R help - Sep 2015 - Pb with date time sequence with period<1sec

Hi,

I'd like to create a date-time seq with a period of 0.05 s, over several
days. 

# try :
start<-strptime(nom_fich,format="%y%m%d")
time<-seq(from=start, by=0.05, length.out = 86400*20*3)
print(as.POSIXlt(time[2])$sec)
# result is 0.04999995 and not 0.05 as expected

But If I am looking at the sequence, the seconds are not separated by 0.05,
but by something very close (0.04999995). Same pb if I want to add a
fraction of seconds to a date-time object :

# try :
start<-strptime(nom_fich,format="%y%m%d")
as.POSIXlt(start+0.05,origin="1970-01-01")$sec
# result is 0.04999995 and not 0.05 as expected

Any idea to solve this pb ?

Thank you in advance !




--
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Looks like R FAQ 7.31 to me.

https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

On Wed, Sep 9, 2015 at 4:19 AM, DE <david.ecotiere at cerema.fr>
wrote:
> Hi,
>
> I'd like to create a date-time seq with a period of 0.05 s, over
several
> days.
>
> # try :
> start<-strptime(nom_fich,format="%y%m%d")
> time<-seq(from=start, by=0.05, length.out = 86400*20*3)
> print(as.POSIXlt(time[2])$sec)
> # result is 0.04999995 and not 0.05 as expected
>
> But If I am looking at the sequence, the seconds are not separated by 0.05,
> but by something very close (0.04999995). Same pb if I want to add a
> fraction of seconds to a date-time object :
>
> # try :
> start<-strptime(nom_fich,format="%y%m%d")
> as.POSIXlt(start+0.05,origin="1970-01-01")$sec
> # result is 0.04999995 and not 0.05 as expected
>
> Any idea to solve this pb ?
>
> Thank you in advance !
>
>

-- 
Sarah Goslee
http://www.functionaldiversity.org

Plus, current dates are an awful lot of seconds since 1970-01-01, so the
relative error on second-scale differences is bigger than you might think:
> as.numeric(Sys.time())
[1] 1441874878

and since relative representation errors are of the order 1e-16, the
corresponding absolute errors are about 1e-7.

(The examples given forgot to tell us what nom_fich is supposed to be, but I
assume something relatively current was meant.)

-pd

On 09 Sep 2015, at 16:50 , Sarah Goslee <sarah.goslee at gmail.com> wrote:
> Looks like R FAQ 7.31 to me.
> 
>
https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
> 
> On Wed, Sep 9, 2015 at 4:19 AM, DE <david.ecotiere at cerema.fr>
wrote:
>> Hi,
>> 
>> I'd like to create a date-time seq with a period of 0.05 s, over
several
>> days.
>> 
>> # try :
>> start<-strptime(nom_fich,format="%y%m%d")
>> time<-seq(from=start, by=0.05, length.out = 86400*20*3)
>> print(as.POSIXlt(time[2])$sec)
>> # result is 0.04999995 and not 0.05 as expected
>> 
>> But If I am looking at the sequence, the seconds are not separated by
0.05,
>> but by something very close (0.04999995). Same pb if I want to add a
>> fraction of seconds to a date-time object :
>> 
>> # try :
>> start<-strptime(nom_fich,format="%y%m%d")
>> as.POSIXlt(start+0.05,origin="1970-01-01")$sec
>> # result is 0.04999995 and not 0.05 as expected
>> 
>> Any idea to solve this pb ?
>> 
>> Thank you in advance !
>> 
>> 
> 
> 
> -- 
> Sarah Goslee
> http://www.functionaldiversity.org
> 
> ______________________________________________
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com