A problem with differentiating

I want to differentiate detk to f.But after I run it,my matrix c is empty. Can anyone tell me what is wrong? Thanks!

f = 0.001;

x = 1:f:5;

w = 2*pi*x;

k = [cos(w), 0; 0, cos(w) ];

detk = k(1,1)*k(2,2)-k(2,1)*k(1,2);

c = diff(detk)/f;

plot(x,detk,x,[0 c],'r:')

Answer 1

[per isakson]

detk is evaluated to a scalar and thus c will be empty.

I find it easier to debug functions than scripts. Thus, I created a function, cssm. Next I set a break point at the line, w = 2*pi*x;, and stepped through the remaining lines. The values of the variables may be inspected in many ways. I kept an eye on the Workspace window while stepping.

    function cssm
        f = 0.001;
        x = 1:f:5;
        w = 2*pi*x;
        k = [cos(w), 0; 0, cos(w) ];
        detk = k(1,1)*k(2,2)-k(2,1)*k(1,2);
        c = diff(detk)/f;
        plot( x,detk, x,[0 c], 'r:' )
    end

Here are some links on debugging in Matlab

• Debugging in MATLAB
• Debugging session
• Debugging Approaches