Multiply rows of matrix by vector?

r vector matrix multiplication
pixel picture pixel · Sep 4, 2010 · Viewed 74.6k times · Source

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

Answer

hatmatrix picture hatmatrix · Sep 6, 2010

I think you're looking for sweep(). 

# Create example data and vector
mat <- 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

vec <- 1:5

# Use sweep to apply the vector with the multiply (`*`) function
#  across columns (See ?apply for an explanation of MARGIN) 
sweep(mat, MARGIN=2, vec, `*`)
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    2    3    4    5
[2,]    2    4    6    8   10
[3,]    3    6    9   12   15

It's been one of R's core functions, though improvements have been made on it over the years.

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