Running like a forever loop

I am working on the following function

function [x,y]=projectile(V_int,theta,dt)
time=dt; %(seconds)
k=0.32; %drag coefficient (given)
g=-9.81; %(m/s^2)
%First time step in x direction
x(1)=0; %initial x position
v_int_x=V_int*cos(theta); %initial velocity in x direction
a_x=v_int_x*k; %initial acceleration in x direction
x(2)=x(1)+v_int_x*time+0.5*a_x*(time^2); %second position in x direction
v_fin_x=v_int_x+a_x*time; 
%First time step in y direction
y(1)=0;
v_int_y=V_int*sin(theta);
a_y=g-v_int_y*k;
y(2)=y(1)+v_int_y*time+0.5*a_x*(time^2);
v_fin_y=v_int_y+a_y*time;
time=time+dt;
i=3; %counter
while y(i-1)>=0
    %calculates x values
    a_x=-v_fin_x*k;
    x(i)=x(i-1)+v_fin_x*time+0.5*a_x*(time^2);
    v_fin_x=v_fin_x+a_x*time;
    %calculates y values
    a_y=v_fin_y*k;
    y(i)=y(i-1)+v_fin_y*time+0.5*a_y*(time^2);
    v_fin_y=v_fin_y+a_y*time;
    %change counter and timestep to continue loop
    i=i+1;
    time=time+dt;
end
plot(x,y)
title('Path of Projectile');
xlabel('Distance of Projectile (meters)');
ylabel('Height of Projectile (meters)');
axis([0 max(x)+50 0 max(y)+50]); %axes for plot
x_distance=max(x);
end

I am trying to run it using this script

for theta=0:pi %degrees
    V_int=sum('karlie');
    dt=0.5; %second(s)
    [x,y]=projectile(V_int,theta,dt);
end
index=(find(x==max(x_)));
max_theta=theta(index);
fprintf('The projectile will travel furthest at an angle of %2.2f degrees\n', max_theta);

However, when I run my script it gets stuck running like a forever loop and I have to terminate it. Not sure why this is happening.

[Star Strider]

your ‘y’ vector increases without bound. After 25 iterations, your projectile has traveled 23.3961e+012 metres, so it is apparently in interstellar space.

Change the first statement of your while loop to something like this:

while y(i-1)>=0 & (i <= 25)

until you figure out what the problem is with your code. That limits the number of iterations, making effective troubleshooting easier.

[Steven Lord]

I suspect it's the introduction of anti-gravity.

    %calculates y values
    a_y=v_fin_y*k;
    y(i)=y(i-1)+v_fin_y*time+0.5*a_y*(time^2);
    v_fin_y=v_fin_y+a_y*time;

Shouldn't these lines of code make use of g somewhere? It might be useful to write the mathematical expression each line or each section of your code implements as a comment before that line or section, so you can easily check if your equation and your code are doing the same thing.