Efficient way to use data frame of indices to initialize matrix
On Dec 7, 2010, at 1:49 PM, Greg Snow wrote:
tmpdf <- data.frame( x = c(1,2,3), y=c(2,3,1), a=c(10,20,30) )
mymat <- matrix(0, ncol=3, nrow=3)
mymat[ as.matrix(tmpdf[,c('x','y')]) ] <- tmpdf$a
cbind is also useful for assembly of arguments to the matrix-`[<-`
function:
tmpdf <- data.frame( x = c(1,2,3), y=c(2,3,1), a=c(10,20,30) )
mymat <- matrix(NA, ncol=max(tmpdf$y), nrow=max(tmpdf$x))
mymat[ cbind(tmpdf$x,tmpdf$y) ] <- tmpdf$a
mymat
[,1] [,2] [,3]
[1,] NA 10 NA
[2,] NA NA 20
[3,] 30 NA NA
-- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.snow at imail.org 801.408.8111
-----Original Message----- From: r-help-bounces at r-project.org [mailto:r-help-bounces at r- project.org] On Behalf Of Cutler, Gene Sent: Tuesday, December 07, 2010 11:31 AM To: r-help at r-project.org Subject: [R] Efficient way to use data frame of indices to initialize matrix I have a data frame with three columns, x, y, and a. I want to create a matrix from these values such that for matrix m: m[x,y] == a Obviously, I can go row by row through the data frame and insert the value a at the correct x,y location in the matrix. I can make that slightly more efficient (perhaps), by doing something like this:
for (each.x in unique(df$x)) m[each.x, df$y[df$x == each.x]] <-
df$a[df$x == each.x] But I feel that there must be a more efficient, or at least more elegant way to do this. -- Gene
______________________________________________ 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.
______________________________________________ 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.
David Winsemius, MD West Hartford, CT