[R] Modifying a data frame based on a vector that contains column numbers

[William Dunlap]

Try looping over columns, as in

fDF <- function (x, column)
{
    stopifnot(length(dim(x))==2, all(column > 0), all(column <= ncol(x)), length(column) == nrow(x))
    u <- unique(column)
    tmp <- split(seq_along(column), factor(column, levels = u))
    for (i in seq_along(tmp)) {
        x[ tmp[[i]],  u[i] ] <- 1
    }
    x
}

> fDF(mydf, myindex)
  c1 c2 c3
1  1 NA NA
2 NA  1 NA
3 NA NA  1
4 NA  1 NA
5  1 NA NA

If you use a matrix instead of a data.frame then the following works and is probably much quicker.
fMat <- function (x, column) 
{
    stopifnot(is.matrix(x), all(column > 0), all(column <= ncol(x)), length(column) == nrow(x))
    x[cbind(seq_len(nrow(x)), column)] <- 1
    x
}

Your problem may be better represented with sparse matrices (see the Matrix package).

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 Dimitri Liakhovitski
> Sent: Wednesday, March 13, 2013 5:11 PM
> To: r-help
> Subject: [R] Modifying a data frame based on a vector that contains column numbers
> 
> Hello!
> 
> # I have a data frame:
> mydf<-data.frame(c1=rep(NA,5),c2=rep(NA,5),c3=rep(NA,5))
> 
> # I have an index whose length is always the same as nrow(mydf):
> myindex<-c(1,2,3,2,1)
> 
> # I need c1 to have 1s in rows 1 and 5 (based on the information in myindex)
> # I need c2 to have 1s in rows 2 and 4 (also based on myindex)
> # I need c3 to have 1 in row 3
> # In other words, I am trying to achieve this result:
> mygoal<-data.frame(c1=c(1,NA,NA,NA,1),c2=c(NA,1,NA,1,NA),c3=c(NA,NA,1,NA,NA))
> 
> I know how to do it with a loop that runs through rows of mydf.
> However, in real life I have a huge data frame with tons of rows, dozens of
> columns (instead of 3 in this example) - I am afraid it'll take forever.
> Any hint on how to do it faster, maybe using subindexing somehow?
> 
> Thank you very much!
> 
> --
> Dimitri Liakhovitski
> 
> 	[[alternative HTML version deleted]]
> 
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