How do I solve this error?

My teacher put in (from what I can tell) an identical code but I can't get it working. Can somebody help me make it work and explain the error?

n1 = 1; %utanför linsen
n2 = 1.5; %i linsen

D = 10; %linsens diameter (cm)
R = D./2; %linsens radie (cm)
d = 10; %fokallängd (cm)

lopt = n1*sqrt(d.^2 + R.^2); %Optiska vägen då strålen inte bryts i linsen

Y = linspace(0, R, 1000);
X = linspace(0, d, 1000);

for ii = 1:length(Y)
    L1(:,ii) = sqrt( (d-X).^2 + Y(ii).^2)*n1;
    L2(:,ii) = X*n2;
    LoptTemp(:,ii) = L1(:,ii) + L2(:,ii);
    diff(:,ii) = abs(LoptTemp(:,ii) - lopt);
    [difference minIndex(ii)] = min(diff(:,ii));
    x_fit(ii) = X(minIndex(ii));
end

figure
plot(x_fit, Y, 'black');
hold on
plot(x_fit.*-1, Y, 'black');
hold on
plot(x_fit, -Y, 'black');
hold on
plot(x_fit.*-1, -Y, 'black');
hold on
ylim([-max(Y)-3, max(Y)+3]);

The error message is:

Conversion to function_handle from double is not possible.

Error in OptikVaglaraUppgift2 (line 75)

LoptTemp(:,ii) = L1(:,ii) + L2(:,ii);

Grateful for help!

Answer 1

[Jan]

The error message means, that the variable LoptTemp has been defined as function handle before.

A pre-allocation solves the problem:

LoptTemp = zeros(length(Y), length(Y));

If you insert this code in a function, the workspace is clean and formerly defined variables to not influence the stability.

[Axelina Lindgren]

Thank you so much for your help! Works perfectly now.