A few comments:
- a matrix of size [52, 141, 43] has 52 rows, 141 columns and 43 pages.
- find is frequently over used and unneeded. The code
B = ex==275 & em==310; %logical array instead of indices
Bpeak = Xgrid(B)
would produce the exact same result faster.
Now to answer your question, you need to permute the dimensions of B so you have a 1x141x43 array, then repmat it across the rows, so:
B = ex==275 & em==310;
B = permute(B, [3 1 2]);
B = repmat(B, 52, 1, 1);
Bpeak = Xgrid(B)
