[R] Date seq question

[Gabor Grothendieck]

On Sat, Jan 21, 2012 at 5:20 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> On Fri, Jan 20, 2012 at 12:12 PM, cameron <raymond.fu at invesco.com> wrote:
>> Can anyone please help me with this?
>> I have a list of business dates.  What I want is to have last day of last
>> month and paste them on next month.
>>
>> What i have                        What i want
>> 5725 2011-09-22
>> 5726 2011-09-23
>> 5727 2011-09-26
>> 5728 2011-09-27
>> 5729 2011-09-28
>> 5730 2011-09-29
>> 5731 2011-09-30
>> 5742 2011-10-17                 2011-09-30
>> 5743 2011-10-18                 2011-09-30
>> 5744 2011-10-19                 2011-09-30
>> 5745 2011-10-20                 2011-09-30
>> 5746 2011-10-21                 2011-09-30
>> 5747 2011-10-24                 2011-09-30
>> 5748 2011-10-25                 2011-09-30
>> *5749 2011-10-26*                 2011-09-30
>> 5765 2011-11-17                 2011-10-26
>> 5766 2011-11-18                 2011-10-26
>> 5767 2011-11-21                 2011-10-26
>> 5768 2011-11-22                 2011-10-26
>> 5769 2011-11-23                 2011-10-26
>> 5770 2011-11-25                 2011-10-26
>> 5771 2011-11-28                 2011-10-26
>> 5772 2011-11-29                 2011-10-26
>> *5773 2011-11-30*                 2011-10-26
>> 5780 2011-12-09                 2011-11-30
>> 5781 2011-12-12                 2011-11-30
>> 5782 2011-12-13                 2011-11-30
>> 5783 2011-12-14                 2011-11-30
>> 5784 2011-12-15                 2011-11-30
>> 5785 2011-12-16                 2011-11-30
>> 5786 2011-12-19                 2011-11-30
>> 5787 2011-12-20                 2011-11-30
>> 5788 2011-12-21                 2011-11-30
>> 5789 2011-12-22                 2011-11-30
>>
>> date <-
>> c("9/22/2011","9/23/2011","9/26/2011","9/27/2011","9/28/2011","9/29/2011","9/30/2011","10/17/2011",
>> "10/18/2011","10/19/2011","10/20/2011","10/21/2011","10/24/2011","10/25/2011","10/26/2011","11/17/2011","11/18/2011","11/21/2011","11/22/2011","11/23/2011","11/25/2011","11/28/2011","11/29/2011","11/30/2011",
>> "12/9/2011","12/12/2011","12/13/2011","12/14/2011","12/15/2011","12/16/2011","12/19/2011","12/20/2011","12/21/2011","12/22/2011")
>>
>
> Try this.  It only uses plain R.  f inputs a "Date" class date and
> returns the index in dt which contains the last date in the prior
> month (or NA if none) assuming that dt is sorted:
>
> dt <- as.Date(date, "%m/%d/%Y")
>
> ym <- format(dt, "%Y %m")
> f <- function(x) tail(c(NA, which(ym < x)), 1)
> dt[sapply(ym, f)]

Just one correction.  f takes a ym-style string, not a Date.

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