The last line is wrong. The system reminds that "Input function must return 'double' or 'single' values. Found 'sym'." How can I obtain the constant matrix K after integral ?

%%%%%I would like to obtain the constant matrix K
clc
clear
h=0.03;
E=1*10^9;
miu=0.3;
df1=[1;0;0];
df2=[0;1;0];
e1=[1;0;0];
e2=[0;1;0];
e3=[0;0;1];
D=(E/(1-miu*miu))*[1,miu,0;miu,1,0;0,0,(1-miu)/2];

syms xta eta  
Bm=zeros(3,3,'sym');
Bb=zeros(3,3,'sym');
K=zeros(3,3,'sym');


M=@(xta)(xta>=0&xta<0.5).*(((0.5-xta)./0.5).*((0.5-xta)./0.5)); %% Define a piecewise function
DM=@(xta)(xta>=0&xta<0.5).*(8.*xta-4);                          %% Define the differential equation of first order
DDM=@(xta)(xta>=0&xta<0.5).*(8);                                %% Define the differential equation of second order

N=@(eta)(eta>=0&eta<0.5).*(((0.5-eta)./0.5).*((0.5-eta)./0.5));  %% Define another piecewise function
DN=@(eta)(eta>=0&eta<0.5).*(8.*eta-4);                           %% Define the differential equation of first order
DDN=@(eta)(eta>=0&eta<0.5).*(8);                                 %% Define the differential equation of second order


        f1=@(xta,eta)DM(xta).*N(eta);   %%%product a new  piecewise function based on previous piecewise functions
        f2=@(xta,eta)M(xta).*DN(eta);   %%%product a new  piecewise function based on previous piecewise functions

%%%%%Caculte the matrix Bm
        Bm=[(f1(xta,eta))*df1'*e1,(f1(xta,eta))*df1'*e2,(f1(xta,eta))*df1'*e3;
            (f2(xta,eta))*df2'*e1,(f2(xta,eta))*df2'*e2,(f2(xta,eta))*df2'*e3;
            ((f2(xta,eta))*df1'+(f1(xta,eta))*df2')*e1,((f2(xta,eta))*df1'+(f1(xta,eta))*df2')*e2,((f2(xta,eta))*df1'+(f1(xta,eta))*df2')*e3];
%%%%%%%%%%%%%%%%%

%%%%%Caculte the matrix Bb
        T=[0;0;1];
        g1=@(xta,eta)DDM(xta).*N(eta);
        g2=@(xta,eta)M(xta).*DDN(eta);
        g3=@(xta,eta)DM(xta).*DN(eta);
        Bb=-[(g1(xta,eta))*T'*e1,(g1(xta,eta))*T'*e2,(g1(xta,eta))*T'*e3;
             (g2(xta,eta))*T'*e1,(g2(xta,eta))*T'*e2,(g2(xta,eta))*T'*e3;
             2*(g3(xta,eta))*T'*e1,2*(g3(xta,eta))*T'*e2,2*(g3(xta,eta))*T'*e3];
%%%%%%%%%%%%%%%%%

        K0=(h.*Bm'*D.*Bm+(h*h*h/12)*Bb'*D.*Bb);
        K=integral2(@(xta,eta)K0,0,1,0,1) %%%calculte the intergration of K0
 %%% In theory,the matrix K should be a constant matrix after the integraion

Answer 1

[Torsten]

So many problems. Had to restructure the code completely.

value = zeros(3,3); 
for i=1:3
  for j=1:3
    value(i,j) = integral2(@(x,y)fun(x,y,i,j),0,1,0,1);
  end
end
value

function values = fun(XTA,ETA,I,J)
  for i=1:size(XTA,1)
    for j=1:size(XTA,2)
      xta=XTA(i,j);
      eta=ETA(i,j);

      h=0.03;
      E=1*10^9;
      miu=0.3;
      df1=[1;0;0];
      df2=[0;1;0];
      e1=[1;0;0];
      e2=[0;1;0];
      e3=[0;0;1];
      D=(E/(1-miu*miu))*[1,miu,0;miu,1,0;0,0,(1-miu)/2];

      M=(xta>=0&xta<0.5).*(((0.5-xta)./0.5).*((0.5-xta)./0.5)); %% Define a piecewise function
      DM=(xta>=0&xta<0.5).*(8.*xta-4);                          %% Define the differential equation of first order
      DDM=(xta>=0&xta<0.5).*(8);                                %% Define the differential equation of second order
      N=(eta>=0&eta<0.5).*(((0.5-eta)./0.5).*((0.5-eta)./0.5));  %% Define another piecewise function
      DN=(eta>=0&eta<0.5).*(8.*eta-4);                           %% Define the differential equation of first order
      DDN=(eta>=0&eta<0.5).*(8);                                 %% Define the differential equation of second order
      f1=DM.*N;   %%%product a new  piecewise function based on previous piecewise functions
      f2=M.*DN;   %%%product a new  piecewise function based on previous piecewise functions
%%%%%Caculte the matrix Bm
      Bm=[(f1)*df1'*e1,(f1)*df1'*e2,(f1)*df1'*e3;
          (f2)*df2'*e1,(f2)*df2'*e2,(f2)*df2'*e3;
          ((f2)*df1'+(f1)*df2')*e1,((f2)*df1'+(f1)*df2')*e2,((f2)*df1'+(f1)*df2')*e3];

%%%%%%%%%%%%%%%%%
%%%%%Caculte the matrix Bb
      T=[0;0;1];
      g1=DDM.*N;
      g2=M.*DDN;
      g3=DM.*DN;
      Bb=-[(g1)*T'*e1,(g1)*T'*e2,(g1)*T'*e3;
           (g2)*T'*e1,(g2)*T'*e2,(g2)*T'*e3;
           2*(g3)*T'*e1,2*(g3)*T'*e2,2*(g3)*T'*e3];
%%%%%%%%%%%%%%%%%
      K0=(h.*Bm'*D.*Bm+(h*h*h/12)*Bb'*D.*Bb);
      values(i,j) = K0(I,J);
    end
  end
end

[Dewen Yu]

Thank you very much! I have learned from it.