MATLAB: Inner matrix dimensions must agree

electrical engineeringinner matrix must agree

This is my first time using Matlab, so I apologize if this is a noob sort of question that gets asked a thousand times.

Here is my m file:

   A1 = 4;
   A2 = -2;
   B1 = 3;
   B2 = 1;
   T1 = .1;
   T2 = .02;
   omega = 40 * pi;
   t = linspace(0,5*T1);
   overdamped = A1*exp(-t./T1)+A2*exp(-t./T2);
   critdamped = A1*exp(-t./T1)+(A2/T1)*t.*exp(-t./T1);
   underdamped = exp(-t./T1)*(B1*cos(omega*t.)+B2*sin(omega*t.));
   plot (t,overdamped)

I am trying to eventually have m file graph overdamped, critdamped, and underdamped on the same plot. I was trying to just get the first one to graph, which I can if I comment out the "critdamped = " and "underdamped = " lines, but if those are in there it throws the error "inner matrix dimensions must agree."

I'm having trouble seeing any errors – to me it seems like all the variables I am working with in those equations are 1×1, where x and t are the only vectors with dimension greater than 1.

Best Answer

That is so excepting when you get to

underdamped = exp(-t./T1)*(B1*cos(omega*t.)+B2*sin(omega*t.));

you're trying to multiply the damping by the waveform all of which are vectors. You're missing the "dot" on the .* to do element-wise multiplication of those two. "+" works without a ".+" operator as the operation is element-wise anyway and there isn't a secondary definition as matrix multiply overloads the "*" operator so have to have two different ones.

underdamped = exp(-t./T1).*(B1*cos(omega*t)+B2*sin(omega*t));

I corrected the extraneous 'dot' after the t in the omega*t terms, too, btw...

Related Solutions

MATLAB: Trying to find root of equation with fzero command in Matlab.

First of all, there is ABSOLUTELY no need to use global variables here. They are a crutch that will make your code poor, impossible to debug, hard to read and follow, etc. Avoid them, instead learning a better programming style at the same time.

Here, I've written dforce to take those variables as inputs. I've postpended the names of several variables with a _i to suggest that they are the i'th element of the external vectors ro and mu.

function W = dforce(v,ro_i,d,mu_i,A)
  Re = (ro_i*v*d)/mu_i;
  Cd = (24/Re)+(6/(1+sqrt(Re)))+0.4;
  W = Cd*0.5*ro_i*v^2*A;
end

See that when the function handle dF is created, it is passed the values of those parameters. I've also preallocated V.

m = 0.5;
g = 9.8;
N = m*g;
R = 287;
P = 101300;
d = 15;
A = (pi*d^2)/4;
T = linspace(-60,60,121);
NT = length(T);
mu = zeros(size(T));
ro = mu;
V = zeros(size(T));
% move the declaration of b1,b2,b3,b4 outside the loop, as they are constants.
b1 = 2.156954157e-14;
b2 = -5.332634033e-11;
b3 = 7.477905983e-8;
b4 = 2.527878788e-7;

Now, the VERY first thing I would do is to test your function. Dopes it make sense? Is there a root to be found? Don't throw a loop at something and expect it to work. Don't write a mess of code and then expect it all to work without any testing.

ALWAYS check things. Check EVERY line if necessary to make sure it makes sense.

i = 1;
mu(i) = b1*T(i)^3+b2*T(i)^2+b3*T(i)+b4;
ro(i) = P/(R*T(i));
dF = @(v)dforce(v,ro(i),d,mu(i),A)- N;

A very good way to check things is with a plot. Does it make sense? If not, then why not? This is how you should use MATLAB. Make sure that you understand everything, that you have made no mistakes. Then when you put it all together at the end, you can be confident that it will indeed work.

ezplot(dF,[-1,1])
Warning: Function failed to evaluate on array inputs; vectorizing the function may speed up its evaluation and avoid the need to loop over array elements. 
> In ezplot>ezplot1 at 484
  In ezplot at 144 
grid on

So, in fact, there is no root at all, at least not for the first value of T.

The point is, careful coding means that you work SLOWLY, one step at a time. Once you get more proficient at coding, you can do bigger chunks without needing to check quite as much.

MATLAB: How to create a vector with if-else statement

Bc(k) >= z(k)

Best Answer

Related Solutions

MATLAB: Trying to find root of equation with fzero command in Matlab.

MATLAB: How to create a vector with if-else statement

Related Question