On 15/11/2010 9:24 AM, Vadim Patsalo wrote:
Patrick and Bert,
Thank you both for you replies to my question. I see how my naïve expectations
fail to floating point arithmetic. However, I still believe there is an
underlying problem.
It seems to me that when asked,
> c(7.7, 7.8, 7.9) %in% seq(4, 8, by=0.1)
> [1] TRUE FALSE TRUE
R should return TRUE in all instances. %in% is testing set membership... in
that way, shouldn't it be using all.equal() (instead of the implicit '=='), as
Patrick suggests the R inferno?
No, because 7.8 is not in the set. Some number quite close to it is
there, but no 7.8. This is true for both meanings of "7.8":
- the number 78/10
- R's representation of that number
Neither one is in the set. What you have there is R's representation of
4 plus 38 times R's representation of 0.1. (R can represent 4 and 38
exactly, but not 0.1, 7.8, pi, or most other numbers.)
Duncan Murdoch
Is there a convenient way to test set membership using all.equal()? In
particular, can you do it (conveniently) when the lengths of the numeric lists
are different?
Thanks again for your reply!
Vadim
On Nov 13, 2010, at 5:46 AM, Patrick Burns wrote:
> See Circle 1 of 'The R Inferno'.
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