Help with Python UnboundLocalError: local variable referenced before assignment

Question

Hi,I have post the similar question before,however,I think I may have misinterpreted my question,so may I just post my origin code here,and looking for someone can help me,I am really stuck now..thanks alot.

from numpy import *
import math as M

#initial condition  All in SI unit
G=6.673*10**-11   #Gravitational constant
ms=1.9889*10**30 #mass of the sun
me=5.9742*10**24 #mass of the earth
dt=10           #time step
#Creat arrays
vs=array([[0,0,0]])     #1st element stand for x component of V of earth
ve=array([[29770,0,0]])
rs=array([[0,0,0]])           
re=array([[0,1.4960*10**11,0]])

#First update velocity in order to start leapfrog approximation
fs=-G*ms*me*((rs-re)/(M.sqrt((rs-re)[0][0]**2+(rs-re)[0][1]**2+(rs-re)[0][2]**2))**3)
fe=-fs
vs=vs+fs*dt/ms 
ve=ve+fe*dt/me

n=input('please enter the number of timestep you want it evolve:')
#update force
def force(n,ms,me,rs,re,G):
    rs,re=update_r(rs,re,n,dt)
    fs=-G*ms*me*((rs-re)/(M.sqrt((rs-re)[0][0]**2+(rs-re)[0][1]**2+(rs-re)[0][2]**2))**3)
    fe=-fs
    return fs,fe

#update velocities
def update_v(n,vs,ve,ms,me,dt,fs,fe):
    fs,fe=force(n,ms,me,rs,re,G)
    i=arange(n)
    vs=vs+fs[:]*i[:,newaxis]*dt/ms
    ve=ve+fe[:]*i[:,newaxis]*dt/me
    return vs,ve

#update position
def update_r(rs,re,n,dt):
    vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe)
    i=arange(n)
    rs=rs+vs[:]*i[:,newaxis]*dt
    re=re+ve[:]*i[:,newaxis]*dt
    return rs,re
#there is start position,v,r,f all have initial arrays(when n=0).
#then it should calculate f(n=1) then use this to update v(n=0)
#to v(n=1),then use v(n=1) update r(n=0) to r(n=1),then use r(n=1)
#update f(n=1) to f(n=2)....and so on until finish n.but this code seems doesnt do this,,how can I make it? –

when i call force python gives:

please enter the number of timestep you want it evolve:4Traceback (most recent call last):
  File "<pyshell#391>", line 1, in <module>
    force(n,ms,me,rs,re,G)
  File "/Users/Code.py", line 24, in force
    rs,re=update_r(rs,re,n,dt)
  File "/Users/Code.py", line 39, in update_r
    vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe)
UnboundLocalError: local variable 'vs' referenced before assignment

can anyone give me some tips?thanks......

Answer 1

where do you call force in this code?

In any event, the problem is in update_r. You reference vs in the first line of update_r even though vs is not defined in this function. Python is not looking at the vs defined above. Try adding

global vs

as the first line of update_r or adding vs to the parameter list for update_r

Answer 2

In the first line of your update_r function, you have: vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe). Look at the function that you are calling. You are calling update_v with a bunch of parameters. One of these parameters is vs. However, that is the first time in that function that vs appears. The variable vs does not have a value associated with it yet. Try initializing it first, and yo0ur error should disappear

Answer 3

On line 39 you do

vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe)

while you are inside a function. Since you defined a global variable called vs, you would expect this to work. It would have worked if you had:

vs_new,ve_new = update_v(n,vs,ve,ms,me,dt,fs,fe)

because then the interpreter knows vs in the function arguments is the global one. But since you had vs in the left hand side, you created an uninitialized local variable.

But dude, you have a much bigger problem in your code: update_r calls update_v, update_v calls force, and force calls update_r - you will get a stack overflow :)