Error in Unable to perform assignment because the left and right sides have a different number of elements.

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Zoe Westcott 2022-7-9

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https://ww2.mathworks.cn/matlabcentral/answers/1756500-error-in-unable-to-perform-assignment-because-the-left-and-right-sides-have-a-different-number-of-el

回答： Image Analyst 2022-7-9

I'm having an issue with trying to work out a error for a finte difference equation but keep getting the error of 'Unable to perform assignment because the left and right sides have a different number of elements'.

 A= (32*pi)/7 ;
f =  @(x) 0.*( x<25/64 & x>39/64) +cos(A*(x-0.5)).*( (25/64)<=x & x<=39/64);
g = @(x) 0.*( x<25/64 & x>=39/64) +-A*sin(A*(x-0.5)).*( (25/64)<x & x<=39/64);
p=-12:1:-5;
h=2.^p;
x = 0:h:1;   
n = length(x);
for m=1:1 
y=(f(x)).^m;
z=g(x).*m.*(f(x).^(m-1));
dy = zeros(n,1);
 for i=3:n-2
    dy(i) = (y(i-2)-8*y(i-1)+8*y(i+1)-y(i+2))./(12*h)        ;
end
e= (abs(z-dy'));     % The error function 
% Now when you plot the derivatives, skip the first and the last point
figure;
plot(x,y,'r');
hold on;
pause
plot(x(2:end-1), dy(2:end-1),'b');
grid on;
pause
plot(x,z,'g');
plot(x,e,'k'); 
legend('cos(x)', 'dy', '-sinx','Error')
hold off  
end
Unable to perform assignment because the left and right sides have a different number of elements.

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Answer 1

Image Analyst 2022-7-9

在 MATLAB Online 中打开

Did you try normal debugging techniques?

Debugging in MATLAB | Doug's MATLAB Video Tutorials

like this:

tempVar = (y(i-2)-8*y(i-1)+8*y(i+1)-y(i+2))./(12*h);
whos h
whos tempVar
dy(i) = (y(i-2)-8*y(i-1)+8*y(i+1)-y(i+2))./(12*h);

Note that

Name Size Bytes Class Attributes

h 1x8 64 double

Name Size Bytes Class Attributes

tempVar 1x8 64 double

So, since h is a vector, tempVar is also a vector of 8 elements. You cannot put those 8 elements into the single i'th element of dy. You can put only one element in. Maybe you want to just pick out one of the 8 values of h to put into the equation?