R help - May 2011 - Extracting months and years from Dates while keeping order

Hello!

I'd like to take Dates and extract from them months and years - but so
that it sorts correctly. For example:

x1<-seq(as.Date("2009-01-01"), length = 14, by = "month")
(x1)
order(x1)  # produces correct order based on full dates

# Of course, I could do "format" - but this way I am losing the
"Date"
quality of the data:

x2<-format(x1,"%Y.%m")
(x2)
order(x2)

Not sure it's possible at all: But is there some other way to extract
just months and years from Dates (while ingoring days) - but so that
it's still somehow a Date object?

Thank you very much!

-- 
Dimitri Liakhovitski
Ninah Consulting
www.ninah.com

On Fri, May 6, 2011 at 4:07 PM, Dimitri Liakhovitski
<dimitri.liakhovitski at gmail.com> wrote:
> Hello!
>
> I'd like to take Dates and extract from them months and years - but so
> that it sorts correctly. For example:
>
> x1<-seq(as.Date("2009-01-01"), length = 14, by =
"month")
> (x1)
> order(x1) ?# produces correct order based on full dates
>
> # Of course, I could do "format" - but this way I am losing the
"Date"
> quality of the data:
>
> x2<-format(x1,"%Y.%m")
> (x2)
> order(x2)
>
> Not sure it's possible at all: But is there some other way to extract
> just months and years from Dates (while ingoring days) - but so that
> it's still somehow a Date object?
>
Try using the yearmon class in zoo.
> library(zoo)
> y <- as.yearmon(x1)
> y
 [1] "Jan 2009" "Feb 2009" "Mar 2009" "Apr
2009" "May 2009" "Jun 2009"
 [7] "Jul 2009" "Aug 2009" "Sep 2009" "Oct
2009" "Nov 2009" "Dec 2009"
[13] "Jan 2010" "Feb 2010"
> order(y)
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14


-- 
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