Fold a square matrix by collapsing lower triangle on upper triangle, or vice versa, through addition.
Usage
fold(x, direction = c("upper", "lower"))Arguments
- x
square numeric matrix
- direction
character, one of
"upper"or"lower", direction of folding
Value
Square matrix of the same dim as x with the lower
(upper) triangle folded onto the upper (lower) triangle.
Details
By default, for direction=="upper", the function takes the values in
the lower triangle of x and adds them symetrically to the values in
the upper triangle. The values on the diagonal remain unchanged. The lower
triangle is filled with 0s. If direction=="lower" the upper triangle
is collapsed on to the lower triangle.
Examples
(m <- matrix(1:4, 2, 2))
#> [,1] [,2]
#> [1,] 1 3
#> [2,] 2 4
(f1 <- fold(m))
#> [,1] [,2]
#> [1,] 1 5
#> [2,] 0 4
(f2 <- fold(m, "lower"))
#> [,1] [,2]
#> [1,] 1 0
#> [2,] 5 4
stopifnot( all.equal(diag(m), diag(f1)) )
stopifnot( all.equal(diag(m), diag(f2)) )
stopifnot( all.equal(f1[1,2], m[2,1] + m[1,2]) )
stopifnot( all.equal(f2[2,1], m[2,1] + m[1,2]) )