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.s...@imail.org
801.408.8111
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-
project.org] On Behalf Of Cutler, Gene
Sent: Tuesday, December 07, 2010 11:31 AM
To: r-help@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
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______________________________________________
R-help@r-project.org mailing list
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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
______________________________________________
R-help@r-project.org mailing list
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
