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

[Gabor Grothendieck]

On Sat, Aug 28, 2010 at 1:32 AM, 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.
>

Try using replace:

   D <- replace(A, row(A) == 3, 2 * A[1,] + A[3,])


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