r - How do I quickly group the time column in a dataframe into intervals?

Question

Assuming in R, I have a data.frame with the first column representing the time (as POSIXct). The rest of the columns (e.g., columns 2) are numeric data.

I would like to group time into 3-minute intervals. Each interval will the the average of values that falls into that particular interval.

Right now, I have a for-loop that iterates through the time column and generate the interval on the fly. I am wondering if there's a more elegant way to accomplish the same thing?

Thanks in advance.

Derek

Answer 1

I think that a command like the following, would return a list of the values that fall into 3 minute intervals. (v is the name of the dataframe and datecol is the name of the date column)

library(plyr)

v<-data.frame(datecol=as.POSIXct(c(
  "2010-01-13 03:02:38 UTC",
  "2010-01-13 03:03:14 UTC",
  "2010-01-13 03:05:52 UTC",
  "2010-01-13 03:07:42 UTC",
  "2010-01-13 03:09:38 UTC",
  "2010-01-13 03:10:14 UTC",
  "2010-01-13 03:12:52 UTC",
  "2010-01-13 03:13:42 UTC",
  "2010-01-13 03:15:42 UTC",
  "2010-01-13 03:16:38 UTC",
  "2010-01-13 03:18:14 UTC",
  "2010-01-13 03:21:52 UTC",
  "2010-01-13 03:22:42 UTC",
  "2010-01-13 03:24:19 UTC",
  "2010-01-13 03:25:19 UTC"
)), x = cumsum(runif(15)*10),y=cumsum(runif(15)*20))


dlply(v,.(cut(datecol,"3 min")),"[")