Hi:

You need to give a function for rollapply() to apply :)

Here's my toy example:

d <- as.data.frame(matrix(rpois(30, 5), nrow = 10))
library(zoo)
d1 <- zoo(d)       # uses row numbers as index

# rolling means of 3 in each subseries (columns)
> rollmean(d1, 3)
        V1       V2       V3
2 3.000000 5.333333 4.333333
3 3.333333 4.333333 4.000000
4 2.666667 3.666667 3.333333
5 4.333333 3.666667 3.333333
6 5.000000 5.000000 5.000000
7 4.666667 4.666667 4.000000
8 5.666667 3.666667 4.000000
9 5.000000 3.000000 2.666667

# create new function
> cv <- function(x) sd(x)/mean(x)

# use new function in rollapply():
> rollapply(d1, 3, FUN = cv)
         V1        V2        V3
2 0.3333333 0.1082532 0.5329387
3 0.1732051 0.4803845 0.6614378
4 0.2165064 0.5677271 0.4582576
5 0.7418193 0.5677271 0.4582576
6 0.6000000 0.3464102 0.4000000
7 0.7525467 0.4948717 0.6614378
8 0.8882158 0.5677271 0.6614378
9 1.0583005 0.3333333 0.2165064

This is a simplistic example, but it should get you started.

HTH,
Dennis
On Fri, Oct 15, 2010 at 2:00 AM, David A. <dasolexa@hotmail.com> wrote:

>  Hi Dennis and Dimitris,
>
> thanks for your answers. I am trying the rollmean() function and also the
> rollapply() function because I also want to calculate CV for the values.
> For this I created a co.var() function. I am having problems using them.
>
> >co.var<-function(x)(
> +sd(x)/mean(x)
> +)
>
>
> > dim(mydata)
> [1] 1710  244
>
> >xxx<-rollmean(mydata,3,by=3)
>
> works fine and creates a vector which I will transform into a matrix. I
> still have to find out how to store the output in my previously created
> 570x244 0's matrix in an ordered way.
>
> but, since the examples in the help page says it´s the same, I tried
>
> > xxx<-rollapply(mydata,3,mean,by=3)
> Error in UseMethod("rollapply") :
>   no applicable method for 'rollapply' applied to an object of class
> "c('matrix', 'integer', 'numeric')"
>
> and, with my created function...
>
> > xxx<-rollapply(ord_raw_filt.df,3,FUN=co.var,by=3)
> Error in UseMethod("rollapply") :
>   no applicable method for 'rollapply' applied to an object of class
> "c('matrix', 'integer', 'numeric')"
>
> Can you help me with the error?
>
> Dave
>
> ------------------------------
> Date: Fri, 15 Oct 2010 00:45:08 -0700
> Subject: Re: [R] using apply function and storing output
> From: djmuser@gmail.com
> To: dasolexa@hotmail.com
> CC: r-help@r-project.org
>
>
> Hi:
>
> Look into the rollmean() function in package zoo.
>
> HTH,
> Dennis
>
> On Fri, Oct 15, 2010 at 12:34 AM, David A. <dasolexa@hotmail.com> wrote:
>
>
> Hi list,
>
> I have a 1710x244 matrix of numerical values and I would like to calculate
> the mean of every group of three consecutive values per column to obtain a
> new matrix of 570x244.  I could get it done using a for loop but how can I
> do that using apply functions?
> In addition to this, do I have to initizalize a 570x244 matrix with 0's to
> store the calculated values or can the output matrix be generated while
> calculating the mean values?
>
> Cheers,
>
> Dave
>
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>
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