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POHelp with Python UnboundLocalError: local variable referenced before assignment
 

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  1. POHelp with Python UnboundLocalError: local variable referenced before assignment
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    <p>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.</p> <pre><code>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? – </code></pre> <p>when i call force python gives:</p> <pre><code>please enter the number of timestep you want it evolve:4Traceback (most recent call last): File "&lt;pyshell#391&gt;", line 1, in &lt;module&gt; 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 </code></pre> <p>can anyone give me some tips?thanks......</p>
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    1. COThe use of "from numpy import *" is a bad practice. It pollutes the global namespace. "import numpy as np" is better. If you have specific functions you use a lot, and you are tired of writing np.sin(), np.cos, etc, you should import those specifically ("from numpy import sin"). cheers.
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    2. COthanks for the tips:)
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