[R] Filling NA with cumprod?

[David L Carlson]

This will loop only as many times as the largest number of consecutive NA's
but uses vectorization within the loop. As currently defined, it will loop
forever if the first value is NA.

a <- c(1, 2, 3, NA, NA, 6, 7, NA, NA, 10)
f <- c(0.9, 1.1, 0.9, 1.1, 0.9, 1.1, 0.9, 1.1, 0.9, 1.1)

a1 <- a
alag <- c(NA, a1[1:length(a1)-1]) 
# change NA to the value to use if the first value in a is NA

while (sum(is.na(a1)) > 0) {
  a1 <- ifelse(is.na(a1), f*alag, a1)
  alag <- c(NA, a1[1:length(a1)-1])
}

----------------------------------------------
David L Carlson
Associate Professor of Anthropology
Texas A&M University
College Station, TX 77843-4352


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Igor Reznikovsky
> Sent: Friday, May 25, 2012 9:08 AM
> To: Petr Savicky
> Cc: r-help at r-project.org
> Subject: Re: [R] Filling NA with cumprod?
> 
> Hello Petr,
> 
> Yes, I was hoping to avoid using loops.  If nothing else works, I will
> take
> approach as the last resort.
> 
> Thank you,
> Igor.
> On May 25, 2012 2:26 AM, "Petr Savicky" <savicky at cs.cas.cz> wrote:
> 
> > On Thu, May 24, 2012 at 08:24:38PM -0700, igorre25 wrote:
> > > Hello,
> > >
> > > I need to build certain interpolation logic using R.
> Unfortunately, I
> > just
> > > started using R, and I'm not familiar with lots of advanced or just
> > > convenient features of the language to make this simpler.  So I
> struggled
> > > for few days and pretty much reduced the whole exercise  to the
> following
> > > problem, which I cannot resolve:
> > >
> > > Assume we have a vector of some values with NA:
> > > a <- c(1, 2, 3, NA, NA, 6, 7, NA, NA, 10)
> > >
> > > and some coefficients as a vector of the same length:
> > >
> > > f <- c(0.9, 1.1, 0.9, 1.1, 0.9, 1.1, 0.9, 1.1, 0.9, 1.1)
> > >
> > > I need to come up with function to get the following output
> > >
> > > o[1] = a[1]
> > > o[2] = a[2]
> > > o[3] = a[3]
> > > o[4] = o[3]*[f3] # Because a[3] is NA
> > > o[5] = o[4]*[f4] # Because a[4] is NA; This looks like recursive
> > > calculations;  If the rest of the elements we NA, I would use a *
> > c(rep(1,
> > > 3), cumprod(f[3:9])), but that's not the case
> > > o[6] = a[6] # Not NA anymore
> > > o[7] = a[7]
> > > o[8] = o[7]*f[7] # Again a[8] is NA
> > > o[9] = o[8]*f[8]
> > > o[10] = a[10] # Not NA
> > >
> > > Even though my explanation may seems complex, in reality the
> requirement
> > is
> > > pretty simple and in Excel is achieved with a very short formula.
> > >
> > > The need to use R is to demonstrate capabilities of the language
> and
> > then to
> > > expand to more complex problems.
> >
> > Hello:
> >
> > How is the output defined, if a[1] is NA?
> >
> > I think, you are not asking for a loop solution. However, in this
> case,
> > it can be a reasonable option. For example
> >
> >  a <- c(1, 2, 3, NA, NA, 6, 7, NA, NA, 10)
> >  f <- c(0.9, 1.1, 0.9, 1.1, 0.9, 1.1, 0.9, 1.1, 0.9, 1.1)
> >  n <- length(a)
> >  o <- rep(NA, times=n)
> >
> >  prev <- 1
> >  for (i in 1:n) {
> >      if (is.na(a[i])) {
> >          o[i] <- f[i]*prev
> >      } else {
> >          o[i] <- a[i]
> >      }
> >      prev <- o[i]
> >  }
> >
> > A more straightforward translation of the Excel formulas is
> >
> >  getCell <- function(i)
> >  {
> >      if (i == 0) return(1)
> >      if (is.na(a[i])) {
> >          return(f[i]*getCell(i-1))
> >      } else {
> >          return(a[i])
> >      }
> >  }
> >
> >  x <- rep(NA, times=n)
> >  for (i in 1:n) {
> >      x[i] <- getCell(i)
> >  }
> >
> >  identical(o, x) # [1] TRUE
> >
> > Petr Savicky.
> >
> > ______________________________________________
> > 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.
> >
> 
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> 
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.