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

[Joshua Wiley]

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, ])

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.
>
>
>
>
>
> --
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>
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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/