Assign Value to Diagonal Entries of Matrix - r

Question

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.

Answer 1

You want the function diag<-
m <- matrix(1:12, nrow=3)
m
[,1] [,2] [,3] [,4]
[1,] 1 4 7 10
[2,] 2 5 8 11
[3,] 3 6 9 12
diag(m) <- 0
m
[,1] [,2] [,3] [,4]
[1,] 0 4 7 10
[2,] 2 0 8 11
[3,] 3 6 0 12

Answer 2

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