[R] Splitting a time + duration into a series of periods

[Phil Spector]

Travers -
    R's date/time abilities are pretty powerful -- you shouldn't have to
resort to outside programs.  Here's how I'd approach the problem:

> dat = read.csv(textConnection('user, login_time, session_min
+ alice, 2010/01/01 10:00, 145
+ bob, 2010/01/01 11:00, 30
+ '),stringsAsFactors=FALSE)
> dat$login_time = as.POSIXct(dat$login_time)
> dat$login_end = dat$login_time + 60 * dat$session_min
> tms = seq(as.POSIXct('2010/01/01 10:00'),
+           as.POSIXct('2010/01/01 12:30'),by=30*60)
> data.frame(time=tms,count=sapply(tms,
+             function(tm)sum(dat$login_time <= tm & dat$login_end > tm)))
                  time count
1 2010-01-01 10:00:00     1
2 2010-01-01 10:30:00     1
3 2010-01-01 11:00:00     2
4 2010-01-01 11:30:00     1
5 2010-01-01 12:00:00     1
6 2010-01-01 12:30:00     0

Hope this helps.

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu



On Fri, 24 Sep 2010, Travers Naran wrote:

> Hi all,
>
> Here's what I have. I have a list of log-in times for users and how
> long their sessions were.
>
> user, login_time, session_min
> alice, 2010/01/01 10:00, 145
> bob, 2010/01/01 11:00, 30
>
> What I want to do is create a bar chart showing, in 1/2 hour segments,
> the number of users logged in at the same time.  For example:
>
> 10:00 1
> 10:30 1
> 11:00 2
> 11:30 1
>
> The only way I can do this now is to send the data through a Perl
> script to generate raw data like:
>
> alice, 2010/01/01, 10:00
> alice, 2010/01/01, 10:30
> alice, 2010/01/01, 11:00
> ...
> bob, 2010/01/01, 11:00
>
> I've looked through the man pages for a couple hours now, and I can't
> find a better of way of doing this directly in R.  Any help or
> pointers?  Thanks in advance.
>
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