On 02/09/12 10:52, CHEL HEE LEE wrote:
I have some trouble to deal the value of 'NaN'. For example,
exp(1e3)
[1] Inf
exp(1e3)*0
[1] NaN
The correct answer should be 0 rather than NaN. I will very appreciate
if anyone can share some technique to get a correct answer.
There is no technique that will consistently give you a "correct" answer.
What you seem to want is "x*0 = 0" if "x" is a number that is so large
that it cannot be stored as a floating point number (e.g. x = exp(1e3))
and hence is represented as "Inf".
At the same time you presumably would want "x*0 = NaN" if "x is genuinely
infinite" (e.g. x = 1/0).
But R will have no way of knowing which sort of "Inf" it is dealing with
when
it is called upon to calculate "x*0".
You simply have to be more careful in your coding and avoid expressions like
exp(a)*b when there is a possibility that "a" could be a large positive
number
and "b" could be 0. E.g. you *might* want to recode exp(a)*b as exp(a +
log(b)).
Note that exp(1e3 + log(0)) does indeed evaluate to 0 as you desire. OTOH
exp(1/0 + log(0)) evaluates to NaN, as it should.
There are still perils lurking in this strategy, I think.
Bottom line: Be very carefully in your coding when overflow/underflow
problems
could potentially arise. There are no general prescriptions; every new
instance
has its own difficulties and its own solution.
HTH
cheers,
Rolf Turner
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