> -----Original Message-----
> From: r-help-boun...@r-project.org
> [mailto:r-help-boun...@r-project.org] On Behalf Of Doran, Harold
> Sent: Thursday, February 25, 2010 10:35 AM
> To: r-help@r-project.org
> Subject: [R] Restructure some data
>
> Suppose I have a data frame like "dat" below. For some
> context, this is the format that represents student's taking
> a computer adaptive test. first.item is the first item that
> student was administered and then score.1 is the student's
> response to that item and so forth.
>
> item.pool <- paste("item", 1:10, sep = "")
> set.seed(54321)
> dat <- data.frame(id = c(1,2,3,4,5), first.item =
> sample(item.pool, 5, replace=TRUE),
> second.item = sample(item.pool,
> 5,replace=TRUE), third.item = sample(item.pool, 5,replace=TRUE),
> score1 = sample(c(0,1), 5,replace=TRUE),
> score2 = sample(c(0,1), 5,replace=TRUE), score3 =
> sample(c(0,1), 5,replace=TRUE))
>
> I need to restructure this into a new format. The new matrix
> df (after the loop) is exactly what I want in the end. But,
> I'm annoyed at myself for not thinking of a more efficient
> way to restructure this without using a loop.
>
> df <- matrix(NA, ncol = length(item.pool), nrow = nrow(dat))
> colnames(df) <- unique(item.pool)
>
> for(i in 1:5){
> for(j in 2:4){
> rr <- which(dat[i,j] == colnames(df))
> df[i,rr] <- dat[i, (j+3)]
> }
> }
>
> Any thoughts?
You can try subscripting by a 2-column matrix, the first
giving the row index and the second the column index. E.g.,
> f <- function(dat) {
allItems <- paste("item", 1:10, sep = "")
items <- as.matrix(dat[2:4])
scores <- as.matrix(dat[, 5:7])
retval <- matrix(NA_real_, nrow = nrow(dat), ncol = 10,
dimnames = list(character(), allItems))
retval[cbind(dat$id, match(items, allItems))] <- scores
retval
}
> identical(f(dat), df)
[1] TRUE
That was a very nice problem description, letting me
reproduce the example data and desired output with
just copy and paste.
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
>
> Harold
>
> [[alternative HTML version deleted]]
>
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