Error iterating with fractions

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Andrew Poissant
Andrew Poissant 2017-4-4
编辑： Stephen23 2017-4-5
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

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Stephen23
Stephen23 2017-4-4
编辑：Stephen23 2017-4-5
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]
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Les Beckham
Les Beckham 2017-4-4
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).

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