R help - Dec 2010 - 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

index m as a vector and do the assignment in one step

i <- df$row + (df$col-1)*nrow(m)
m[i] <- df$a

or something along those lines.

-Whit


On Tue, Dec 7, 2010 at 1:31 PM, Cutler, Gene <gcutler at amgen.com>
wrote:
> 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.
>

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
> 
> ______________________________________________
> 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.