Problem in converting expression into double array and error in converting from sym to double

Hi,

I have written a code to get Lagrange Interpolation functions used in FInite element analysis. Below is the code.

% lagrange interpolation functions
syms r;
r_e=input('enter the natural co-ordinate values of r in the order of nodes as row vector=');
for i=1:length(r_e)
    for j=1:length(r_e)
        if j~=i
            N_R(j)=((r-r_e(j)))/(r_e(i)-r_e(j));
        else
            N_R(j)=1;
        end
    end
    N_r(i)=prod(N);
end

But it shows these error when i enter the inputs!

enter the natural co-ordinate values of r in the order of nodes as row vector=[-1 1]

The following error occurred converting from sym to double:

Unable to convert expression into double array.

Error in lagrange (line 8)

N_R(j)=((r-r_e(j)))/(r_e(i)-r_e(j));

Please suggest me a solution.

[madhan ravi]

What should be the final size?

[SUBASH K]

N_r(j) size is 1*1

what am trying to do is to fromulate this

If size is the Problem then where am i doing wrong, how to maitain vector or matrix size in loops?

Answer 1

[Walter Roberson]

Initialize N_R= sym(0) before the loops

[SUBASH K]

Thank you very much.It works flawlessly.

Since the final output of N_R is symbolic, so we are supposed to initilize in the form of symbols. Is this could be the reason?

[Walter Roberson]

The type of a variable is determined by the first assignment. In most cases the assignment is an expression involving the symbolic variable r, but you start with i and j both 1 so you first do the else of i~=j and you assign 1 in that case which is numeric not symbolic. With the array not having been initialized that made the whole array numeric.