[R] Column-by-column division

[Harold Doran]

Some timings of the different suggestions below

> library(microbenchmark)
> x <- matrix(1:20, nrow=10)
> s <- c(1,2)
> option1 <- sapply(1:2, function(i) x[,i]/s[i])
> option2 <- x %*% diag(1/s)
> option3 <- sweep(x, 2, s, '/')
> all.equal(option1,option2)
[1] TRUE
> all.equal(option2,option3)
[1] TRUE
> microbenchmark(sapply(1:2, function(i) x[,i]/s[i]), x %*% diag(1/s), sweep(x, 2, s, '/'))
Unit: microseconds
                                 expr    min      lq     mean  median     uq      max neval
sapply(1:2, function(i) x[, i]/s[i]) 18.425 19.8800 37.42147 21.5765 22.789 1583.518   100
                      x %*% diag(1/s)  1.456  2.1830  2.86150  2.9100  3.395    9.213   100
                  sweep(x, 2, s, "/") 32.486 34.1825 37.31964 35.3950 36.364  131.395   100

From: Eric Berger <ericjberger using gmail.com>
Sent: Wednesday, March 3, 2021 3:19 PM
To: Harold Doran <harold.doran using cambiumassessment.com>
Cc: Steven Yen <styen using ntu.edu.tw>; R-help Mailing List <r-help using r-project.org>
Subject: Re: [R] Column-by-column division

Why not use standard matrix multiplication which is straightforward here:

x %*% diag(1/s)

HTH,
Eric


On Wed, Mar 3, 2021 at 7:13 AM Harold Doran <harold.doran using cambiumassessment.com<mailto:harold.doran using cambiumassessment.com>> wrote:
To make sure the scalar is used instead of using the recycled vector s, maybe like this

x <- matrix(1:20, nrow=10)
s <- c(1,2)
sapply(1:2, function(i) x[,i]/s[i])

-----Original Message-----
From: R-help <r-help-bounces using r-project.org<mailto:r-help-bounces using r-project.org>> On Behalf Of Steven Yen
Sent: Wednesday, March 3, 2021 6:00 AM
To: R-help Mailing List <r-help using r-project.org<mailto:r-help using r-project.org>>
Subject: [R] Column-by-column division

I have a 10 x 2 matrix x. Like to divide the first column by s[1] and second column by s[2]. The following lines work but are clumsy. Any idea? Thanks.

 > x
       [,1] [,2]
  [1,]    1   11
  [2,]    2   12
  [3,]    3   13
  [4,]    4   14
  [5,]    5   15
  [6,]    6   16
  [7,]    7   17
  [8,]    8   18
  [9,]    9   19
[10,]   10   20
 > s
[1] 1 2
 > t(t(x)/s)
       [,1] [,2]
  [1,]    1  5.5
  [2,]    2  6.0
  [3,]    3  6.5
  [4,]    4  7.0
  [5,]    5  7.5
  [6,]    6  8.0
  [7,]    7  8.5
  [8,]    8  9.0
  [9,]    9  9.5
[10,]   10 10.0

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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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