What causes an "Argument to dynamic structure reference must evaluate to a valid field name" error please? code attached.

This is my code. I have matlab error saying "Argument to dynamic structure reference must evaluate to a valid field name. Can anyone see a problem please?

 [x, y, z]=meshgrid(-0.3:0.01:3, -0.3:0.01:3, 0:0.01:1);

 s=0.6;

 zh=1;

 a=0;

   xa = -s/2;
   xb = s/2;
   ya = (-1/3)*s*cos(30);
   yb = (-1/3)*s*cos(30);
   yc = (2/3)*s*cos(30);
for j=1:length(x)
    for k=1:length(y)
        for h=1:length(z)
            a(j, k, h)=sqrt((xa-j).^2+(ya-k).^2+(-h).^2);
            t=90+asin(h./a)-asin((a/0.5)*(sin(90-asin(h./a))));
            ha=sqrt((a.^2+0.25)-(cos.(t).*a));
            hz=zh-ha;
        end
    end
end

 surf(x, y, hz)

Answer 1

[Walter Roberson]

Look at your line

ha=sqrt((a.^2+0.25)-(cos.(t).*a));

notice you have a "." between the "cos" and the "(t)". That syntax is generally valid but it has to do with structures; for your purpose just get rid of that "."

[Firew Biruu]

Thank you dear.