[R] Help with vectors and rollapply

William Dunlap wdunlap at tibco.com
Tue Jul 10 18:22:17 CEST 2012

It looks like you already have the zoo package loaded so you can use its na.locf(),
which replaces NA's with the last non-NA value.  Convert the 0s to NAs with
replace() and feed the result into na.locf():
  a  <- c(-2,0,0,0,1,0,0,3,0,0,-4)
  aOut <- c(-2,-2,-2,-2,1,1,1,3,3,3,-4)
  na.locf(replace(a, a==0, NA) )
  #  [1] -2 -2 -2 -2  1  1  1  3  3  3 -4
  all.equal(aOut, .Last.value)
  # [1] TRUE

If you need to treat NA and 0 differently you will need to do more work.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
> Behalf Of Raghuraman Ramachandran
> Sent: Tuesday, July 10, 2012 8:23 AM
> To: r-help at r-project.org
> Subject: [R] Help with vectors and rollapply
> Hello
> I have a vector a =(-2,0,0,0,1,0,0,3,0,0,-4)
> I want to replace all zeros into previous non-zero state. So for instance the above
> vector should be converted into:
> a= (-2,-2,-2,-2,1,1,1,3,3,3,-4)
> I tried many things and finally concluded that probably(?) rollapply may be the best
> way?
> I tried
> f= function(x){
>                 ifelse(x==0,Lag(x),x)
> }
> And then, rollappy(a,1,f) and that didn't work. Can someone help please?
> Thx
> R
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