two-sided moving average?

Question

how to get two sided "moving average" that is a function that averages n numbers from right and left of a vector and gives them weights according to their distance from center value ?

I tried to use TTR but its moving averages works only from left to right and set leftmost values as NA. So I cannot use that smoothed vector as a input to smooth.spline

Answer 1

In the zoo package rollmean and rollapply have arguments that allow numerous variations.

library(zoo)
x <- seq(10)^2

# no NAs at end
rollmean(x, 3)

# NAs at ends
rollmean(x, 3, na.pad = TRUE)

# weighted mean
rollapply(zoo(x), 3, function(x) c(1, 2, 1) %*% x / 4) 

# at ends take means of less than 3 points - needs devel version
# partial= is in development and at this point must use na.rm = TRUE to use partial
source("http://r-forge.r-project.org/scm/viewvc.php/*checkout*/pkg/zoo/R/rollapply.R?revision=802&root=zoo")
rollapply(zoo(x), 3, mean, partial = TRUE, na.rm = TRUE)

EDIT:

Note that since this was written the development version of zoo was changed so that instead of writing partial = TRUE one writes rule = "partial" or rule = 3. The problem was that as new end rules were added to the development version (there are now 3 and a 4th will be added before its released) having a separate argument for each one clutters the user interface. Also rule is more consistent with approx in the core of R. In fact, rule=1 and rule=2 will have the same meaning in rollapply and in approx (from the core of R) for better consistency and ease of use. The parentheses around mean in the example below are currently required in the development version to prevent it from calling rollmean, where rule="partial" has not yet been implemented, but the need to do that will be eliminated by the time its officially released.

source("http://r-forge.r-project.org/scm/viewvc.php/*checkout*/pkg/zoo/R/rollapply.R?revision=815&root=zoo")
rollapply(zoo(x), 3, (mean), rule = "partial")

Answer 2

Look at the filter() function, and particularly the sides argument:

filter                  package:stats             R Documentation

Linear Filtering on a Time Series

Description:

     Applies linear filtering to a univariate time series or to each
     series separately of a multivariate time series.

Usage:

     filter(x, filter, method = c("convolution", "recursive"),
            sides = 2, circular = FALSE, init)

Arguments:
[...] 
   sides: for convolution filters only. If ‘sides=1’ the filter
          coefficients are for past values only; if ‘sides=2’ they are
          centred around lag 0. In this case the length of the filter
          should be odd, but if it is even, more of the filter is
          forward in time than backward.