MATLAB symbolic to double error

Hi, I am trying to run this code but there are errors. I don't know how to remove them. Here is code: This error comes when converting symbolic to double. Any help much appreciated.

clear all;
clc;

nel=4; % number of elements 
totlength=10; 

nne=3; 
nnp=2*nel+1; 
le=totlength/nel; 
x=0:le:totlength; 

element_nodes=[1:2:(nnp-2);2:2:(nnp-1);3:2:nnp]';

%element_nodes=[1:nel;2:nnp]'; 

B=[];

syms xlc

%Finding out B matrix
for i=1:1:nel
    j=element_nodes(i,1)
    B(i)=[(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)];
end

Any help or suggestions? Thanks.

Answer 1

[Walter Roberson]

Experiment with starting with

B = sym([]);

I think you are going to have trouble with your assignment to B(i) anyhow, as you appear to be attempting to assign a list of 3 elements into a single storage location.

[Andrew]

This gives different error, some index mismatch.

[Walter Roberson]

Right. Because you are trying to store three elements into a single location. Notice the [] on the right hand side of your B(i): it indicates you are building a list of values. You think more about what your want your B to look like at the end.

[Andrew]

I want B to be matrix at the end, n rows by 3 columns. If I write code,

[(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)]

it gives correct answer, but I want to build a matrix.

[Andrew]

I modified my code. Instead of writing 3 values in 1 row, I created two nested loops to write values at each location. But still error is present. Here is code:

clear all;
clc;

nel=4; % number of elements 
totlength=10; 

nne=3; 
nnp=2*nel+1; 
le=totlength/nel; 
x=0:le:totlength; 

element_nodes=[1:2:(nnp-2);2:2:(nnp-1);3:2:nnp]';

%element_nodes=[1:nel;2:nnp]'; 

B=[];

syms xlc

%Finding out B matrix
for i=1:1:nel
    k=1:1:nne
    j=element_nodes(i,1);
    if(k==1)
        B(i,k)=vpa((2*xlc-x(i+2)-x(j+1))/(2*le^2));
    elseif (k==2)
        B(i,k)=vpa((2*xlc-x(j+2)-x(i))/(-1*le^2));
    else
        B(i,k)=vpa((2*xlc-x(j+1)-x(i))/(2*le^2));
    end
    %B(i)=[(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)];
end

[Walter Roberson]

 B = sym([]);

And in the loop,

B(i,:) = [(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)];