CODEWITHSUNDEEP
rmatrix
RoyalTS
RoyalTS

Reputation: 10203

Filling in a matrix from a list of row,column,value

I have a data frame that contains a list of row positions, column positions and values, like so:

combs <- as.data.frame(t(combn(1:10,2)))
colnames(combs) <- c('row','column')
combs$value <- rnorm(nrow(combs))

I would like to fill in a matrix with these values such that every value appears in the matrix in exactly the position specified by row and column. I suppose I could do this manually

mat <- matrix(nrow=10,ncol=10)
for(i in 1:nrow(combs)) {
  mat[combs[i,'row'],combs[i,'column']] <- combs[i,'value']
}

But surely there is a more elegant way to accomplish this in R?

Upvotes: 3

Views: 2273

Answers (1)

flodel
flodel

Reputation: 89097

Like this:

mat <- matrix(nrow = 10, ncol = 10)
mat[cbind(combs$row, combs$column)] <- combs$value

You could also consider building a sparse matrix using the Matrix package:

library(Matrix)
mat <- sparseMatrix(i = combs$row, j = combs$column, x = combs$value,
                    dims = c(10, 10))

Upvotes: 5

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