(tempmat.^2,2);
[m,index] = min(tempmat1);
m = sqrt(m); %# optional, only if you need the actual numerical value

This avoids calculating sqrt on the whole array, since the minumum of the squared differences will have the same index. It also uses the second output of min to avoid the second pass of find.