[R] Outer product from matrix by row and a vector

[David Winsemius]

On Aug 7, 2012, at 9:02 PM, Ingo Reinhold wrote:

> Dear all,
>
> I am facing a challenge when applying the outer product with a  
> matrix by rows.
>
> What I have is a rather big matrix, which I would like to convert  
> into a different matrix by doing something like
>
> outer(matrix_row, vector, function(x,y) x[1]+5*x[4]/y)


>
> In order to get there, I tried to get the matrix into a list using  
> the mefa package, as subsetting the original data appears too  
> complicated. However, if I apply the following code I always get a  
> dimensional error, as the unlist() function is applied to the whole  
> list.
>
> library(mefa)
> a<-matrix(c(1:6),ncol=3)
> b<-c(1,2,3,4)
> a<-mat2list(a,MARGIN=1)
>
> outer(a,b,function(x,y){unlist(a)[1]+unlist(a)[3]/b})

(You really should not be referring to a and b inside that function  
body. It's arguments are now named x and y.)

Can you tell us what the answer _should_ be for the first entry? And a  
step by step illustration of how you got there?

If this is not what it should be, then you need to read up on the  
expected result of the division of a scalar by a vector in R.

  apply(a, 1, function(x) { x[1] + x[3]/ b } )

          [,1] [,2]
[1,] 6.000000  8.0
[2,] 3.500000  5.0
[3,] 2.666667  4.0
[4,] 2.250000  3.5

>
>
> Any idea how to do this

Do "this" is still somewhat nebulous.

> more elegantly than a nested for-loop?
>
> Many thanks,
>
> Ingo
>

David Winsemius, MD
Alameda, CA, USA