[R] aggregate a Date column does not work?

[Tan, Richard]

Thanks, add as.Date('1970-01-01') to the result column works.

Richard

-----Original Message-----
From: David Winsemius [mailto:dwinsemius at comcast.net] 
Sent: Monday, November 22, 2010 3:51 PM
To: Tan, Richard
Cc: r-help at r-project.org
Subject: Re: [R] aggregate a Date column does not work?


On Nov 22, 2010, at 3:39 PM, Tan, Richard wrote:

> Hi, I am trying to aggregate max a Date type column but have weird
> result, how do I fix this?

In the process of getting max() you coerced the Dates to numeric and  
now you need to re-coerce them back to Dates

?as.Date
as.Date(<your result>)  (possibly with an origin it the default  
"1970-01-01" doesn't get used.

-- 
David.
>
>
>
>> a <- rbind(
>
> + data.frame(name='Tom', payday=as.Date('1999-01-01')),
>
> + data.frame(name='Tom', payday=as.Date('2000-01-01')),
>
> + data.frame(name='Pete', payday=as.Date('1998-01-01')),
>
> + data.frame(name='Pete', payday=as.Date('1999-01-01'))
>
> + )
>
>> a
>
>  name     payday
>
> 1  Tom 1999-01-01
>
> 2  Tom 2000-01-01
>
> 3 Pete 1998-01-01
>
> 4 Pete 1999-01-01
>
>> aggregate(a$payday, list(a$name), max)
>
>  Group.1     x
>
> 1     Tom 10957
>
> 2    Pete 10592
>
>
>
> Thanks,
>
> Richard
>
>
>
>
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
West Hartford, CT