Get the row and column indices of upper and lower trianges of a matrix

Given the number of rows in a symmetric matrix, calculate the row and column indices of the upper or lower triangles.

TriIndex(Nrow, which = "lower")

Arguments

Nrow	The number of rows
which	Specify which = "lower" or which = "upper". Defaults to "lower".

Value

A two-column matrix.

Note

A straightforward way to do this is to use which(lower.tri(YourMatrix), arr.ind = TRUE), however, this can be quite slow as the number of rows increases.

References

http://stackoverflow.com/a/20899060/1270695

Author

Ananda Mahto

Examples

TriIndex(4)
#>      row col
#> [1,]   2   1
#> [2,]   3   1
#> [3,]   4   1
#> [4,]   3   2
#> [5,]   4   2
#> [6,]   4   3
TriIndex(4, "upper")
#>      row col
#> [1,]   1   2
#> [2,]   1   3
#> [3,]   2   3
#> [4,]   1   4
#> [5,]   2   4
#> [6,]   3   4

m <- matrix(0, nrow = 4, ncol = 4)
which(lower.tri(m), arr.ind = TRUE)
#>      row col
#> [1,]   2   1
#> [2,]   3   1
#> [3,]   4   1
#> [4,]   3   2
#> [5,]   4   2
#> [6,]   4   3