MATLAB: Replace the elements of a matrix under specific conditions

Answer 1

You can't do this

-f_y/Es < Esi < f_y/Es

You need to do it in two steps, like (-f_y/Es < Esi) & (Exi < f_y/Es). So try this:

fsi=zeros(size(Esi));
fsi(Esi>=f_y/Es)=f_y;
fsi(Esi<=-f_y/Es)=-f_y;
fsi((-f_y/Es < Esi) & (Exi < f_y/Es)) = Esi * Es;

Or perhaps a little more readable and less cryptic for people reading your code.

fsi = zeros(size(Esi));
indexesToReplace = Esi >= f_y/Es;
fsi(indexesToReplace) = f_y;
indexesToReplace = Esi <= -f_y/Es;
fsi(indexesToReplace) = -f_y;
indexesToReplace = (-f_y/Es < Esi) & (Exi < f_y/Es);
fsi(indexesToReplace) = Esi * Es;