subtract a constant vector from each row in a matrix in r

Question

I have a matrix with 5 columns and 4 rows. I also have a vector with 3 columns. I want to subtract the values in the vector from columns 3,4 and 5 respectively at each row of the matrix.

b <- matrix(rep(1:20), nrow=4, ncol=5)
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    5    9   13   17
[2,]    2    6   10   14   18
[3,]    3    7   11   15   19
[4,]    4    8   12   16   20

c <- c(5,6,7)

to get

     [,1] [,2] [,3] [,4] [,5]
[1,]    1    5    4    7   10
[2,]    2    6    5    8   11
[3,]    3    7    6    9   12
[4,]    4    8    7   10   13

Answer 1

This is exactly what sweep was made for:

b <- matrix(rep(1:20), nrow=4, ncol=5)
x <- c(5,6,7)

b[,3:5] <- sweep(b[,3:5], 2, x)
b

#     [,1] [,2] [,3] [,4] [,5]
#[1,]    1    5    4    7   10
#[2,]    2    6    5    8   11
#[3,]    3    7    6    9   12
#[4,]    4    8    7   10   13

..or even without subsetting or reassignment:

sweep(b, 2, c(0,0,x))