How to do a row-wise replacement of values using R? I have a Matrix and I would like to replace some of its values using an index vector. The problem is that R automatically does a column-wise extraction of the values as opposed to a row-wise. You will find my code and results below:
Matrix=matrix(rep(0,42),nrow=6,ncol=7,byrow=TRUE)
v=c(1,7,11,16,18)
Matrix[v]=1
Matrix
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 1 1 0 0 0 0 0
[2,] 0 0 0 0 0 0 0
[3,] 0 0 0 0 0 0 0
[4,] 0 0 1 0 0 0 0
[5,] 0 1 0 0 0 0 0
[6,] 0 0 1 0 0 0 0
What I actually want to get is the row-wise version of this meaning:
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 1 0 0 0 0 0 1
[2,] 0 0 0 1 0 0 0
[3,] 0 1 0 1 0 0 0
[4,] 0 0 0 0 0 0 0
[5,] 0 0 0 0 0 0 0
[6,] 0 0 0 0 0 0 0
>
Apparently R does a column-wise replacement of values by default. What is the best way to obtain a row-wise replacement of the values?
Thanks!
You could recalculate the onedimensional indizes to row- and column-indices. Supposing you have calculated the row-indices in the first column of the matrix Ind and the columnindices in the second column of Ind you can do Matrix[Ind] <- 1
Matrix <- matrix(rep(0,42),nrow=6,ncol=7,byrow=TRUE)
v <- c(1,7,11,16,18)
Row <- (v-1) %/% ncol(Matrix) +1
Col <- (v-1) %% ncol(Matrix) +1
Matrix[cbind(Row,Col)] <- 1
Matrix
# [,1] [,2] [,3] [,4] [,5] [,6] [,7]
# [1,] 1 0 0 0 0 0 1
# [2,] 0 0 0 1 0 0 0
# [3,] 0 1 0 1 0 0 0
# [4,] 0 0 0 0 0 0 0
# [5,] 0 0 0 0 0 0 0
# [6,] 0 0 0 0 0 0 0