问题
This question already has an answer here:
- How to calculate any negative number to the power of some fraction in R? 2 answers
How can it be that the expression
> (exp(17.118708 + 4.491715 * -2)/-67.421587)^(-67.421587)
results in
[1] NaN
while
> -50.61828^(-67.421587)
which should basically have the same outcome, gives me
[1] -1.238487e-115
This is driving me crazy, I spent hours searching for the Error. "-2", in this case, is a Parameter of the function. I really can't think of a solution. Thanks for your help!
EDIT:
I see that when I add brackets
> (-50.61828)^(-67.421587)
it also results in
[1] NaN
...but that does not solve my Problem.
回答1:
It is because of the implementation of pow under C99 standard.
Let alone OP's example: (-50.61828)^(-67.421587), the mathematically justified (-8)^(1/3) = -2 does not work in R:
(-8)^(1/3)
# [1] NaN
Quoted from ?"^":
Users are sometimes surprised by the value returned, for example
why ‘(-8)^(1/3)’ is ‘NaN’. For double inputs, R makes use of IEC
60559 arithmetic on all platforms, together with the C system
function ‘pow’ for the ‘^’ operator. The relevant standards
define the result in many corner cases. In particular, the result
in the example above is mandated by the C99 standard. On many
Unix-alike systems the command ‘man pow’ gives details of the
values in a large number of corner cases.
I am on Ubuntu LINUX, so can help get relevant part of man power printed here:
If x is a finite value less than 0, and y is a finite noninteger, a
domain error occurs, and a NaN is returned.
回答2:
From what I can tell, -50.61828^(-67.421587) is evaluating as -(50.61828^(-67.421587)). (-50.61828)^(-67.421587) also results in NaN.
来源：https://stackoverflow.com/questions/38061082/r-expression-results-in-nan-for-no-obvious-reason