# [R] setting off-diagonals to zero

David Winsemius dwinsemius at comcast.net
Thu Jan 24 08:32:59 CET 2013

```On Jan 23, 2013, at 5:31 PM, David Winsemius wrote:

>
> On Jan 23, 2013, at 7:13 AM, emorway wrote:
>
>> The following 1460 x 1460 matrix can be throught of as 16 distinct
>> 365 x 365
>> matrices.  I'm trying to set off-diaganol terms in the 16 sub-
>> matrices with
>> indices more than +/- 5 (days) from each other to zero using some
>> for loops.
>> This works well for some, but not all, of the for loops.  The R
>> code I"m
>> using follows.  For some reason the third loop below zero's-out
>> everything
>> in the sub-quadrant it is targeting, which is readily observable when
>> viewing the matrix ("View(MAT)").
>>
>> library(Matrix)
>> MAT<-matrix(rnorm(1460*1460,mean=0,sd=1),nrow = 1460, ncol = 1460)
>>
>
> The way to do that in a single 365 x 365 matrix is:
>
> Mat <- matrix( 1:(365*365), 365, 365)
> Mat[ abs( col(Mat)-row(Mat) ) > 5 ] <- 0
> Mat
>
> The way to propagate that pattern is to use rep(), so here is a one-
>
> MAT[ rep( abs( col(Mat)-row(Mat) ) > 5, 16) ] <- 0

That might not construct the bands in the blocks correctly: This might
be better:

MAT [ kronecker( matrix(1, 4,4), abs( col(Mat)-row(Mat) ) > 5,
FUN="&" ) ] <- 0

--
David.
>
>
> Didn't test on you gigantuan matrix;  used smaller example:
>
> Mat <- matrix( 1:(16*16), 16, 16)
> test <- rbind(Mat, Mat)
> test[rep( abs( col(Mat)-row(Mat) ) > 2 , 2)] <- 0
> test
>
> --
> David.
>
>
>> #works great
>> for (i in 1:365) {
>> SEQ <- (i - 5):(i + 5)
>> SEQ <- SEQ[SEQ > 0 & SEQ < 366]
>> MAT[(1:365)[-SEQ], i] <- 0
>> }
>>
>> #works great
>> for (i in 1:365) {
>> SEQ <- (i - 5):(i + 5)
>> SEQ <- SEQ[SEQ > 0 & SEQ < 366]
>> MAT[(1:365)[-SEQ], i + 365] <- 0
>> }
>>
>> #zero's out everything, including main-diagonal and near-main-
>> diagonal
>> terms???
>> for (i in 731:1095) {
>> SEQ <- (i - 5):(i + 5)
>> SEQ <- SEQ[SEQ > 730 & SEQ < 1096]
>> MAT[(731:1095)[-SEQ], i + 365] <- 0
>> }
>>
>> View(MAT)
>>
>> I'm not sure why the third FOR loop above is not leaving the main-
>> diagonal
>> and near-main-diagonal terms alone?
>>
>>
>>
>> --
>> View this message in context: http://r.789695.n4.nabble.com/setting-off-diagonals-to-zero-tp4656407.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius
> Alameda, CA, USA
>
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