On 6/11/2009 12:34 PM, Dan Ruderman wrote:
Hopefully this is straightfoward.
I have an matrix which is mostly zeroes. I want to assign it
some non-zero elements whose rows, columns, and values I know.
As a simple example, say I create a 3x2 matrix of zeros:
m <- matrix(rep(0,6),nrow=3)
Now say I want to make the [1,1] and [3,2] elements of this
matrix be non-zer, so I create two vectors, one for rows and one for cols:
rows <- c(1,3)
cols <- c(1,2)
And I have two values to be put in these locations:
vals <- c(-1,1)
What I'd like to do is something like:
m[rows,cols] <- vals
But what I get instead is:
m
[,1] [,2]
[1,] -1 -1
[2,] 0 0
[3,] 1 1
What I hoped to see is:
m
[,1] [,2]
[1,] -1 0
[2,] 0 0
[3,] 0 1
If anyone can offer some advice I'd be most thankful.
If the index to a matrix is a two column matrix, then the first column
is taken to be a row number, the second a column number. So you get
what you want with
m[cbind(rows,cols)] <- vals
This is discussed (and other indexing methods too) in the R Language
Definition manual in the section on indexing matrices and arrays.
Duncan Murdoch
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