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Editorial Team
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Asked: 2026-06-17T09:48:57+00:00

I need to access and assign single slots of an m*n matrix inside a

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I need to access and assign single slots of an m*n matrix inside a for loop. The code so far:

rowCount <- 9
similMatrix = matrix(nrow = rowCount - 1, ncol = rowCount)
show(similMatrix)
for(i in (rowCount - 1)){
  for (j in rowCount)
    if (i == j){
      similMatrix[i == j] <- 0;
    }
}
show(similMatrix)

so if i = j the NA value in the matrix needs to be replaced with 0.

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1 Answer

  1. Editorial Team

    For the purpose of setting the “diagonal” elements to zero you have already been given an answer but I wonder if you were hoping for something more general. The reasons for lack of success with that code were two-fold: the construction of your indices were flawed and the indexing was wrong. This would have succeeded:

    for(i in 1:(rowCount - 1)){  # need an expression that retruns a sequence
  for (j in 1:rowCount)      # ditto
    if (i == j){
      similMatrix[i,j] <- 0;  # need to index the matrix with two element if using i,j
    }
}
#----------
> show(similMatrix)
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,]    0   NA   NA   NA   NA   NA   NA   NA   NA
[2,]   NA    0   NA   NA   NA   NA   NA   NA   NA
[3,]   NA   NA    0   NA   NA   NA   NA   NA   NA
[4,]   NA   NA   NA    0   NA   NA   NA   NA   NA
[5,]   NA   NA   NA   NA    0   NA   NA   NA   NA
[6,]   NA   NA   NA   NA   NA    0   NA   NA   NA
[7,]   NA   NA   NA   NA   NA   NA    0   NA   NA
[8,]   NA   NA   NA   NA   NA   NA   NA    0   NA

    But resorting to loops in R is generally considered a last resort (sometimes for the wrong reasons.) There is a much more compact way of doing the same “loop” operation and it generalizes more widely than just setting the diagonal.

    similMatrix[ row(similMatrix) == col(similMatrix) ] <- 0
> similMatrix
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,]    0   NA   NA   NA   NA   NA   NA   NA   NA
[2,]   NA    0   NA   NA   NA   NA   NA   NA   NA
[3,]   NA   NA    0   NA   NA   NA   NA   NA   NA
[4,]   NA   NA   NA    0   NA   NA   NA   NA   NA
[5,]   NA   NA   NA   NA    0   NA   NA   NA   NA
[6,]   NA   NA   NA   NA   NA    0   NA   NA   NA
[7,]   NA   NA   NA   NA   NA   NA    0   NA   NA
[8,]   NA   NA   NA   NA   NA   NA   NA    0   NA

    If you wanted to set the subdiagonal to zero you could just use:

    similMatrix[ row(similMatrix)-1 == col(similMatrix) ] <- 0

    You can avoid generating the extra row and col matrices using this:

     mind <- min( dim(similMatrix) )
 # avoid going outside dimensions if not symmetric
 similMatrix[ cbind( seq(maxd),seq(maxd) ) <- 0
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