RuntimeWarning: invalid value encountered in divide
Question:
I have to make a program using Euler’s method for the “ball in a spring” model
from pylab import*
from math import*
m=0.1
Lo=1
tt=30
k=200
t=20
g=9.81
dt=0.01
n=int((ceil(t/dt)))
km=k/m
r0=[-5,5*sqrt(3)]
v0=[-5,5*sqrt(3)]
a=zeros((n,2))
r=zeros((n,2))
v=zeros((n,2))
t=zeros((n,2))
r[1,:]=r0
v[1,:]=v0
for i in range(n-1):
rr=dot(r[i,:],r[i,:])**0.5
a=-g+km*cos(tt)*(rr-L0)*r[i,:]/rr
v[i+1,:]=v[i,:]+a*dt
r[i+1,:]=r[i,:]+v[i+1,:]*dt
t[i+1]=t[i]+dt
#print norm(r[i,:])
plot(r[:,0],r[:,1])
xlim(-100,100)
ylim(-100,100)
xlabel('x [m]')
ylabel('y [m]')
show()
I keep getting this error:
a=-g+km*cos(tt)*(rr-L0)*r[i,:]/rr
RuntimeWarning: invalid value encountered in divide
I can’t figure it out, what is wrong with the code?
Answers:
You are dividing by rr which may be 0.0. Check if rr is zero and do something reasonable other than using it in the denominator.
I think your code is trying to “divide by zero” or “divide by NaN”. If you are aware of that and don’t want it to bother you, then you can try:
import numpy as np
np.seterr(divide='ignore', invalid='ignore')
For more details see:
Python indexing starts at 0 (rather than 1), so your assignment “r[1,:] = r0” defines the second (i.e. index 1) element of r and leaves the first (index 0) element as a pair of zeros. The first value of i in your for loop is 0, so rr gets the square root of the dot product of the first entry in r with itself (which is 0), and the division by rr in the subsequent line throws the error.
To prevent division by zero you could pre-initialize the output ‘out’ where the div0 error happens, eg np.where does not cut it since the complete line is evaluated regardless of condition.
example with pre-initialization:
a = np.arange(10).reshape(2,5)
a[1,3] = 0
print(a) #[[0 1 2 3 4], [5 6 7 0 9]]
a[0]/a[1] # errors at 3/0
out = np.ones( (5) ) #preinit
np.divide(a[0],a[1], out=out, where=a[1]!=0) #only divide nonzeros else 1
I have to make a program using Euler’s method for the “ball in a spring” model
from pylab import*
from math import*
m=0.1
Lo=1
tt=30
k=200
t=20
g=9.81
dt=0.01
n=int((ceil(t/dt)))
km=k/m
r0=[-5,5*sqrt(3)]
v0=[-5,5*sqrt(3)]
a=zeros((n,2))
r=zeros((n,2))
v=zeros((n,2))
t=zeros((n,2))
r[1,:]=r0
v[1,:]=v0
for i in range(n-1):
rr=dot(r[i,:],r[i,:])**0.5
a=-g+km*cos(tt)*(rr-L0)*r[i,:]/rr
v[i+1,:]=v[i,:]+a*dt
r[i+1,:]=r[i,:]+v[i+1,:]*dt
t[i+1]=t[i]+dt
#print norm(r[i,:])
plot(r[:,0],r[:,1])
xlim(-100,100)
ylim(-100,100)
xlabel('x [m]')
ylabel('y [m]')
show()
I keep getting this error:
a=-g+km*cos(tt)*(rr-L0)*r[i,:]/rr
RuntimeWarning: invalid value encountered in divide
I can’t figure it out, what is wrong with the code?
You are dividing by rr which may be 0.0. Check if rr is zero and do something reasonable other than using it in the denominator.
I think your code is trying to “divide by zero” or “divide by NaN”. If you are aware of that and don’t want it to bother you, then you can try:
import numpy as np
np.seterr(divide='ignore', invalid='ignore')
For more details see:
Python indexing starts at 0 (rather than 1), so your assignment “r[1,:] = r0” defines the second (i.e. index 1) element of r and leaves the first (index 0) element as a pair of zeros. The first value of i in your for loop is 0, so rr gets the square root of the dot product of the first entry in r with itself (which is 0), and the division by rr in the subsequent line throws the error.
To prevent division by zero you could pre-initialize the output ‘out’ where the div0 error happens, eg np.where does not cut it since the complete line is evaluated regardless of condition.
example with pre-initialization:
a = np.arange(10).reshape(2,5)
a[1,3] = 0
print(a) #[[0 1 2 3 4], [5 6 7 0 9]]
a[0]/a[1] # errors at 3/0
out = np.ones( (5) ) #preinit
np.divide(a[0],a[1], out=out, where=a[1]!=0) #only divide nonzeros else 1