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$a-- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare [email protected] 801.408.8111 > -----Original Message----- > From: [email protected] [mailto:r-help-boun...@r- > project.org] On Behalf Of Cutler, Gene > Sent: Tuesday, December 07, 2010 11:31 AM > To: [email protected] > Subject: [R] Efficient way to use data frame of indices to initialize > matrix > > I have a data frame with three columns, x, y, and a. I want to create > a matrix from these values such that for matrix m: > m[x,y] == a > > Obviously, I can go row by row through the data frame and insert the > value a at the correct x,y location in the matrix. I can make that > slightly more efficient (perhaps), by doing something like this: > > for (each.x in unique(df$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 > > ______________________________________________ > [email protected] 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. ______________________________________________ [email protected] 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.
