R help - Jan 2012 - Calculating rolling mean by group

Hello all,

I am trying to determine how to calculate rolling means in R using a
grouping variable. Say I have a dataframe like so:

dat1 <- data.frame(x = runif(2190, 0, 125), year=rep(1995:2000,
each=365), jday=1:365, site="here")
dat2 <- data.frame(x = runif(2190, 0, 200), year=rep(1995:2000,
each=365), jday=1:365, site="there")
dat <- rbind(dat1,dat2)

## What I would like to do is calculate a rolling 7 day mean
separately for each site. I have looked at both
## rollmean() in the zoo package and running.mean() in the igraph
package but neither seem to have led
## me to calculating a rolling mean by group. My first thought was to
use the plyr package but I am confused
## by this output:

library(plyr)
library(zoo)

ddply(dat, c("site"), function(df) return(c(roll=rollmean(df$x, 7))))

## Can anyone recommend a better way to do this or shed some light on
this output?

Thanks so much in advance!

Sam

On Mon, Jan 9, 2012 at 6:39 PM, Sam Albers <tonightsthenight at gmail.com>
wrote:
> Hello all,
>
> I am trying to determine how to calculate rolling means in R using a
> grouping variable. Say I have a dataframe like so:
>
> dat1 <- data.frame(x = runif(2190, 0, 125), year=rep(1995:2000,
> each=365), jday=1:365, site="here")
> dat2 <- data.frame(x = runif(2190, 0, 200), year=rep(1995:2000,
> each=365), jday=1:365, site="there")
> dat <- rbind(dat1,dat2)
>
> ## What I would like to do is calculate a rolling 7 day mean
> separately for each site. I have looked at both
> ## rollmean() in the zoo package and running.mean() in the igraph
> package but neither seem to have led
> ## me to calculating a rolling mean by group. My first thought was to
> use the plyr package but I am confused
> ## by this output:
>
> library(plyr)
> library(zoo)
>
> ddply(dat, c("site"), function(df) return(c(roll=rollmean(df$x,
7))))
>
> ## Can anyone recommend a better way to do this or shed some light on
> this output?
>
Using dat in the question, try this:

library(zoo)
z <- read.zoo(dat, index = 2:3, split = 4, format = "%Y %j")
zz <- rollmean(z, 7)

The result, zz, is a multivariate zoo series with one column per group.

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