From: Richard M. Heiberger <rmh_at_temple.edu>

Date: Fri, 22 Apr 2011 14:38:08 -0400

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. Received on Fri 22 Apr 2011 - 18:44:19 GMT

Date: Fri, 22 Apr 2011 14:38:08 -0400

*> system.time(for (i in 1:1000)
*

+ mat[seq(1, nrow(mat), by=2), ]+mat[seq(2, nrow(mat), by=2), ]
+ )

user system elapsed

0.18 0.00 0.19

*>
**> system.time(for (i in 1:1000)
*

+ mat[seqn <- seq(1, nrow(mat), by=2), ]+mat[seqn+1, ]
+ )

user system elapsed

0.08 0.00 0.08

*>
**> system.time(for (i in 1:1000)
*

+ mat[seqn <- seq(1, length=nrow(mat)/2, by=2), ]+mat[seqn+1, ]
+ )

user system elapsed

0.05 0.00 0.05

*>
**> system.time(for (i in 1:1000)
*

+ {mat2 <- mat; dim(mat2) <- c(2,3,6); mat2[1,,]+mat2[2,,]}
+ )

user system elapsed

0.01 0.00 0.02

*>
*

On Fri, Apr 22, 2011 at 12:28 PM, David Winsemius <dwinsemius_at_comcast.net>wrote:

*>
*

> On Apr 22, 2011, at 12:13 PM, Christine SINOQUET wrote:

*>
**> Hello,
**>>
**>> mat1 only consists of 0s and 1s:
**>> 0 0 1 0 0 0
**>> 1 1 0 1 1 0
**>> 1 1 1 0 1 0
**>> 0 1 1 0 0 1
**>> 1 0 0 1 0 0
**>> 0 1 0 1 0 1
**>>
**>> N = 3
**>> M = 6
**>>
**>> I would like to "compress" mat1 every two rows, applying summation over
**>> the two rows (per column), at each step, to yield:
**>>
**>> mat2
**>> 1 1 1 1 1 0
**>> 1 2 2 0 1 1
**>> 1 1 0 2 0 1
**>>
**>
**> > mat[seq(1, nrow(mat), by=2), ]+mat[seq(2, nrow(mat), by=2), ]
**> [,1] [,2] [,3] [,4] [,5] [,6]
**> [1,] 1 1 1 1 1 0
**> [2,] 1 2 2 0 1 1
**> [3,] 1 1 0 2 0 1
**>
**>
**>
*

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