MATLAB: What is meant by this syntax : ( another variable in brackets after a variable)

Question

Jmatf11(fmag>0) = (fx(fmag>0)./fmag(fmag>0)).^2 .* Jmf(fmag>0);

Answer 1

variable(expression) is indexing. In that expression it is "logical indexing". See https://www.mathworks.com/help/matlab/math/matrix-indexing.html#bq7egb6-1