Number of elements error

p = [2 3 4 5 6 7 8 9 10];
L2 = zeros(size(p));
xchange = zeros(size(p));
f = @(x) (cos(2*x)).*(sin(x)-0.5*x.^2);

for j = 1:length(p)
    n = 2^(p(j));
    x = linspace(-3,3,n);
    dx = x(2)-x(1);
    L = 0;
    ff = [];
    for i = 3:(length(x)-2)
        ff = (-f(x(i+2))+6*f(x(i+1))-3*f(x(i))-2*f(x(i-1)))/(6*dx);
        %ff = (-1*x(i+2)+6*x(i+1)-3*x(i)-2*x(i-1))/(6*dx);
        L(i) = dx*((f(x(i))-ff)^2);
        L = L + L(i);
        
    end
    %ff(1:2) = ff(3);
    %ff(n-1:n) = ff(n-2);
    L2(j) = L.^0.5;
    xchange(j) = dx;
end

plot(log(xchange),log(L2))
hold on

Hi, can anyone help me with this. I have been spending hours to solve the error printed in the command window below;

"Unable to perform assignment because the left and right sides have a different number of

elements.

Error in Untitled2 (line 24)

L2(j) = L.^0.5;"

[Geoff Hayes]

Mohammad - in the line of code

L2(j) = L.^0.5;

L2 is a 9x9 matrix and L is an array of different lengths (depending upon p). So the above code is trying to assign an array to a scalar element of your L2 matrix. Do you really mean to assign an array here? Should L2 be a cell array so that you can assign differently sized vectors/arrays to it?

[Mohammad Ezzad Hamdan]

Thank you for your reply,

i have changed the two lines below....

L2 = zeros(1,length(p));

xchange = zeros(1,length(p));

... and yet still getting the same error message.

Answer 1

[Matt J]

    for i = 3:(length(x)-2)

        ff(i) = (-1*x(i+2)+6*x(i+1)-3*x(i)-2*x(i-1))/(6*dx);
        
        L = L + dx*((f(x(i))-ff(i))^2);
        
    end

[Mohammad Ezzad Hamdan]

x(i+2) , x(i+1), x(i) and x(i-1) terms should be substituted in f from the 4th line.

[Matt J]

Please Accept-click the answer if you are not getting the error anymore.

[Mohammad Ezzad Hamdan]

still getting error message

[Matt J]

It runs fine for me.

p = [2 3 4 5 6 7 8 9 10];
L2 = zeros(size(p));
xchange = zeros(size(p));
f = @(x) (cos(2*x)).*(sin(x)-0.5*x.^2);
for j = 1:length(p)
    n = 2^(p(j));
    x = linspace(-3,3,n);
    dx = x(2)-x(1);
    L = 0;
    ff = [];
    for i = 3:(length(x)-2)
        ff = (-f(x(i+2))+6*f(x(i+1))-3*f(x(i))-2*f(x(i-1)))/(6*dx);
        %ff = (-1*x(i+2)+6*x(i+1)-3*x(i)-2*x(i-1))/(6*dx);
      
        L = L + dx*((f(x(i))-ff)^2);
        
    end
    %ff(1:2) = ff(3);
    %ff(n-1:n) = ff(n-2);
    L2(j) = L.^0.5;
    xchange(j) = dx;
end
plot(log(xchange),log(L2))