[R] matrix manipulation

[cowboy]

thank you, Petr.
This is exactly what I'm looking for in my post.
An related question can be how to get an arbitrary weight, say if row1
and row 2 have 1 common value 1, then assign a weight 10, if row 1 and
row 2 have 2 common value 1, then assign a weight 12. I'm not so sure
how to expand your method.
Really  need to refresh my linear algebra;-)
regards,
cal at cowbo

On 6/14/12, Petr Savicky <savicky at cs.cas.cz> wrote:
> 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.
>
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