For-Loop: Iterations using non-integer indices

I am creating a function to solve for Projectile Motion using Quadratic Drag... I am new to MATLAB and have written a function for Quadratic Drag that yields Vdotx (acceleration in x-direction) and Vdoty (acceleration in y-direction) both varying with time. I need to write a for loop to evaluate velocity in both directions using:

%Vx(t+dt)= Vx(t)+Vdotx(t)dt

%Vy(t+dt)= Vy(t)+Vdoty(t)dt

And then solving for position using:

%x(t+dt) = x(t)+Vx(t)dt

%y(x+dt) = y(t)+Vy(t)dt

The issue I can't get around is when using "For loops" a real positive integer is needed for indices. For example I would like my for loop to evaluate the x and y positions for a dt of 0.001. To do this I have written:

dt = 0.001;

Vo = 10;

theta = (pi/180)*33.3;

Vox   = Vo*cos(theta);

Voy   = Vo*sin(theta);

Vx(1) = Vox;

Vy(1) = Voy;

y(1) = 10;

x(1) = 0;

for t = 1:dt:8

      Vx(t+dt)=Vx(t)+Vdotx*dt;

      Vy(t+dt)=Vx(t)+Vdotx*dt;

      x(t+dt) = x(t)+Vx(t)*dt

      y(t+dt) = x(t)+Vy(t)*dt

end

This does not work because the indice (dt) is not an integer. Please help.

Thank you

Answer 1

[KSSV]

You should follow like this:

dt = 0.001;
Vo = 10;
theta = (pi/180)*33.3;
Vox   = Vo*cos(theta);
Voy   = Vo*sin(theta);

t = 1:dt:8 ;
nt = length(t) ;

Vx = zeros(1,nt) ;
Vy = zeros(1,nt) ;
x = zeros(1,nt) ;
y = zeros(1,nt) ;
Vx(1) = Vox;
Vy(1) = Voy;
y(1) = 10;
x(1) = 0;

Vdotx = rand ;  % not defined so I  have taken random value
for i = 2:length(t)
      Vx(i)= Vx(i-1)+Vdotx*dt;
      Vy(i)=Vx(i-1)+Vdotx*dt;
      x(i) = x(i-1)+Vx(i)*dt ;
      y(i) = x(i-1)+Vy(i)*dt ;
end

Answer 2

[Walter Roberson]

tvals = 0:dt:8;
nt = length(tvals) ;
x = zeros(1, nt) 
y = zeros(1, nt) 
for tidx = 1 : nt
  t = tvals(tidx) 
  x(tidx+1) = x(tidx) +..... 
end