I have a problem with square bracket in solving ode45

Hello, as in the title I have small problem with square brackets. I try to solve 6 ordinary equations using function ode45 but it gives me an error "dimensions of matrices being concatenated are not consistent" and i check that 3 equations give only one value, but other 3 equations give two values (first number is the results of solving equation and second number is 0). Here is my question how to remove this zero value as result. I check these equations and without square bracket they give one value.

here is my code

function xprime = lorenz(t,x);
alfa = 0.017; 
beta = 0.067; 
tau_e = 0.017; 
tau_l = 0.0001;
tau_j =0.00000001;
tau_k =0.00000001;
c = 0.000001;
p = 0.000001;
d = 0.000001;

xprime = [c*x(2) + x(3)*tau_k -(tau_l*x(1) - tau_j*x(1)); 
    d*x(3) + x(1)*tau_j - (tau_e*x(2)-c*x(2)); 
    tau_l*x(1) + tau_e*x(2) + beta*x(4) -(x(3)*alfa + x(3)*d + x(3)*tau_k); 
    x(3)*alfa + x(5)*tau_e + tau_l*x(6) -(x(4)*beta+x(4)*p+x(4)*tau_k); 
    x(4)*p + x(6)*tau_j - (x(5)*tau_e + x(5)*c); 
    x(5)*c + x(4)*tau_k-(x(6)*tau_l + x(6)*tau_j)];

I run the code this way [t,x]=ode45(@lorenz,[0 500],[0 0 0.8 0.2 0 0])

Answer 1

[Torsten]

function xprime = lorenz(t,x);

alfa = 0.017; 
beta = 0.067; 
tau_e = 0.017; 
tau_l = 0.0001;
tau_j =0.00000001;
tau_k =0.00000001;
c = 0.000001;
p = 0.000001;
d = 0.000001;

xprime = zeros(6,1);
xprime(1) = c*x(2) + x(3)*tau_k -(tau_l*x(1) - tau_j*x(1)); 
xprime(2) = d*x(3) + x(1)*tau_j - (tau_e*x(2)-c*x(2)); 
xprime(3) = tau_l*x(1) + tau_e*x(2) + beta*x(4) -(x(3)*alfa + x(3)*d + x(3)*tau_k); 
xprime(4) = x(3)*alfa + x(5)*tau_e + tau_l*x(6) -(x(4)*beta+x(4)*p+x(4)*tau_k); 
xprime(5) = x(4)*p + x(6)*tau_j - (x(5)*tau_e + x(5)*c); 
xprime(6) = x(5)*c + x(4)*tau_k-(x(6)*tau_l + x(6)*tau_j);

Best wishes

Torsten.

[Paulina Urban]

Thank You :) now everything work

Answer 2

[Jan]

This is an evil behavior of Matlab, which tries to be smart:

a = 1;
b = 2;
c1 = [a - b]   % -1
c2 = [a -b]    % [1, -2]
c3 = [a-b]     % -1

It is even hard to describe this in clear words: If a minus occurs without a separating space before an expression, but a space before, it is assumed to be a unary minus. Then including the expression in square brackets the corresponding term is assumed to be an extra element. To avoid this use commas to separate elements in a vector concatenated by square brackets and insert two space around each operator:

xprime = [c * x(2) + x(3) * tau_k - (tau_l * x(1) - tau_j * x(1)); ...
  d * x(3) + x(1) * tau_j - (tau_e * x(2) - c * x(2)); ...
  tau_l * x(1) + tau_e * x(2) + beta * x(4) - (x(3) * alfa + x(3) * d + x(3) * tau_k); ...
  x(3) * alfa + x(5) * tau_e + tau_l * x(6) - (x(4) * beta + x(4) * p+x(4) * tau_k); ...
  x(4) * p + x(6) * tau_j - (x(5) * tau_e + x(5) * c); ...
  x(5) * c + x(4) * tau_k - (x(6) * tau_l + x(6) * tau_j)];

This differs from your version only by spaces and to be secure the line continuation is written explicitly by "..." .

See also the not obvious meaning of:

2./a

Is this the elementwise division of 2 by the vector 2 (valid) or is it the abbreviated 2.0 divided by the vector (error)?

Summary: Insert spaces around operators with 2 arguments. Saving some key-presses during typing is not useful, if it causes ambiguities.