I want to subtract ith value of the vector from non zero values of the ith row of a sparse matrix for e.g.
[,1] [,2] [,3] [,4]
[1,] 0 0 4 0
[2,] 0 5 0 3
[3,] 1 2 0 0
and here is the vector that i am trying to subtract:
[1] 1 2 3
so what i need in the end is:
[,1] [,2] [,3] [,4]
[1,] 0 0 3 0
[2,] 0 3 0 1
[3,] -2 -1 0 0
I have tried this using apply but haven't been able to figure the problem out, it does not return me quite what i want. The dimensions of the matrix are too large and I do not want to use loops. Thanks and regards.
Since subtracting a vector from a matrix is performed column-wise, mat-vec does the necessary subtraction. Since you only want to use this when the original matrix was non-zero (and return 0 for the elements that were originally 0), you could multiply by mat != 0, which is a 1/0 (TRUE/FALSE) matrix stating whether the original element was non-zero.
(mat - vec) * (mat != 0)
# [,1] [,2] [,3] [,4]
# [1,] 0 0 3 0
# [2,] 0 3 0 1
# [3,] -2 -1 0 0
If you instead wanted to do it for a sparse matrix:
library(Matrix)
(mat <- sparseMatrix(i=c(3, 2, 3, 1, 2), j=c(1, 2, 2, 3, 4), x=c(1, 5, 2, 4, 3)))
# 3 x 4 sparse Matrix of class "dgCMatrix"
# [1,] . . 4 .
# [2,] . 5 . 3
# [3,] 1 2 . .
vec <- c(1, 2, 3)
mat@x <- mat@x - vec[mat@i+1]
mat
# 3 x 4 sparse Matrix of class "dgCMatrix"
# [1,] . . 3 .
# [2,] . 3 . 1
# [3,] -2 -1 . .