CODEWITHSUNDEEP
rmatrixscalar
Christofer Östlin
Christofer Östlin

Reputation: 33

Multiplying one matrix with a set of scalars

I have an R script that systematically perform changes to a 3x3 matrix by scalar multiplication as follows

R <- matrix(rexp(9, rate=.1), ncol=3);
for (i in 1:360) {
K = i*R;
...
}

and use the modified matrix for further calculations within the loop. However, the loop itself is nested inside two other for loops, making the script very slow. So my question is, how can I vectorize this innermost loop such that the result instead is a three-dimensional array A of size 3x3x360 where

A[,,i] = i*R;

for all i ranging from 1 to 360?

Upvotes: 3

Views: 70

Answers (1)

MrFlick
MrFlick

Reputation: 206421

How about some basic multiplication and reshaping

set.seed(15) #for reproducibility
R <- matrix(rexp(9, rate=.1), ncol=3);
R
#           [,1]      [,2]      [,3]
# [1,]  2.042281  1.760375 2.9230182
# [2,] 19.466458  6.628580 0.1818078
# [3,]  2.544348 27.541514 4.1325714

dd <- array(sapply(1:360, `*`, R), dim=c(dim(R), 360))
dd[,,1]
#           [,1]      [,2]      [,3]
# [1,]  2.042281  1.760375 2.9230182
# [2,] 19.466458  6.628580 0.1818078
# [3,]  2.544348 27.541514 4.1325714
dd[,,2]
#           [,1]      [,2]      [,3]
# [1,]  4.084562  3.520749 5.8460364
# [2,] 38.932916 13.257161 0.3636157
# [3,]  5.088697 55.083027 8.2651427
dd[,,10]
#           [,1]      [,2]      [,3]
# [1,]  20.42281  17.60375 29.230182
# [2,] 194.66458  66.28580  1.818078
# [3,]  25.44348 275.41514 41.325714

Upvotes: 4

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