You haven't quite given us enough information about your code, because that little code snippet may or may not work, depending on what D looks like before the loop. In the code below, I show three possible examples of D. The first two work, and the third one does not. I expect your full code defines D to be the wrong size going into the loop.
% Some made-up data
rng default
X = rand(120,120);
% D is not yet defined
disp("D not defined")
% Your code
n=20;
for i = 1:n
D(i,:) = min(X(i,:)) + max(X(i,:)) - X(i,:);
end
% An example of D that works
D = rand(1,120);
disp("D is 1x30")
% Your code
n=20;
for i = 1:n
D(i,:) = min(X(i,:)) + max(X(i,:)) - X(i,:);
end
% An example of D that does not work
D = rand(1,1);
disp("D is 1x1")
% Your code
n=20;
for i = 1:n
D(i,:) = min(X(i,:)) + max(X(i,:)) - X(i,:);
end
