Error : '' In an assignment A(I) = B, the number of elements in B and I must be the same ''

I want to draw the phase portrait of pendulum but its giving an error. code is like this

    function derivative_x = myfunc(t,x)
    derivative_x=zeros(2,1);

% prameters

    g=10;
    m= 2;
    M= 8;
    a= 1/10;
    l=1/2;
    K1 = [- 429.6222 -122.8607] ;
    K2 = [- 913.7748 - 284.1638];

              derivative_x(1)= x(2);
 error ==>>   derivative_x(2)= (g .* sin(x(1)) - 0.5.*a.*m.*l.*(x(2)^2).*sin(2*x(1))-    (a.*cos(x(1)).*(K1.*x(1) +K2.*x(2))))/(1.33.*l-a.*m.*l.*(cos(x(1))^2));

Initial_Time=0;
Final_Time=20;
[x1,x2] = meshgrid(-2.5:0.9:4);
for i = 1:length(x1)
for j = 1:length(x1)
x10 = x1(i,j);
x20 = x2(i,j);
[t,x] = ode45(@myfunc, [Initial_Time Final_Time], [x10 x20]);
plot(x(:,1),x(:,2))
axis([-2.5 4 -2.5 4])
xlabel('x1','fontsize',30,'fontweight','bold')
ylabel('x2','fontsize',30,'fontweight','bold')
hold on
end
end

but its giving an error In an assignment A(I) = B, the number of elements in B and I must be the same. kindly help....!

[pfb]

that means exactly what it says. You're "equalling" two objects that have a different number of elements.

The error message should come with a line number, which should help you (and us) to pinpoint it.

[Ahmad Sheikh]

error is at this line

derivative_x(2)= (g .* sin(x(1)) - 0.5.*a.*m.*l.*(x(2)^2).*sin(2*x(1))-(a.*cos(x(1)).*(K1.*x(1) +K2.*x(2))))/(1.33.*l-a.*m.*l.*(cos(x(1))^2));

Answer 1

[Jan]

This is a scalar:

derivative_x(2)

This is scalar also:

 (g .* sin(x(1)) - 0.5.*a.*m.*l.*(x(2)^2).*sin(2*x(1))- ...

This is a [1 x 2] vector:

(a.*cos(x(1)).*(K1.*x(1) +K2.*x(2))))/ ...

and the rest is scalar again:

(1.33.*l-a.*m.*l.*(cos(x(1))^2));

Therefore the right hand side is a [1 x 2] vector and cannot be assigned to a scalar.

[Ahmad Sheikh]

Thanks Jan your info helped...!