Got error after using main function Python 2.7.2

Question

I'm trying to use main function in order to make the trajectory of the projectile with air resistance. The problem is, when I'm using main function, I got an error but not when I use function without main, what I did it wrong?

    v=float(input("initial velocity: "))
    theta=float(input("Angle from the horizontal of initial velocity: "))
    B=float(input("normalised drag coefficient: "))
    t=float(input("the step interval in seconds: " ))

    vx = v * math.cos(theta*math.pi/180.0)
    print(str(vx))
    vy = v * math.sin(theta*math.pi/180.0)
    sx = 0
    sy = 0
    t0 = 0
    ax = 0
    ay = 0
    g= 9.81

    def AX(ax):
        ax= -B*v*vx
        return ax
    def AY(ay):
        ay= -(B*v*vy)-g
        return ay
    def VX(vx):
        vx=vx+ax*t
        return vx
    def VY(vy):
        vy=vy+ay*t
        return vy
    def SX(sx):
        sx= sx + vx*t
        return sx
    def SY(sy):
        sy= sy + vy*t
        return sy
    Xdisplacement=[]
    Ydisplacement=[]
    def main():
        while sy>=0 :
            Xdisplacement.append(float(sx))
            Ydisplacement.append(float(sy))
            sx=SX(sx)
            sy=SY(sy)
            ax=AX(ax)
            ay=AY(ay)
            vx=VX(vx)
            vy=VY(vy)
            v= ((vx**2.0)+(vy**2.0))**(1.0/2.0)

        plt.plot(Xdisplacement,Ydisplacement)
        plt.title("trajectory")
        plt.xlabel("x")
        plt.ylabel("y")
        plt.show()
    main()

Why did you start indenting the code at the `vx = ...` line? — Barmar, Oct 30 '16 at 00:46
I'm sorry. It was the problem when I copied the code and post. Speaking of which, if I don't put main() everything works fine. But if I put main() like this, I got "UnboundLocalError: local variable 'sy' referenced before assignment" — Nai, Oct 30 '16 at 08:30
The problem is that within the scope of the `main` function you are making a reference to `sy` before you assign it. Depending on what behavior you are after, you may want to initialize it to zero within your main function. — kabdulla, Oct 30 '16 at 10:59

score 0 · Accepted Answer · answered Oct 30 '16 at 11:17

If you insist on using global variables and don't want to explicitly send them to a function you have to explicitly define them as global:

def main():
    global sy,sx,ax,ay,vx,vy

and then the rest. You may have seen answers this is unnecessary for read operations on global variables (as in your while sy >= 0) but python will assume the variable is local if you assign to it anywhere within the function, and you do. Hence, as a local variable it is not defined at the start of the while.

I have added the global variable in main function (including v) and it works now. And thank you for the advice. — Nai, Oct 31 '16 at 13:52

Got error after using main function Python 2.7.2

1 Answers1