[R] Conditionally adding a constant

[jim holtman]

Here is a way of doing it without loops:

> df <- data.frame(x = c(1,2,3,4,5), y = c(10,20,30,NA,NA))
>
> require(zoo)  # need na.locf to fix the NAs
>
> # replace NA with preceeding values
> df$y <- na.locf(df$y)
> df
  x  y
1 1 10
2 2 20
3 3 30
4 4 30
5 5 30
>
> # assuming that you want to increment the counts when x > 3
> inc <- cumsum(df$x > 3) * 2
> inc
[1] 0 0 0 2 4
>
> df$y <- df$y + inc
> df
  x  y
1 1 10
2 2 20
3 3 30
4 4 32
5 5 34
>
>
>
>


On Mon, Jan 2, 2012 at 1:59 PM, Joshua Wiley <jwiley.psych at gmail.com> wrote:
> Here is another approach.  Probably with some thought and fingerwork,
> rle() could be used to avoid the while loop, but that should only slow
> things down if there are long runs of NAs --- there can be a lot of
> NAs as long as they are spaced apart and it should still be quite
> efficient.
>
> f <- function(x, y) {
>  i <- which(x > 3)
>  cond <- TRUE
>  while (cond) {
>    y[i] <- y[i - 1] + 2L
>    cond <- any(is.na(y))
>  }
>  return(y)
> }
>
> df <- data.frame(x = c(1,2,3,4,5), y = c(10,20,30,NA,NA))
>
> df$y <- f(df$x, df$y)
>
> Cheers,
>
> Josh
>
> On Mon, Jan 2, 2012 at 4:47 AM, Rui Barradas <ruipbarradas at sapo.pt> wrote:
>> Hello,
>>
>> I believe this works.
>>
>> f1 <- function(x){
>>        for(i in 2:length(x)) x[i] <- ifelse(x[i-1] > 3, x[i-1] + 2, x[i])
>>        x
>> }
>>
>> f2 <- function(x){
>>        for(i in 2:length(x)) x[i] <- ifelse(is.na(x[i]) & (x[i-1] > 3), x[i-1] +
>> 2, x[i])
>>        x
>> }
>>
>> df <- data.frame(x = c(1,2,3,4,5), y = c(10,20,30,NA,NA))
>>
>> apply(df, 2, f1)      # df$x[4] > 3, df$x[5] also changes
>> apply(df, 2, f2)      # only df$y has NA's
>>
>> Maybe there's a better way, avoiding the loop.
>>
>> Rui Barradas
>>
>>
>> --
>> View this message in context: http://r.789695.n4.nabble.com/Conditionally-adding-a-constant-tp4253049p4253125.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
> Programmer Analyst II, Statistical Consulting Group
> University of California, Los Angeles
> https://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.



-- 
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?
Tell me what you want to do, not how you want to do it.