[R] Replacing values without looping

[Petr PIKAL]

Hi

> Hi,
> 
> If you truly have an array, this is option that should be much faster
> than a loop:
> 
> index <- which(is.na(dat))
> dat[index] <- dat[index - 1]
> 
> the only catch is that when there previous value is NA, you may have
> to go through the process a few times to get them all.  One way to
> automate this would be:
> 
> index <- which(is.na(dat))
> 
> while (any(index)) {
>   dat[index] <- dat[index - 1]
>   index <- which(is.na(dat))
> }
> 
> If your dataset has many adjacent missing values, then it would be
> worth it to use a fancier technique that looks for the first previous
> nonmissing value.  There could even be a clever way with indexing that
> I am missing.

Package zoo and function ?na.locf can be used for this type of task.

Regards
Petr


> 
> HTH,
> 
> Josh
> 
> On Thu, Jun 16, 2011 at 5:13 AM, wuffmeister <hvemhva at gmail.com> wrote:
> > I got an array similar to the one below, and want to replace all NAs 
with the
> > previous value.
> > 99 8.2 b
> > NA 8.3 x
> > NA 7.9 x
> > 98 8.1 b
> > NA 7.7 x
> > 99 9.3 b
> > ...
> >
> > i.e. the first two NAs should be replaced to 99, whereas the last one 
should
> > be 98.
> >
> > I would like to apply a function to reach row, checking if the value 
in col
> > 1 is NA, and if it is, set the value to the previous row's col 1 
value.
> >
> > Haven't been able to do this without looping, which gets very slow for 
large
> > datasets...
> >
> > --
> > View this message in context: http://r.789695.n4.nabble.com/Replacing-
> values-without-looping-tp3602247p3602247.html
> > Sent from the R help mailing list archive at Nabble.com.
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
> 
> 
> 
> -- 
> Joshua Wiley
> Ph.D. Student, Health Psychology
> University of California, Los Angeles
> http://www.joshuawiley.com/
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.