CODEWITHSUNDEEP
rmatrixmatrix-multiplicationmultiplication
Jane
Jane

Reputation: 47

Multiply one column of matrix with fixed factor

this is probably a really trivial question, but I have not found a workable solution to it yet. There is a similar question posted here, but it did not get a workable answer, so I am grateful for any advice.

I have a simple 3x3 matrix and want to multiply the 2nd column by a fixed factor. Example:

m<-matrix(rep(c(1,2,3),1*3),nrow=3)
m
     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    2    2
[3,]    3    3    3

If I try to do m[,2]*5, it gives me [1] 5 10 15

The result I would like looks like this: 

     [,1] [,2] [,3]
[1,]    1    5    1
[2,]    2    10   2
[3,]    3    15   3

It can't be that difficult, can it?

Upvotes: 2

Views: 163

Answers (2)

IRTFM
IRTFM

Reputation: 263301

I think a more general approach than single column assignment would be a sweep with the vector c(1,5,1):

 sweep(m, 2, c(1,5,1), "*")
     [,1] [,2] [,3]
[1,]    1    5    1
[2,]    2   10    2
[3,]    3   15    3

Then you could apply a single vector to a matrix and not need to do repeated single column operations:

sweep(m, 2, c(2,5,10), "*")
     [,1] [,2] [,3]
[1,]    2    5   10
[2,]    4   10   20
[3,]    6   15   30

@akrun's solution in the comments m*c(1, 5, 1)[col(m)] seems to do this efficiently, but I had difficulty understanding how it works. It seems to depend on the precedence of operations and would have been easier for me to understand if it had been written: c(1, 5, 1)[col(m)]*m. That makes it clearer that the vector c(1, 5, 1)[col(m)] is computed first rather than m*c(2,5,10) being extracted by [col(m)].

Upvotes: 0

PinkFluffyUnicorn
PinkFluffyUnicorn

Reputation: 1306

Just assign it like this m[,2] <- m[,2]*5

Upvotes: 1

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