[R] Number of digits of a value for problem 7.31 RFAQ SOLVED

[Matthieu Stigler]

Richard.Cotton at hsl.gov.uk a e'crit :
>>> What I mean is if R shows 2.3456 I want to obtain the info that 
>>>       
> digits=4 
>   
>>> even if in facts the value has more (internal) digits.
>>>       
>> Try:
>> x = 1.23456789
>> format(x, nsmall=20)
>> # [1] "1.23456788999999989009"
>>     
>
> I've just re-read the question.  I suspect what you really wanted was 
> something like:
>
> getndp <- function(x, tol=2*.Machine$double.eps)
> {
>   ndp <- 0
>   while(!isTRUE(all.equal(x, round(x, ndp), tol=tol))) ndp <- ndp+1 
>   if(ndp > -log10(tol)) warning("Tolerance reached, ndp possibly 
> underestimated.")
>   ndp 
> }
>
> x = 0.123456
> getndp(x)
> # [1] 6
>
> x2 <- 3
> getndp(x2)
> # [1] 0 
>
> x3 <- 0.1234567890123456789
> getndp(x3)
> # [1] 16
> # Warning message:
> # In getndp(x3) : Tolerance reached, ndp possibly underestimated.
>
> Regards,
> Richie.
>
> Mathematical Sciences Unit
> HSL
>
>   

Nice! Thanks a lot!!
Now I can solve my problem that:

target<-0.223423874568437
target<=target+0.1-0.1 #TRUE
target>=target+0.1-0.1 #FALSE!!
target==target+0.1-0.1 #FALSE!!

by:
target<=round(target+0.1-0.1, getndp(target)) #TRUE
target>=round(target+0.1-0.1, getndp(target)) #TRUE!!
target==round(target+0.1-0.1, getndp(target)) #TRUE!!