Wrong computation of time differenze in POSIXct - additional digits

[Julia-2]

Hello,

I wanted to compute the time differenze between to times:

first =as.POSIXct( "2012-06-15 16:32:39.0025 CEST")
second  = as.POSIXct("2012-06-15 16:32:39.0086 CEST")
second - first

The result is
Time difference of 0.006099939 secs

instead of just 0.0061 secs
So R adds aditional  numbers after the result.
I know I could round it in this case.
But I am working with a large data set and need to always get the
correct result.

difftime() does not work correct either.

Has anybody a suggestion how to get the correct result?

Thank you
Julia

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[David Winsemius]

On Jun 15, 2012, at 12:49 PM, Julia wrote:

> Hello,
>
> I wanted to compute the time differenze between to times:
>
> first =as.POSIXct( "2012-06-15 16:32:39.0025 CEST")
> second  = as.POSIXct("2012-06-15 16:32:39.0086 CEST")
> second - first
>
> The result is
> Time difference of 0.006099939 secs
>
> instead of just 0.0061 secs
> So R adds aditional  numbers after the result.

It's a floating point representation issue.  You don't really want to  
change that value, but are asking to see something different:

 > round ( second - first, 4)
Time difference of 0.0061 secs

> I know I could round it in this case.
> But I am working with a large data set and need to always get the
> correct result.
>
> difftime() does not work correct either.
>
> Has anybody a suggestion how to get the correct result?

Use a computer system that runs on exact arithmetic?

Read FAQ 7.31
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

(And expect to read about 4-6 similar messages in the next hour.)

>
> Thank you
> Julia
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT

______________________________________________
[hidden email] mailing list
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[arun kirshna]

Hi,

I checked the same with strptime.  It is the rounding issue.

Try this:

dat1<-data.frame(datetime=c("2012-06-15 16:32:39.0025 CEST","2012-06-15 16:32:39.0086 CEST"))
 op<-options(digits.secs=4)
 dat1$datetime<- strptime(dat1$datetime, "%Y-%m-%d %H:%M:%OS")


formatC(as.numeric(difftime(dat1[2,1],dat1[1,1],units="secs")),format="f",digits=4)
[1] "0.0061"
#or,
 formatC(as.numeric(diff(dat1$datetime)),format="f",digits=4)
[1] "0.0061"

A.K.




----- Original Message -----
From: Julia <[hidden email]>
To: [hidden email]
Cc:
Sent: Friday, June 15, 2012 12:49 PM
Subject: [R] Wrong computation of time differenze in POSIXct - additional digits

Hello,

I wanted to compute the time differenze between to times:

first =as.POSIXct( "2012-06-15 16:32:39.0025 CEST")
second  = as.POSIXct("2012-06-15 16:32:39.0086 CEST")
second - first

The result is
Time difference of 0.006099939 secs

instead of just 0.0061 secs
So R adds aditional  numbers after the result.
I know I could round it in this case.
But I am working with a large data set and need to always get the
correct result.

difftime() does not work correct either.

Has anybody a suggestion how to get the correct result?

Thank you
Julia

______________________________________________
[hidden email] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.


______________________________________________
[hidden email] mailing list
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and provide commented, minimal, self-contained, reproducible code.

[Rui Barradas]

Hello,

A classic of floating-point accuracy is

 > 3/5 - 3/5
[1] 0
 > 3/5 - (2/5 + 1/5)
[1] -1.110223e-16
 > 3/5 - 2/5 - 1/5
[1] -5.551115e-17

Rui Barradas

Em 15-06-2012 18:18, David Winsemius escreveu:

>
> On Jun 15, 2012, at 12:49 PM, Julia wrote:
>
>> Hello,
>>
>> I wanted to compute the time differenze between to times:
>>
>> first =as.POSIXct( "2012-06-15 16:32:39.0025 CEST")
>> second  = as.POSIXct("2012-06-15 16:32:39.0086 CEST")
>> second - first
>>
>> The result is
>> Time difference of 0.006099939 secs
>>
>> instead of just 0.0061 secs
>> So R adds aditional  numbers after the result.
>
> It's a floating point representation issue.  You don't really want to
> change that value, but are asking to see something different:
>
>  > round ( second - first, 4)
> Time difference of 0.0061 secs
>
>> I know I could round it in this case.
>> But I am working with a large data set and need to always get the
>> correct result.
>>
>> difftime() does not work correct either.
>>
>> Has anybody a suggestion how to get the correct result?
>
> Use a computer system that runs on exact arithmetic?
>
> Read FAQ 7.31
> http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
>
>
> (And expect to read about 4-6 similar messages in the next hour.)
>
>>
>> Thank you
>> Julia
>>
>> ______________________________________________
>> [hidden email] mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

______________________________________________
[hidden email] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

[Peter Dalgaard-2]

On Jun 15, 2012, at 20:33 , Rui Barradas wrote:

> Hello,
>
> A classic of floating-point accuracy is
>
> > 3/5 - 3/5
> [1] 0
> > 3/5 - (2/5 + 1/5)
> [1] -1.110223e-16
> > 3/5 - 2/5 - 1/5
> [1] -5.551115e-17
>
> Rui Barradas
>

Yes. There are only about 16 significant digits in (64 bit) floating point. One further point is that times are stored internally as seconds since Jan 1 1970, of which there has been quite a few by now:

> unclass(Sys.time())
[1] 1339793894

with already 10 digits before the decimal point, you can only expect fractional seconds to be accurate to about 6 digits.

-pd

> Em 15-06-2012 18:18, David Winsemius escreveu:
>>
>> On Jun 15, 2012, at 12:49 PM, Julia wrote:
>>
>>> Hello,
>>>
>>> I wanted to compute the time differenze between to times:
>>>
>>> first =as.POSIXct( "2012-06-15 16:32:39.0025 CEST")
>>> second  = as.POSIXct("2012-06-15 16:32:39.0086 CEST")
>>> second - first
>>>
>>> The result is
>>> Time difference of 0.006099939 secs
>>>
>>> instead of just 0.0061 secs
>>> So R adds aditional  numbers after the result.
>>
>> It's a floating point representation issue.  You don't really want to
>> change that value, but are asking to see something different:
>>
>> > round ( second - first, 4)
>> Time difference of 0.0061 secs
>>
>>> I know I could round it in this case.
>>> But I am working with a large data set and need to always get the
>>> correct result.
>>>
>>> difftime() does not work correct either.
>>>
>>> Has anybody a suggestion how to get the correct result?
>>
>> Use a computer system that runs on exact arithmetic?
>>
>> Read FAQ 7.31
>> http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
>>
>>
>> (And expect to read about 4-6 similar messages in the next hour.)
>>
>>>
>>> Thank you
>>> Julia
>>>
>>> ______________________________________________
>>> [hidden email] mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>> ______________________________________________
>> [hidden email] mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: [phone]
Email: [hidden email]  Priv: [hidden email]

______________________________________________
[hidden email] mailing list
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and provide commented, minimal, self-contained, reproducible code.