Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

I have an "unable to perform assignment because the indices on the left side are not compatible with the size of the right side" error when i try to run this code. Could you help how can i solve this problem?

Error in (line 15)

z2(i,j)=f0+A0'*X;

clc;
clear;
a=[0.3; 0.7];
x0=[1; 1];
f0= 2*(a'*x0)+log(a'*x0);
A0= 2+(1/(a'*x0)) ;
B0= -(1/(a'*x0).^2);
x = 0.1: 0.05: 0.5;
[x1,x2] = meshgrid(x,x);
z1 = 2*(a(1,1)*x1+a(2,1)*x2)+log(a(1,1)*x1+a(2,1)*x2);
n=length(x1);
for i=1:n   
    for j=1:n
    X=[x1(i,j)-x0(1,1); x2(i,j)-x0(2,1)];
    z2(i,j)=f0+A0'*X;
    z3(i,j)=f0+A0'*X+0.5*X'*B0*X;
    end
end
colormap(gray);
mesh(x1,x2,z1)
hold on
mesh(x1,x2,z2)
mesh(x1,x2,z3)
xlabel('x1');
ylabel('x2');
hold off

Answer 1

[KSSV]

Your LHS is a single elememnt and RHS is a 1x2 matrix, which cannot be saved.

May be you want this?

a=[0.3; 0.7];
x0=[1; 1];
f0= 2*(a'*x0)+log(a'*x0);
A0= 2+(1/(a'*x0)) ;
B0= -(1/(a'*x0).^2);
x = 0.1: 0.05: 0.5;
[x1,x2] = meshgrid(x,x);
z1 = 2*(a(1,1)*x1+a(2,1)*x2)+log(a(1,1)*x1+a(2,1)*x2);
n=length(x1);
z2 = zeros(size(x1)) ;
z3 = zeros(size(x2)) ; 
for i=1:n   
    for j=1:n
        X=[x1(i,j)-x0(1,1); x2(i,j)-x0(2,1)];
        z2(i,j)=f0+A0'*X(1);
        z3(i,j)=f0+A0'*X(2)+0.5*X'*B0*X;
    end
end
colormap(gray);
mesh(x1,x2,z1)
hold on
mesh(x1,x2,z2)
mesh(x1,x2,z3)
xlabel('x1');
ylabel('x2');
hold off

[Anastasiya Burak]

Thank you, it works!

Answer 2

[Anis Syafiqah]

I also encounter the same "Unable to perform assignment because the indices on the left side are not compatible with the size of the right side" error. I tried preallocating it by using the zeros for numeric arrays but I still cannot solve it.

The error is at the %Exact Solution part (line 44)

FF(j)=fex(x);

Could somebody help me solve this problem?

% Runge Kutta of order 5

clc
clear

% y''=-4y+2, y(0)=1, y'(0)=1

fy=@(x,y,z) -2;
fz=@(x,y,z) 2-4*y;
fex =@(x)  ((1/2*sin(2*x))+(1/2*cos(2*x))+1/2);

x(1)=0;
z(1)=1;
y(1)=1;
h=0.1;
xfinal=1;
N=ceil((xfinal-x(1))/h);

for j=1:N
    x(j+1)=x(j)+h;

    k1y=fy(x(j),y(j),z(j));
    k1z=fz(x(j),y(j),z(j));
    k2y=fy(x(j),(y(j)+(1/3*h*k1y)),(z(j)+(1/3*h*k1y)));
    k2z=fz(x(j),(y(j)+(1/3*h*k1z)),(z(j)+(1/3*h*k1z)));
    k3y=fy(x(j),(y(j)+(2/3*h*k2y)),(z(j)+(2/3*h*k2y)));
    k3z=fz(x(j),(y(j)+(2/3*h*k2z)),(z(j)+(2/3*h*k2z)));
    k4y=fy(x(j),(y(j)+(h*((-167765027/45900120)*k1y + (43549/7217)*k2y-(30361/14840)*k3y))),(z(j)+(h*((-167765027/45900120)*k1y + (43549/7217)*k2y-(30361/14840)*k3y))));
    k4z=fz(x(j),(y(j)+(h*((-167765027/45900120)*k1z + (43549/7217)*k2z-(30361/14840)*k3z))),(z(j)+(h*((-167765027/45900120)*k1z + (43549/7217)*k2z-(30361/14840)*k3z))));
    k5y=fy(x(j),(y(j)+(h*((-51638854921283/28366716018615)*k1y + (35525/9169)*k2y - (27646/19955)*k3y- (10643/155037)*k4y))),(z(j)+(h*((-51638854921283/28366716018615)*k1y + (35525/9169)*k2y - (27646/19955)*k3y- (10643/155037)*k4y))));
    k5z=fz(x(j),(y(j)+(h*((-51638854921283/28366716018615)*k1z + (35525/9169)*k2z - (27646/19955)*k3z- (10643/155037)*k4z))),(z(j)+(h*((-51638854921283/28366716018615)*k1z + (35525/9169)*k2z - (27646/19955)*k3z- (10643/155037)*k4z))));
    k6y=fy(x(j),(y(j)+(h*((-9030268043/1401332565)*k1y + (736810/53619)*k2y - (28702/5227)*k3y- (7/5)*k4y +(3/5)*k5y))),(z(j)+(h*((-9030268043/1401332565)*k1y + (736810/53619)*k2y - (28702/5227)*k3y- (7/5)*k4y +(3/5)*k5y))));
    k6z=fz(x(j),(y(j)+(h*((-9030268043/1401332565)*k1z + (736810/53619)*k2z - (28702/5227)*k3z- (7/5)*k4z +(3/5)*k5z))),(z(j)+(h*((-9030268043/1401332565)*k1z + (736810/53619)*k2z - (28702/5227)*k3z- (7/5)*k4z +(3/5)*k5z))));
 
    y=zeros(size(k6y));
    z=zeros(size(k6z));

    y(j+1)=y(j)+(h/144)*((14*k1y)+(48*k2y)+(162*k3y)+(33*k4y)-(125*k5y)+(12*k6y));
    z(j+1)=z(j)+(h/144)*((14*k1z)+(48*k2z)+(162*k3z)+(33*k4z)-(125*k5z)+(12*k6z));

    %Exact solution
    FF(j)=fex(x);
    xx(j)=x;

    
    %Error
    Err(j) = abs(FF(j)-y(j));

  fprintf('%.1f\t\t%7.8f\t\t%7.8f\t\t%7.8f\n', xx(j), FF(j), y(j), Err(j))

end