MATLAB: Hi, I’m new to Matlab. I keep getting Index exceeds matrix dimensions error. Maybe someone can help me with this.

jacobianmatrices

function [ J ] = J_springs( x )
%Jacobian Matrix Jf
%df/dx(i) df/dx(i-1) df/dx(i+1)
%Initialization
x=1:20
N=20
K=zeros(N,1);
b=zeros(N,1);
J=zeros(N,1);
for i = 1:N
    K(i) = (100+i-N/2); %N/cm
    b(i) = (-10+0.1*(i-N/2)); %N/cm^3 
end
for i = 2:N-2
    J(i,i) = -(K(i,i)+K(i+1,i))+3*b(i,i)*(x(i,i)-x(i-1,i))^2+3*b(i+1,i)*(x(i+1,i)-x(i,i))^2; 
end
%

for i = 2:N-1
    J(i,i-1) = K(i,i-1)-3*b(i,i-1)*(x(i,i-1)-x(i-1,i-1))^2;
end
%
for i = 1:N-2
    J(i,i+1) = K(i+1,i+1)-3*b(i+1,i+1)*(x(i+1,i+1)-x(i,i+1))^2; 
end
%  
end

Best Answer

K=zeros(N,1);
So K is a column vector with N rows.
for i = 1:N
  K(i) = (100+i-N/2); %N/cm
  b(i) = (-10+0.1*(i-N/2)); %N/cm^3 
end
so the first N entries in K are initialized (that is, all of it.)
for i = 2:N-2
  J(i,i) = -(K(i,i)+K(i+1,i))+3*b(i,i)*(x(i,i)-x(i-1,i))^2+3*b(i+1,i)*(x(i+1,i)-x(i,i))^2; 
end
So i starts from 2, so the right hand side immediate starts trying to evaluate K(i,i) which is K(2,2) but K only has one column.
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