Multiply rows of matrix by vector?

user_296427 0
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I have a numeric matrix with 25 columns and 23 rows, and a vector of length 25. How can I multiply each row of the matrix by the vector without using a for loop?

The result should be a 25x23 matrix (the same size as the input), but each row has been multiplied by the vector.

Added reproducible example from @hatmatrix's answer:

matrix <- matrix(rep(1:3,each=5),nrow=3,ncol=5,byrow=TRUE)

     [,1] [,2] [,3] [,4] [,5]
[1,]    1    1    1    1    1
[2,]    2    2    2    2    2
[3,]    3    3    3    3    3

vector <- 1:5

Desired output:

     [,1] [,2] [,3] [,4] [,5]
[1,]    1    2    3    4    5
[2,]    2    4    6    8   10
[3,]    3    6    9   12   15
multiplication vector r matrix
6 Answers

For speed one may create matrix from the vector before multiplying

mat <-  matrix(rnorm(1e6), ncol=1e4)
vec <- c(1:1e4)
mat * matrix(vec, dim(mat)[1], length(vec))

library(microbenchmark)
microbenchmark(
  transpose = t(t(mat) * vec), 
  make_matrix = mat * matrix(vec, dim(mat)[1], length(vec), byrow = TRUE),
  sweep = sweep(mat,MARGIN=2,vec,`*`))
#Unit: milliseconds
#       expr      min        lq     mean    median       uq      max neval cld
#  transpose 9.940555 10.480306 14.39822 11.210735 16.19555 77.67995   100   b
#make_matrix 5.556848  6.053933  9.48699  6.662592 10.74121 74.14429   100   a 
#      sweep 8.033019  8.500464 13.45724 12.331015 14.14869 77.00371   100   b
user_44662550

These solutions using outer() or collapse::TRA() are is significantly faster than anything suggested here.

user_110017510

If you want speed, you can use Rfast::eachrow. It is the fastest from all...

user_31616250
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