Subscript indices must either be real positive integers or logicals.

Hi all.

I tried to run a Matlab script where I have to plot a certain graph, but I keep getting the same error message, Subscript indices must either be real positive integers or logicals.. My code is below:

syms s
De = 56;%mm, Do is also De
Di = 28.5; %mm
t = 2;
h = 1.6;
FN = 4440;
E =210e3;
nu = 0.3;
delta = De/Di 
Cm = 4*E/(1-nu^2);
C4 = 1
C1 = (1/pi)*(((delta-1)/delta)^2/(((delta+1)/(delta-1))-(2/log(delta))))
F = Cm*t*s/(C1*De^2)*C4^2*(C4^2*(h-s)*(h-0.5*s)+t^2)
s1 = linspace(0,1.2,50); %1.2 because 0.75h, 0.75*1.6
plot(s1,F(s1),'-k','LineWidth',2), grid on, xlabel('s [mm]'), ylabel('F [N]')
set(gca,'FontSize',18)

Could anyone explain what the error means and where I should correct it?

Answer 1

[Star Strider]

In this instance (using the Symbolic Math Toolbox), it will work as you want if you create ‘F’ as a function:

F(s) = Cm*t*s/(C1*De^2)*C4^2*(C4^2*(h-s)*(h-0.5*s)+t^2)

If you were not using the Symbolic Math Toolbox, you would create ‘F(s)’ as an anonymous function:

F = @(s) Cm*t*s./(C1*De^2)*C4^2.*(C4^2*(h-s).*(h-0.5*s)+t^2);

Note the need to vectorise the multiplication and division operations using element-wise operations. See the documentation on Vectorization (link) for details.

You would call them the same way in your plot call.