Function Error: Array indices must be positive integers or logical values.

% Inputs
m = 1.327E20; %gravitational parameter of central body
ri = 1.51E11; %inner radius in meters
ro = 5.77E10; %outer radius in meters


%Helper Equations
R = (ro)/(ri);
a = ((ro)+(ri))/2;

%Delta V Equation
v(ro,ri) = (sqrt(m/(ri)))*((1/sqrt(R))-((sqrt(2)*(1-R))/(sqrt(R(a+R))))-1);

Array indices must be positive integers or logical values.

fprintf('\n Delta V = %g m', v(ro,ri))

I'm trying to solve for a lengthy homework problem by using MATLAB and I keep getting this error. I've solved with the initial conditions to be sure that I'm using a valid equation. The eqaution is supposed to be:

I have a feeling that my missuse of parenthesis is causing this issue. I'm getting confused about the array error because I'm a beginner.

Answer 1

[James Tursa]

When you have parentheses appear on the left hand side of an assignment like this:

v(ro,ri) = (sqrt(m/(ri)))*((1/sqrt(R))-((sqrt(2)*(1-R))/(sqrt(R(a+R))))-1);

then MATLAB thinks you are trying to index into a variable called v. And since ro and ri are not positive integers, you get the error. Also this line has a typo with a missing * so you will get the same error because of that.

Instead, what you want to do is calculate v directly:

v = (sqrt(m/ri))*(1/sqrt(R)-((sqrt(2)*(1-R))/(sqrt(R*(a+R))))-1);

And then just print v in your printf statement:

fprintf('\n Delta V = %g \n', v)

[Liam Hoyle]

Thanks so much!!!

I was halfway done the problem using my calculator, but the code is wayyy faster (:

[James Tursa]

Note that if you wanted a function to calculate v, you could create a file called deltav.m (or some other name of your choosing) with the following code:

% deltav calculates delta velocity for the orbit problem blah blah blah
function v = deltav(m,ri,ro)
R = ro / ri;
a = (ro + ri) / 2;
v = (sqrt(m/ri))*(1/sqrt(R)-((sqrt(2)*(1-R))/(sqrt(R*(a+R))))-1);
end