[R] How to define new matrix based on an elementary row operation in a single step?

[David Winsemius]

On Aug 28, 2010, at 2:54 AM, Joshua Wiley wrote:

> Is this sufficiently single steppish for you?
>
> D <- A <- matrix(1:16, 4)
> D[3, ] <- 2 * D[1, ] + D[3, ]
>
> # Alternately, you could do this
> # but it is much messier, and I do not see how
> # two steps is really an issue
> # you want to end up with two matrices anyways
> # so it's not like you save memory by only making one assignment
> A2 <- matrix(1:16, 4)
> D2 <- rbind(A2[1:2, ], 2 * A2[1, ] + A2[3, ], A2[4, ])

You can do it the "matrix-ey"  way as well by creating a row extractor/ 
multiplier operator. This matrix multiplication copies 2 times the 1st  
row to the third row:
The operation:
 > matrix( c(0,0,2,rep(0,13)), 4) %*% A2
      [,1] [,2] [,3] [,4]
[1,]    0    0    0    0
[2,]    0    0    0    0
[3,]    2   10   18   26
[4,]    0    0    0    0

So it can be used thusly:

  D2 <- A2 + matrix (c(0,0,2,rep(0,13)), 4) %*% A2
  D2
      [,1] [,2] [,3] [,4]
[1,]    1    5    9   13
[2,]    2    6   10   14
[3,]    5   17   29   41
[4,]    4    8   12   16

This method does not pull the "donor" matrix apart.

-- 
David
>
> all.equal(A, A2)
> all.equal(D, D2)
>
> Cheers,
>
> Josh
>
> On Fri, Aug 27, 2010 at 10:32 PM, Cheng Peng <cpeng at usm.maine.edu>  
> wrote:
>>
>> Sorry for possible misunderstanding:
>>
>> I want to define a matrix (B) based on an existing matrix (A) in a  
>> single
>> step and keep A unchanged:
>>
>>> #Existing matrix
>>> A=matrix(1:16,ncol=4)
>>> A
>>     [,1] [,2] [,3] [,4]
>> [1,]    1    5    9   13
>> [2,]    2    6   10   14
>> [3,]    3    7   11   15
>> [4,]    4    8   12   16
>>> # New matrix B is defined to be the submatrix after row1 and  
>>> column1 are
>>> deleted.
>>> B=A[-1,-1]    # this single step deletes row1 nad column 1 and  
>>> assigns the
>>> name to the resulting submatrix.
>>> B                 # check the new matrix B
>>     [,1] [,2] [,3]
>> [1,]    6   10   14
>> [2,]    7   11   15
>> [3,]    8   12   16
>>> A                 # check the original matrix A
>>     [,1] [,2] [,3] [,4]
>> [1,]    1    5    9   13
>> [2,]    2    6   10   14
>> [3,]    3    7   11   15
>> [4,]    4    8   12   16
>>
>>
>> Question: How can I do define a new matrix (D) by adding 2*row1 to  
>> row3 in A
>> in a single step as what was done in the above example?
>>
>> If you do:  A[3,]=2*A[1,]+A[3,],  the new A is not the original A;  
>> if you
>> D=A first, then D[3,]=2*D[1,]+D[3,], you used two step!
>>
>> Hope this clarifies my original question. Thanks again.
>>
>>
>>
>>
>>
>> --
>> View this message in context: http://r.789695.n4.nabble.com/How-to-define-new-matrix-based-on-an-elementary-row-operation-in-a-single-step-tp2341768p2352538.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> -- 
> Joshua Wiley
> Ph.D. Student, Health Psychology
> University of California, Los Angeles
> http://www.joshuawiley.com/
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.