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 <cp...@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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--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/
______________________________________________
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and provide commented, minimal, self-contained, reproducible code.
______________________________________________
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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.
