[R] matrix manipulation

[Petr Savicky]

On Thu, Jun 14, 2012 at 01:11:45PM +0000, G. Dai wrote:
> Dear Rlisters,
> I'm writing to ask how to manipulate a matrix or dataframe in a specific way.
> 
> To elaborate, let's consider an example. Assume we have the following
> 3 by 4 matrix A with elements either 0 or 1,
> 0  1  1  0
> 1  0  1  1
> 0  0  0  1
> 
> >From the original matrix A, I'd like to generate a new matrix B satisfing
> 1) B is initially set to be equal to A, and then
> 2) Each row of B is a weighted sum of rows of A
> 3) The weight is given by the number of common 1 shared by the rows.
> For row 1:
>         it has one common 1 with row 2, thus the weight is 1;
>         it has zero common 1 with row 3, thus the weight is 0;
> For row 2:
>         it has one common 1 with row 1, thus the weight is 1;
>         it has one common 1 with row 3, thus the weight is 1;
> For row 3:
>         it has zero common 1 with row 1, thus the weight is 0;
>         it has one common 1 with row 2, thus the weight is 1;
> 4) In this way, the new matrix B is
> 1  1  2  1
> 1  1  2  2
> 1  0  1  2

Hi.

If i understand correctly, each row of B is obtained from the
corresponding row of A by adding mutliples of other rows using
the weights as described. Try the following.

  a <- rbind(
  c(0, 1, 1, 0),
  c(1, 0, 1, 1),
  c(0, 0, 0, 1))

  w <- a %*% t(a)
  diag(w) <- 1

  w %*% a

       [,1] [,2] [,3] [,4]
  [1,]    1    1    2    1
  [2,]    1    1    2    2
  [3,]    1    0    1    2

The entries of w are the weights except of the diagonal, which
is all ones, since each row of A contributes to the same row of B
with weight one. Is this, what you are looking for?

Petr Savicky.