Incorrect Dimensions for matrix multiplication

Ld=(0:0.001:5);
L=0.007;
Ta=25;
h=375;
Kf=175;
Kd=0.032;
Tb=75;
Ap=3.02*10^-3;
N=238;
Ac=10^-6;
m=92.58;
Lf=L-Ld;
Rtf=(cosh(m*Lf)+(h/(m*Kf))*sinh(m*Lf))/(sqrt(4*h*(1)^3)*(sinh(m*Lf)+h/(m*Kf))*cosh(m*Lf));

Error using *
Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To operate on each element of the matrix individually, use TIMES (.*) for elementwise multiplication.

disp(Rtf)

why do i keep getting an error on the second to last line?

Answer 1

[James Tursa]

Because you are using matrix operators instead of element-wise operators. Try this:

Rtf=(cosh(m.*Lf)+(h./(m.*Kf)).*sinh(m.*Lf))./(sqrt(4*h.*(1)^3).*(sinh(m.*Lf)+h./(m.*Kf)).*cosh(m.*Lf));

Notice the dots. * and / are matrix operations, and .* and ./ are element-wise operations.

Not sure why you have (1)^3

Answer 2

[Sam Chak]

Ld=(0:0.001:5);     % <-- array is first defined here, but does not appear in Rtf
L=0.007;
Ta=25;
h=375;
Kf=175;
Kd=0.032;
Tb=75;
Ap=3.02*10^-3;
N=238;
Ac=10^-6;
m=92.58;
Lf=L-Ld;            % <-- Lf array is derived from Ld array, and it appears in Rtf

% To perform element-wise division & multiplication of arrays, the dots (.) are in indicated below
Rtf=(cosh(m*Lf)+(h/(m*Kf))*sinh(m*Lf))./(sqrt(4*h*(1)^3)*(sinh(m*Lf)+h/(m*Kf)).*cosh(m*Lf));
%                                     ^                                       ^

% disp(Rtf)
plot(Ld, Rtf), grid on, xlabel('Ld'), ylabel('Rtf')

Answer 3

[Torsten]

You multiply and divide arrays elementwise when you compute Rtf. Thus you have to use elementwise multiplication (.*) and elementwise division (./) :

Rtf=(cosh(m*Lf)+(h/(m*Kf))*sinh(m*Lf))./(sqrt(4*h*(1)^3)*(sinh(m*Lf)+h/(m*Kf)).*cosh(m*Lf));

instead of

Rtf=(cosh(m*Lf)+(h/(m*Kf))*sinh(m*Lf))/(sqrt(4*h*(1)^3)*(sinh(m*Lf)+h/(m*Kf))*cosh(m*Lf));

Take a look here for more details:

https://uk.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html