MATLAB says my matrices on both side of equal sign are not equal in size

Asked Nov 14 '22 at 04:06

Active Nov 14 '22 at 15:00

Viewed 31 times

This is my code

syms xi eta
    N1 = ((1+xi)*(1+eta))/4;
    N2 = ((1-xi)*(1+eta))/4;
    N3 = ((1-xi)*(1-eta))/4;
    N4 = ((1+xi)*(1-eta))/4;
    Nxieta = [N1 N2 N3 N4];
    difNxieta = sym(zeros(2,4))
    for i = 1:4
        difNxieta(1,i) = diff(Nxieta(1:i),xi);
        difNxieta(2,i) = diff(Nxieta(1:i),eta);
    end

And this is the error it showed me

Unable to perform assignment because the size of the left side is 1-by-1 and the
size of the right side is 1-by-2.

Error in sym/privsubsasgn (line 1128)
                L_tilde2 =
                builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in sym/subsasgn (line 965)
            C = privsubsasgn(L,R,inds{:});

Error in Ae420_FEA_project>localstiffnessmatrix (line 55)
        difNxieta(1,i) = diff(Nxieta(1:i),xi);

But I don't understand. diff(Nxieta(i:1),xi) is not an array.

edited Nov 14 '22 at 15:00

beaker

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asked Nov 14 '22 at 04:06

Faito Dayo

1

If you put a breakpoint on this line, and then assign `diff(Nxieta(1:i),xi)` to a temp variable, what does it give you? – BillBokeey Nov 14 '22 at 10:36
temp = eta/4 + 1/4, but this is one thing, not a 2 element vector – Faito Dayo Nov 14 '22 at 13:50
1

This is the case only for the first iteration in your loop. On the second iteration you get `[eta/4 + 1/4, - eta/4 - 1/4]` – BillBokeey Nov 14 '22 at 13:54
1

`dbstop if error` is your friend – Ben Voigt Nov 14 '22 at 15:27

MATLAB says my matrices on both side of equal sign are not equal in size

0 Answers0