i'm getting this error "Unable to perform assignment because the left and right sides have a different number of elements"

clear
syms X1 X2 X3 X4 
xx = [X1, X2, X3, X4]
d = length(xx);
sum = 0;

for ii = 1:(d/4)
	term1 = (xx(4*ii-3) + 10*xx(4*ii-2))^2;
	term2 = 5 * (xx(4*ii-1) - xx(4*ii))^2;
	term3 = (xx(4*ii-2) - 2*xx(4*ii-1))^4;
	term4 = 10 * (xx(4*ii-3) - xx(4*ii))^4;
	sum = sum + term1 + term2 + term3 + term4;
end

yb1 =  sum
xa1(1)=1;
xa2(1)=1;
xa3(1)=1;
xa4(1)=1;
xa5(1)=1;
xa6(1)=1;
xa7(1)=1;
xa8(1)=1;

error = 10^(-4); 
i = 1; 
dyb1_dx1 = diff(yb1, X1);
dyb1_dx2 = diff(yb1, X2);
dyb1_dx3 = diff(yb1, X3)
dyb1_dx4 = diff(yb1, X4)


J2 = [subs(dyb1_dx1,[X1,X2,X3,X4], [xa1(1),xa2(1),xa3(1),xa4(1)]) subs(dyb1_dx2, [X1,X2,X3,X4], [xa1(1),xa2(1),xa3(1),xa4(1)]) subs(dyb1_dx3,[X1,X2,X3,X4],[xa1(1),xa2(1),xa3(1),xa4(1)]) subs(dyb1_dx4,[X1,X2,X3,X4],[xa1(1),xa2(1),xa3(1),xa4(1)] ) ] % Gradient
S2 = -(J2)
while (norm(J2))>error
    I2 = [xa1(i),xa2(i),xa3(i),xa4(i)]';
    syms h;
    g2 = subs(yb1, [X1,X2,X3,X4], [xa1(i)+h*S2(1),xa2(i)+h*S2(2),xa3(i)+h*S2(3),xa4(i)+h*S2(4)])
    
    dg_dh = diff(g2,h);
    h = vpasolve(dg_dh==0, h); 
    xa1(i+1)=I2(i)+h*S2(1)
    xa2(i+1)=I2(i)+h*S2(2)
    xa3(i+1)=I3(i)+h*S2(3)
    xa4(i+1)=I4(i)+h*S2(4)
   
    %xa1(i+1) = I2(1)+h*S2(1); 
    %xa2(i+1) = I2(2)+h*S2(2);
    %xa3(i+1) = I2(3)+h*S2(3);
    %xa4(i+1) = I2(4)+h*S2(4);
    
    i = i+1;
    J2 = [subs(dyb1_dx1,[X1,X2,X3,X4], [xa1(i),xa2(i),xa3(i),xa4(i)]) subs(dyb1_dx2, [X1,X2,X3,X4], [xa1(i),xa2(i),xa3(i),xa4(i)]) subs(dyb1_dx3,[X1,X2,X3,X4],[xa1(i),xa2(i),xa3(i),xa4(i)]) subs(dyb1_dx4,[X1,X2,X3,X4],[xa1(i),xa2(i),xa3(i),xa4(i)])];
    S2 = -(J2); 
end

[Jan]

Please post the complete error message, which mentions the failing line of code. It is hard to guess, where the error occurs.

Note: redefining important Matlab functions causes bugs frequently. "sum" and "error" are shadowed by variables in your code.

[alok ranjan]

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

Error in hoja (line 143)

xa1(i+1)=I2(i)+h*S2(1)

last part of the code im getting error..

Answer 1

[Stephan]

Problem 1

h returns more than one solution which produces the error because the result of

xa1(i+1)=I2(i)+h*S2(1)

with h being a vector produces a vector while xa1 is a scalar.

- to get only real solutions use:

h = vpasolve(dg_dh==0, h, [-Inf Inf]); 

or if you need one of the complex solutions choose them appropriate.

Problem 2

I3 and I4 are not defined - if i use the section that is commented out instead it works, because no undefined variables are in use. So i guess change:

    xa1(i+1)=I2(i)+h*S2(1)
    xa2(i+1)=I2(i)+h*S2(2)
    xa3(i+1)=I3(i)+h*S2(3)
    xa4(i+1)=I4(i)+h*S2(4)
   
%     xa1(i+1) = I2(1)+h*S2(1); 
%     xa2(i+1) = I2(2)+h*S2(2);
%     xa3(i+1) = I2(3)+h*S2(3);
%     xa4(i+1) = I2(4)+h*S2(4);

to:

%     xa1(i+1)=I2(i)+h*S2(1)
%     xa2(i+1)=I2(i)+h*S2(2)
%     xa3(i+1)=I3(i)+h*S2(3)
%     xa4(i+1)=I4(i)+h*S2(4)
   
    xa1(i+1) = I2(1)+h*S2(1); 
    xa2(i+1) = I2(2)+h*S2(2);
    xa3(i+1) = I2(3)+h*S2(3);
    xa4(i+1) = I2(4)+h*S2(4);

If i do so and set the error to 10e-1 (because of calculation time) the code works for me.

[alok ranjan]

@Stephanthankyou sir,, now it is working!

[Stephan]

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