[R] setting off-diagonals to zero

[emorway]

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)

#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?



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