I wrote a function which returns me a certain array. Whenever I just print a certain value of this function, it works fine. However, if I try to remember certain values of the function as variables, it works for one variable, but afterwards it returns me an error, saying: “ValueError: setting an array element with a sequence.”

My code is as follows:

N=5
steps=10

delta_t=0.01    #these are just a bunch of constants
k_B=1 
T=1 
m=1
gamma=1 
sigma=np.sqrt(2*k_B*T*m*gamma/delta_t)
a=16    
q0=2*np.pi/a

def diff_array(V0, k_c, x0):
    pos_array=np.empty([N+steps+1])
    for i in range(N+steps+1):
        pos_array[i]=0
    def potential_force(x):
        return (V0*np.pi)/a*np.sin(q0*x+np.pi)
    for i in range(N):
        x1=0
        x2=0
        v1=0
        v2=0
        x=0
        G1_old=k_c*(-x0)+np.random.normal(0,sigma)+potential_force(0)
        G2_old=-k_c*(-x0)+np.random.normal(0,sigma)+potential_force(0)
        for j in range(steps):
            F=k_c*(x2-x1-x0)
            W1=np.random.normal(0,sigma)
            W2=np.random.normal(0,sigma)
            U1=potential_force(x1)
            U2=potential_force(x2)
            G1=F+W1+U1
            G2=-F+W2+U2
            x1=x1+delta_t*v1*(1-delta_t*gamma/2)+(delta_t)**2*G1/(2*m)
            v1=(v1*(1-delta_t*gamma/2)+delta_t*(G1_old+G1)/(2*m))/(1+delta_t*gamma/2)
            x2=x2+delta_t*v2*(1-delta_t*gamma/2)+(delta_t)**2*G2/(2*m)
            v2=(v2*(1-delta_t*gamma/2)+delta_t*(G2_old+G2)/(2*m))/(1+delta_t*gamma/2)
            G1_old=G1
            G2_old=G2
            x=(x1+x2)/2
            pos_array[N+j+1]=pos_array[N+j+1]+(x**2-pos_array[N+j+1])/(i+1)
        pos_array[i]=x1**2
    return pos_array

Apparently, calling this function in a second variable causes the variable x to be seen as a sequence. But if I just call this function, i.e. in a print, it works just fine. What is going on?