Oh OK, missed that.

Here is a solution using base: (already posted)

I didn't sort out the issue in  my email below but here is a (not very
R'ish) solution:

> pm = function(x) {
+   for(i in 1:7){
+      if(format(as.Date(Sys.Date()-
i),'%w') == 1){
+      d = Sys.Date() - i;
+     }
+   }
+   d
+ }
> pm(Sys.Date())
[1] "2011-08-29"


On Fri, Sep 2, 2011 at 9:59 AM, Marc Schwartz <marc_schwartz@me.com> wrote:

> On Sep 2, 2011, at 10:35 AM, Ben qant wrote:
>
> > Hello,
> >
> > I'm attempting to return the date (in form '%Y-%m-%d') of the Monday
> > previous to the current date. For example: since it is 2011-09-02 today,
> I
> > would expect 2011-08-29 to be the return value.
> >
> > I found the following in:
> > http://www.mail-archive.com/r-help@r-project.org/msg144184.html
> >
> > Start quote from link:
> > 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"
> >
> > End quote from link.
> >
> > But when I do it I get:
> >> prevmonday <- function(x) 7 * floor(as.numeric(x-1+4) / 7) +
> as.Date(1-4)
> >> prevmonday(Sys.Date())
> > Error in as.Date.numeric(1 - 4) : 'origin' must be supplied
> >
> > I've tried setting the 'origin' argument in as.Date() in different ways,
> but
> > it returns inaccurate results.
> >
> > Thanks,
> >
> > Ben
>
>
> If memory serves, this is because Gabor used the version of as.Date() from
> his 'zoo' package in that post, which does not require an origin to be
> specified, whereas the default as.Date() function in R's base package does:
>
> prevmonday <- function(x) 7 * floor(as.numeric(x-1+4) / 7) + as.Date(1-4)
>
> > prevmonday(Sys.Date())
> Error in as.Date.numeric(1 - 4) : 'origin' must be supplied
>
> > require(zoo)
> Loading required package: zoo
>
> Attaching package: 'zoo'
>
> The following object(s) are masked from 'package:base':
>
>    as.Date
>
> > prevmonday(Sys.Date())
> [1] "2011-08-29"
>
>
> # Remove 'zoo' to use the base function
> detach(package:zoo)
>
> > prevmonday(Sys.Date())
> Error in as.Date.numeric(1 - 4) : 'origin' must be supplied
>
>
> # Fix the function to use base::as.Date()
> prevmonday <- function(x) 7 * floor(as.numeric(x-1+4) / 7) + as.Date(1-4,
> origin = "1970-01-01")
>
> > prevmonday(Sys.Date())
> [1] "2011-08-29"
>
>
> See ?as.Date
>
> HTH,
>
> Marc Schwartz
>
>

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