For loop with symbolic expressions

I am trying to make a for loop that put symbolic expressions inside a matrix ( in positions given by index of for loop) and i get this error:

    ??? The following error occurred converting from sym to double:
Error using ==> mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use the VPA function instead.

Can you help me ?

Thanks in advance,

[sixwwwwww]

Can you please share your code to locate the error?

[rodrigo]

Yes, my code is above:

%%%%%%%%%%%%%%%%%%%%%%%%%

k=1; matriz_incognitas=zeros(6+4*k,1);

syms x syms theta L=2

Ni=1-(x/L) Nj=x/L

matriz_N=zeros(3,6+4*k);

for m=1:k

    matriz_N(1,4+m-1)= 0
    matriz_N(1,4+m+k-1)=sprintf(Ni*cos(k+1)*theta

    matriz_N(2,4+m-1)= -Ni*sin(k+1)*theta*(1/(k+1))
    matriz_N(2,4+m+k-1)= 0

    matriz_N(3,4+m-1)=  Ni*cos(k+1)*theta
    matriz_N(3,4+m+k-1)= 0

    matriz_N(1,4+m-1)= 0
    matriz_N(1,4+m+k-1)=Nj*cos(k+1)*theta

    matriz_N(2,4+m-1)= -Nj*sin(k+1)*theta*(1/(k+1))
    matriz_N(2,4+m+k-1)= 0

    matriz_N(3,4+m-1)=  Nj*cos(k+1)*theta
    matriz_N(3,4+m+k-1)= 0

end

%%%%%%%%%%%%%%%%%%%%%%%%%%

Answer 1

[Walter Roberson]

Do not initialize the matrix to zeros(): that requires numeric values instead of symbolic.

Try

matriz_N = sym(zeros(3, 6+4*k));