I wrote the following script:
import numpy d = numpy.array([[1089, 1093]]) e = numpy.array([[1000, 4443]]) answer = numpy.exp(-3 * d) answer1 = numpy.exp(-3 * e) res = answer.sum()/answer1.sum() print res
But I got this result and with the error occurred:
nan C:\Users\Desktop\test.py:16: RuntimeWarning: invalid value encountered in double_scalars res = answer.sum()/answer1.sum()
It seems to be that the input element were too small that python turned them to be zeros, but indeed the division has its result.
How to solve this kind of problem?
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~ Asked on 2015-01-05 17:15:04
You can't solve it. Simply
answer1.sum()==0, and you can't perform a division by zero.
This happens because
answer1 is the exponential of 2 very large, negative numbers, so that the result is rounded to zero.
nan is returned in this case because of the division by zero.
Now to solve your problem you could:
scipy/numpyfunction that does exactly what you want! Check out @Warren Weckesser answer.
Here I explain how to do some math manipulation that helps on this problem. We have that for the numerator:
exp(-x)+exp(-y) = exp(log(exp(-x)+exp(-y))) = exp(log(exp(-x)*[1+exp(-y+x)])) = exp(log(exp(-x) + log(1+exp(-y+x))) = exp(-x + log(1+exp(-y+x)))
x=3* 1089 and
y=3* 1093. Now, the argument of this exponential is
-x + log(1+exp(-y+x)) = -x + 6.1441934777474324e-06
For the denominator you could proceed similarly but obtain that
log(1+exp(-z+k)) is already rounded to
0, so that the argument of the exponential function at the denominator is simply rounded to
-z=-3000. You then have that your result is
exp(-x + log(1+exp(-y+x)))/exp(-z) = exp(-x+z+log(1+exp(-y+x)) = exp(-266.99999385580668)
which is already extremely close to the result that you would get if you were to keep only the 2 leading terms (i.e. the first number
1089 in the numerator and the first number
1000 at the denominator):
For the sake of it, let's see how close we are from the solution of Wolfram alpha (link):
Log[(exp[-3*1089]+exp[-3*1093])/([exp[-3*1000]+exp[-3*4443])] -> -266.999993855806522267194565420933791813296828742310997510523
The difference between this number and the exponent above is
+1.7053025658242404e-13, so the approximation we made at the denominator was fine.
The final result is
'exp(-266.99999385580668) = 1.1050349147204485e-116
From wolfram alpha is (link)
1.105034914720621496.. × 10^-116 # Wolfram alpha.
and again, it is safe to use numpy here too.
~ Answered on 2015-01-05 17:19:19
You can use
np.logaddexp (which implements the idea in @gg349's answer):
In : d = np.array([[1089, 1093]]) In : e = np.array([[1000, 4443]]) In : log_res = np.logaddexp(-3*d[0,0], -3*d[0,1]) - np.logaddexp(-3*e[0,0], -3*e[0,1]) In : log_res Out: -266.99999385580668 In : res = exp(log_res) In : res Out: 1.1050349147204485e-116
Or you can use
In : from scipy.special import logsumexp In : res = np.exp(logsumexp(-3*d) - logsumexp(-3*e)) In : res Out: 1.1050349147204485e-116
~ Answered on 2015-01-05 21:06:12