Efficient way to use data frame of indices to initialize matrix

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
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
> 
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Efficient way to use data frame of indices to initialize matrix

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