Message-ID: <9BBCFAD1-33D6-475F-B3BC-47472D02973C@uos.de>
Date: 2009-02-11T11:59:17Z
From: Stefan Evert
Subject: How to handle large numbers?
In-Reply-To: <Pine.LNX.4.43.0902110305200.22148@hymn13.u.washington.edu>
>> 2, I need to calculate e.g. exp(a)/(exp(b)+c), where both a and b
>> are very
>> large numbers (>>1000, e.g a=1000, b=1007, and c=5). R gives me NaN
>> when I
>> use the following command:
>>
>>> exp(1000)/(exp(1007)+5)
>> [1] NaN
>>
>> I am pretty sure this should be close to zero.
>
> No. It should be close to 1. Try 1000/(1000+5) for a simpler example.
Shouldn't it rather be 1 / exp(7), since exp(1007) = exp(1000) * exp(7)?
You can get the correct solution if you rewrite the fraction
algebraically first, dividing all terms by exp(1000).
1 / (exp(7) + 5 * exp(-1000))
[1] 0.000911882
Of course, you have to know which of the numbers are large in order to
do this right ...
Best regards,
Stefan Evert
[ stefan.evert at uos.de | http://purl.org/stefan.evert ]