[R] Removing rows with earlier dates

[Joshua Wiley]

Hi,

On Fri, Dec 24, 2010 at 5:45 AM, Ali Salekfard <salekfard at googlemail.com> wrote:
[snip]
> I have a data frame that contains mapping rules in this way:
>
> ACCOUNT, RULE COLUMNS, Effective Date
>
>
> The dataframe comes from a database that stores all dates. What I would like
> to do is to create a data frame with only the most recent rule for each

Assuming that "Effective Date" is a date class (you can use
str(yourdataframe) you tell):

with(YourDataFrame, tapply(`Effective Date`, `RULE COLUMNS`, which.max))

this will return the location of the maximum `Effective Date`

If you want the actual value, rather than the location:

with(YourDataFrame, tapply(`Effective Date`, `RULE COLUMNS`,
  function(x) x[which.max(x)]))

should do the trick (assuming Effective Date is a date class, not
character).  By the way, you will find your life more convenient if
you avoid object or column names with spaces or special characters.

Cheers,

Josh

> account. In traditional programming languages I would loop through each
> account find the most recent rule(s) and fill up my updated data frame.
>
> Does anyone have any better idea to use R's magic (Its syntax is still
> magical to me) for this problem?
>
> By the way the list of rules is quite extensive (144643 lines to be
> precise), and there are usually 1-3 most recent rules (rows) for each
> account.
>
> Thanks.
>
>        [[alternative HTML version deleted]]
>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/