[R] matrix column division by vector

[David L Carlson]

Bert wins the race:

> system.time(replicate(1e5, m/rep(v,e=2)))
   user  system elapsed 
   0.25    0.00    0.25 
> system.time(replicate(1e5, m/matrix( v, ncol=ncol(m), nrow=nrow(m), byrow=TRUE)))
   user  system elapsed 
   0.42    0.00    0.42 
> system.time(replicate(1e5, t(t(m)/v)))
   user  system elapsed 
   1.31    0.00    1.33 
> system.time(replicate(1e5, sweep(m, 2, v, "/")))
   user  system elapsed 
   3.39    0.00    3.40 
> system.time(replicate(1e5, t(apply(m, 1, function(x) x/v))))
   user  system elapsed 
   5.04    0.01    5.06

David C

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Jeff Newmiller
Sent: Wednesday, May 14, 2014 10:28 AM
To: carol white; carol white; r-help at stat.math.ethz.ch
Subject: Re: [R] matrix column division by vector

Please post in plain text... your email is getting distorted and hard to read by the HTML.

I don't know how to use do.call for this, but when you understand how vectors recycle and matrices and arrays are laid out in memory (read the Introduction to R document if not) then the following comes to mind:

mat2 <- m / matrix( v, ncol=ncol(m), nrow=nrow(m), byrow=TRUE )

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On May 14, 2014 7:51:36 AM PDT, carol white <wht_crl at yahoo.com> wrote:
>Hi,
>What is the elegant script to divide the columns of a matrix by the
>respective position of a vector elements?
>
>m=rbind(c(6,4,2),c(3,2,1))
>
>v= c(3,2,1)
>
>res= 6/3�� 4/2� 2/1
>������� 3/3�� 2/2 �� 1/1
>
>
>this is correct�
>mat2 = NULL
>
>for (i in 1: ncol(m))
>
>��� mat2 = cbind(mat2, m[,i]/ v[i])
>
>
>but how to do more compact and elegant with for ex do.call?
>
>Many thanks
>
>Carol
>	[[alternative HTML version deleted]]
>
>
>
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>
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