R help - Aug 2011 - Convert week value to date

Hello all,
I'm hoping to convert a decimal value for week of the year back to a date
object.
Eg:
strptime(paste(2010,1:52,sep=" "),format="%Y %W")

I expected (hoped?) this would give me the date for Monday of each week. 
Instead, it's giving me 52 values of today's date.

Where am I erring?
Thanks
Michael

_______________________________________________________
Michael Folkes
Salmon Stock Assessment
Canadian Dept. of Fisheries & Oceans     
Pacific Biological Station
3190 Hammond Bay Rd.
Nanaimo, B.C., Canada
V9T-6N7
Ph (250) 756-7264 Fax (250) 756-7053  Michael.Folkes@dfo-mpo.gc.ca


	[[alternative HTML version deleted]]

On Aug 17, 2011, at 4:52 PM, Folkes, Michael wrote:
> Hello all,
> I'm hoping to convert a decimal value for week of the year back to a  
> date object.
> Eg:
> strptime(paste(2010,1:52,sep=" "),format="%Y %W")
Yeah, agree that seems as though it should have been successful. I  
cannot get any of my invocations using %W to work.

 > strptime(Sys.Date(), "%Y-%m-%d-%W")
[1] NA
 > strptime(Sys.Date(), "%Y-%m-%d")
[1] "2011-08-17"
 > strptime(Sys.Date(), "%Y-%m-%d %W")
[1] NA

Oh well.

seq.POSIXt(as.POSIXlt("2010-01-01"), by="week", length=52)
>
> I expected (hoped?) this would give me the date for Monday of each  
> week.  Instead, it's giving me 52 values of today's date.
>
> Where am I erring?
> Thanks
> Michael
>
David Winsemius, MD
West Hartford, CT

Hi Michael

I am not sure of your decimal format of the week but if you can 
convert it to a calendar day of the year by multiplying by 7
then you could you could use eg if 321 was the calendar day

  strptime(paste(2010, 321,sep=" "),format="%Y %j")
[1] "2010-11-17"

Regards

Duncan


Duncan Mackay
Department of Agronomy and Soil Science
University of New England
ARMIDALE NSW 2351
Email: home mackay at northnet.com.au



At 06:52 18/08/2011, you wrote:
>Hello all,
>I'm hoping to convert a decimal value for week of the year back to a 
>date object.
>Eg:
>strptime(paste(2010,1:52,sep=" "),format="%Y %W")
>
>I expected (hoped?) this would give me the date for Monday of each 
>week.  Instead, it's giving me 52 values of today's date.
>
>Where am I erring?
>Thanks
>Michael
>
>_______________________________________________________
>Michael Folkes
>Salmon Stock Assessment
>Canadian Dept. of Fisheries & Oceans
>Pacific Biological Station
>3190 Hammond Bay Rd.
>Nanaimo, B.C., Canada
>V9T-6N7
>Ph (250) 756-7264 Fax (250) 756-7053  Michael.Folkes at dfo-mpo.gc.ca
>
>
>         [[alternative HTML version deleted]]
>
>______________________________________________
>R-help at r-project.org 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.

On Wed, Aug 17, 2011 at 4:52 PM, Folkes, Michael
<Michael.Folkes at dfo-mpo.gc.ca> wrote:
> Hello all,
> I'm hoping to convert a decimal value for week of the year back to a
date object.
> Eg:
> strptime(paste(2010,1:52,sep=" "),format="%Y %W")
>
> I expected (hoped?) this would give me the date for Monday of each week.
?Instead, it's giving me 52 values of today's date.
>
If the objective is to get all the Mondays in 2011 then:

s <- seq(as.Date("2011-01-01"), as.Date("2011-12-31"), by
= "day")
s[format(s, "%w") == "1"]

If the objective is to get the previous Monday relative to an input
date (or the same date if its already Monday) then in the zoo-quickref
vignette is a function nextfri which outputs the next Friday on or
after the input date.  We can readily modify that to get the prior
Monday on or before the current date:

prevmonday <- function(x) 7 * floor(as.numeric(x-1+4) / 7) + as.Date(1-4)

For example,
> prevmonday(Sys.Date())
[1] "2011-08-15"
> prevmonday(prevmonday(Sys.Date()))
[1] "2011-08-15"

-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com