[R] Find out the day of week for a chron object?

[Ajay Shah]

On Wed, Mar 03, 2004 at 06:58:02AM -0500, Gabor Grothendieck wrote:
> 
> Here are three different ways (using x as defined in your 
> post):
> 
>  with( month.day.year(x), day.of.week(month,day,year) )
> 
>  do.call( "day.of.week", month.day.year(x) )
> 
>  as.numeric(x-3)%%7   # uses fact that chron(3) is Sunday

Thanks! Using this, I wrote --

  library(chron);
  prevFriday <- function(x) {
    repeat {
      x <- x - 1;
      if (5 == with(month.day.year(x), day.of.week(month,day,year))) break;
    }
    return(x);
  }
  x = dates("12-02-04", format="d-m-y")
  print(prevFriday(x))

and it works. :-)

Could someone give me a glimmer into HOW and WHY that expression 
with(month.day.year(x), day.of.week(month,day,year))
works? :-)

-- 
Ajay Shah                                                   Consultant
ajayshah at mayin.org                      Department of Economic Affairs
http://www.mayin.org/ajayshah           Ministry of Finance, New Delhi