Number of elements error

Question

Mohammad Ezzad Hamdan 2019-3-18

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/450750-number-of-elements-error

评论： Matt J 2019-3-18

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;"

2 个评论
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Geoff Hayes 2019-3-18

在 MATLAB Online 中打开

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 2019-3-18

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.

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Answer 1

Matt J 2019-3-18

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/450750-number-of-elements-error#answer_365945

编辑：Matt J 2019-3-18

在 MATLAB Online 中打开

    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

4 个评论
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Mohammad Ezzad Hamdan 2019-3-18

still getting error message

Matt J 2019-3-18

在 MATLAB Online 中打开

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))