Error iterating with fractions

I want to do something pretty simple but am not sure how to do because I can't iterate using fractions. What I want is to solve for x(i), y(i), and z(i). All three are dependent on the previous value. I have attached an attempt to solve this below but naturally I can't iterate using fractions. My tf value is 3.5272 so naturally I want more than 3 or 4 iterations. Any ideas? I just want to solve the three equations for a time step of 0.1 and eventually plot it. I get an error in the line where I define x(i) saying "subscript must be either real positive integers or logicals."

theta = 50; % deg
phi = 0; % deg
u = 16; % m/s
u_x = u*cosd(theta)*cosd(phi);
u_y = u*sind(theta)*cosd(phi);
u_z = u*sind(phi);
x(1) = 0;
y(1) = 0;
z(1) = 60;
tf = 3.5272;
for i = 1.1:0.1:roundn(tf,-1)
    x(i) = u_x*i + 0.5*ax*i^2 + x(i-0.1);
    y(i) = u_y*i + 0.5*ay*i^2 + y(i-0.1);
    z(i) = -u_z*i + 0.5*az*i^2 + z(i-0.1);
end

Answer 1

[Stephen23]

Fractions cannot be used as indices. Put the fractional values in a vector and make the indices integers:

 vec = 1.1:0.1:roundn(tf,-1);
 for k = 2:numel(vec)
      val = vec(k);
      x(k) = +u_x*val + 0.5*ax*val^2 + x(k-1);
      y(k) = +u_y*val + 0.5*ay*val^2 + y(k-1);
      z(k) = -u_z*val + 0.5*az*val^2 + z(k-1);
  end

[edited based on comments below]

[Andrew Poissant]

I am still getting the error "subscript indices must be real positive integers or logicals". Is the code working on your end?

[Andrew Poissant]

Maybe it should be x(k-1) instead of x(val-0.1)?

[Les Beckham]

I suspect that is what Stephen intended. You will also need to change the for loop to start at 2 instead of 1 (so you don't try to index into x with a zero).

[Andrew Poissant]

Got it, thanks!