Nice way to multiply each row of a matrix by a vector in r

Question

I know this is a really basic question, sorry.

I want to multiply each row of a matrix by a vector. So I have:

mat=matrix(1,2,4)
vec=c(1,2,3,4)

#works but ugly:
new.mat=mat
for(i in 1:nrow(mat)){
  new.mat[i,]=mat[i,]*vec
}

I thought I'd found the answer with 'apply' but I couldn't get it to work the same way.

score 12 · Accepted Answer · answered Jul 18 '13 at 15:35

I think this is what you are looking for...

t( t(mat) * vec )
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    1    2    3    4

* like most other operators in R is vectorised. The t is necessary because R will recycle column-wise. The apply solution is:

t( apply( mat , 1 , `*` , vec ) )
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    1    2    3    4

score 4 · Answer 2 · answered Jul 18 '13 at 15:37

4

using apply

> t(apply(mat,1 , function(x) x*vec))
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    1    2    3    4

answered Jul 18 '13 at 15:37

Jilber Urbina

Very elegant way, like Simon. I upvoted yours because I like the function to be contained in a single part instead of being splited by a comma – Yohan Obadia May 25 '16 at 16:07

score 2 · Answer 3 · answered Jul 18 '13 at 15:38

2

I'd have to say the most elegant way to go about this is:

mat=matrix(1,2,4)
vec=1:4
new.mat=t(vec*t(mat))

answered Jul 18 '13 at 15:38

nwknoblauch

score 2 · Answer 4 · answered Jul 18 '13 at 16:16

2

I prefer sweep, although I think it's marginally slower:

sweep(mat,MARGIN=2,vec,"*")

MARGIN=2 specifies that each column (margin 2) should be multiplied by the corresponding element of vec.

answered Jul 18 '13 at 16:16

Ben Bolker

4 Answers4