R help - Jul 2011 - Calculating difference in variable values (e.g. elapsed time) in data frame

Hello,

I have a data frame containing time (e.g. GMT), and I would like to 
create/add a new variable that would be the computation of the elapsed 
time since the first observation.  Does anyone have a suggestion for an 
easy way to do this?  I am having trouble creating a new variable that 
would contain just the first time observation (then I could take 
difference between actual time and initial time).

e.g. - here's a brief representation of the data:

time <-  
c("19:36:11","19:36:12","19:36:13","19:36:14","19:36:15","19:36:16")
strptime(time, "%H:%M:%S")
y<-c(197,194,189,179,166,150)
mydata<-data.frame(time=time,y=y)

OK, now how do I create a new variable, say, time_el, that would 
calculate the elapsed time since 19:36:11?  I assume that I need the 
strptime() function to make sure R treats the character strings as time.

Thank you very much for any & all assistance!

-Philippe

Hi,

On Thu, Jul 28, 2011 at 11:38 AM, Philippe Hensel
<Philippe.Hensel at noaa.gov> wrote:
> Hello,
>
> I have a data frame containing time (e.g. GMT), and I would like to
> create/add a new variable that would be the computation of the elapsed time
> since the first observation. ?Does anyone have a suggestion for an easy way
> to do this? ?I am having trouble creating a new variable that would contain
> just the first time observation (then I could take difference between
actual
> time and initial time).
>
> e.g. - here's a brief representation of the data:
>
> time <-
>
?c("19:36:11","19:36:12","19:36:13","19:36:14","19:36:15","19:36:16")
> strptime(time, "%H:%M:%S")
> y<-c(197,194,189,179,166,150)
> mydata<-data.frame(time=time,y=y)
>
> OK, now how do I create a new variable, say, time_el, that would calculate
> the elapsed time since 19:36:11? ?I assume that I need the strptime()
> function to make sure R treats the character strings as time.
Thank you for providing a small reproducible example!

You missed one step, which is assigning the result of strptime() to a variable.
After that, you can just subtract.

time <- 
c("19:36:11","19:36:12","19:36:13","19:36:14","19:36:15","19:36:16")
time <- strptime(time, "%H:%M:%S")
y<-c(197,194,189,179,166,150)
timeel <- time - time[1]
mydata<-data.frame(time=time, y=y, timeel=timeel)

Sarah
-- 
Sarah Goslee
http://www.functionaldiversity.org