Question

I am getting this error while running my code:

Error using ==> mldivide Matrix dimensions must agree.

Here is my code :

%make the plots of phase and group velocity vs discreteness of the grid
c=1;

a=input('Please enter the ratio cdt/dx : ')

figure(1)
R=2:40;
plot(R,phase_vel(R,a)/c)
xlabel('R=l/dx')
ylabel('u_phase/c')

%figure(2)
%plot(group_vel(R,a),R,0,40)
%xlabel('R=l/dx')
%ylabel('u_group/c')

and here are my functions :

function phase_velocity = phase_vel(R,a)
    %numerical phase velocity of the discrete wave 
    c=1;
    phase_velocity=(2*pi*c)/(R*knum(R,a));
end

function group_velocity =group_vel(R,a )
    %numerical group velocity of the discrete wave
    c=1;
    group_velocity=(a*sin(knum(R,a)))/(sin(2*pi*a/R))
end

function knumber = knum(R,a)
    %This is the k wave number
    knumber=acos((1/a)^2*(cos(2*pi*a/R)-1)+1);
end

How can I resolve this error?

EDIT: I used . operator in every equation and i changed the limits of R=4:40

Answer 1

If your goal is to apply your formulas to each individual value in the vector R then you should be performing all of your computations using the element-wise arithmetic operators .*, ./, and .^ instead of the matrix operators *, /, and ^.

Your error is probably occurring in the first call to your function knum, specifically when you try to compute 2*pi*a/R. Since 2*pi*a is a single scalar value, you get an error when trying to perform matrix right division / using the row vector R. The really weird thing is the error message:

??? Error using ==> mldivide
Matrix dimensions must agree.

which implies you are using the matrix left division operator \, which you clearly aren’t. I tested this in MATLAB R2010b and I get the same incorrect function name appearing in my message. I think this may just be a typo in the error message, and I’ve dropped a note to the MATLAB folks to take a look at it and clear it up.