CODEWITHSUNDEEP
rmatrixmatrix-indexing
Kinin
Kinin

Reputation: 55

R programming: How to do replace values by row?

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!

Upvotes: 0

Views: 1515

Answers (3)

akrun
akrun

Reputation: 887008

We can do

+(matrix(seq_along(Matrix) %in% v, ncol=ncol(Matrix), nrow=nrow(Matrix), byrow=TRUE))
#     [,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

Upvotes: 1

jogo
jogo

Reputation: 12559

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

Upvotes: 2

Jason
Jason

Reputation: 1569

You could redo your 1's to make them row-wise or you can do the following:

Matrix=matrix(rep(0,42),nrow=6,ncol=7,byrow=TRUE)
v=c(1,7,11,16,18)
Matrix<-t(Matrix)
Matrix[v]=1
Matrix<-t(Matrix)
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

Upvotes: 0

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