MATLAB: I am not able to get passed through shape function in matlab. Shape function contains symbolic variable which needs to be converted to vpa function and how to do that

galerkinpost

%this is half part of the code which is not running because of this following 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.
Error in Galerkin (line 10)
    N(i)=(x.^2-2*x).^i;
This is my code:
syms pi p q E A l x
n=3;
k=(n:n);
r=(1:n);
a=(1:n);
N=(1:n);
pi=eval(pi);
p=q*x/E*A*l;
for i=1:n  
      N(i)=(x.^2-2*x).^i;  %Shape function
      %N(i)=sin((2*i-1)/2*pi*x);
      %N(i)=piecewise(x>=(i-1)/n & x<1/n+(i-1)/n,n*(x-(i-1)/n),x>=i/n & x<=(i+1)/n & x<1,-n*x+i+1,0);
      Y1=(N(i));
      Y2=diff(N(i),x);
      Y3=diff(N(i),x,2);
      x=linspace(0,1);
      plot(x,Y1)
      hold on
      plot(x,Y2)
      hold on
      plot(x,Y3)
      hold off
      Legend(N,N',N'');
end

Best Answer

syms pi p q E A l x
n=3;
k=(n:n);
r=(1:n);
a=(1:n);
N = sym(1:n);
pi=eval(pi);
p=q*x/E*A*l;
for i=1:n  
        syms x
        N(i)=(x.^2-2*x).^i;  %Shape function
        %N(i)=sin((2*i-1)/2*pi*x);
        %N(i)=piecewise(x>=(i-1)/n & x<1/n+(i-1)/n,n*(x-(i-1)/n),x>=i/n & x<=(i+1)/n & x<1,-n*x+i+1,0);
        Y1=(N(i));
        Y2=diff(N(i),x);
        Y3=diff(N(i),x,2);
        x=linspace(0,1);
        plot(x,subs(Y1))
        hold on
        plot(x,subs(Y2))
        hold on
        plot(x,subs(Y3))
        hold off
        legend('N','N''','N''''');
        pause(1);
end
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