[R] Element-by-element division

[Sarah Goslee]

Hi,

It's a good idea to keep discussion on R-help, so others can
participate and the results make it into the archives.

On Mon, Jul 27, 2015 at 9:11 PM, Steven Yen <syen04 at gmail.com> wrote:
> Thanks Sarah. That serves my need. I however find ?sweep hard to comprehend.

Heh. The help makes it seem more complicated than it really is. It
isn't that hard:

sweep(a, 2, b, "/")

a: the object to act on
2: the direction to go (1 for rows, 2 for columns - a has 2 columns
and b is of length 2, so you need to choose columns)
b: the vector to use (?sweep calls this "the summary statistic"
because the use case was originally conceived of as being: "divide
columns by standard deviation" and such)
"/": the function to use

so to add vector x to the rows of a, you'd do:
sweep(a, 1, x, "+")

The default FUN is "-", so the first example in the help subtracts the
median from the columns:
     require(stats) # for median
     med.att <- apply(attitude, 2, median)
     sweep(data.matrix(attitude), 2, med.att)  # subtract the column medians

The complicated bits come in if b is an array instead of a vector, or
if the dimensions aren't identical. For your case, and for most cases,
none of that matters.

Sarah

> What am I missing? The S language?
> Steven Yen
>
> On 7/27/2015 4:17 PM, Sarah Goslee wrote:
>
> Hi,
>
> See ?sweep
>
> For instance, to get your matrix two:
>
> sweep(a, 2, b, "/")
>
>      [,1] [,2]
> [1,]    2   24
> [2,]    4   28
> [3,]    6   32
> [4,]    8   36
> [5,]   10   40
>
>
> Sarah
>
> On Mon, Jul 27, 2015 at 4:04 PM, Steven Yen <syen04 at gmail.com> wrote:
>
> I need help with element-by-element division. Below, matrices a and c are
> both 5 x 2 and element-by-element division works as (I) expected. What if
> matrix is 1 by 2: to divide first column of a by b[1] and second column of
> a by b[2]. I had to go around (two ways) to make it work. In Gauss, these
> can be dine by a./b and a./c. Any such simple way in R? Thank!
>
> a<-matrix(1:10,nrow=5); a
>
>      [,1] [,2]
> [1,]    1    6
> [2,]    2    7
> [3,]    3    8
> [4,]    4    9
> [5,]    5   10
>
> b<-matrix(c(0.5,0.25),nrow=1); b
>
>      [,1] [,2]
> [1,]  0.5 0.25
>
> c<-matrix(rep(c(0.5,0.25),5),nrow=5,byrow=T); c
>
>      [,1] [,2]
> [1,]  0.5 0.25
> [2,]  0.5 0.25
> [3,]  0.5 0.25
> [4,]  0.5 0.25
> [5,]  0.5 0.25
>
> one<-a/c; one     [,1] [,2]
>
> [1,]    2   24
> [2,]    4   28
> [3,]    6   32
> [4,]    8   36
> [5,]   10   40
>
>
> two<-a/b
>
> Error in a/b : non-conformable arrays
>
> two<-cbind(a[,1]/b[1],a[,2]/b[2]); two
>
>      [,1] [,2]
> [1,]    2   24
> [2,]    4   28
> [3,]    6   32
> [4,]    8   36
> [5,]   10   40
>
> b2<-matrix(rep(b,5),nrow=5,byrow=T); b2     [,1] [,2]
>
> [1,]  0.5 0.25
> [2,]  0.5 0.25
> [3,]  0.5 0.25
> [4,]  0.5 0.25
> [5,]  0.5 0.25> a/b2     [,1] [,2]
> [1,]    2   24
> [2,]    4   28
> [3,]    6   32
> [4,]    8   36
> [5,]   10   40
>
>