Write a function to build the layout matrix

[sfcheung]

In semPaths, to specify the layout of nodes, we need to prepare a matrix:

m <- matrix(c("x1",   NA,  NA,   NA,
                NA, "x3",  NA, "x4",
              "x2",   NA,  NA,   NA), byrow = TRUE, 3, 4)

This will become inefficient to create and modify if the number of variables is large. We can write a function to create the matrix this way:

m <- fct(x1 = c(2, 1),
         x2 = c(3, 1),
         x3 = c(2, 2),
         x4 = c(2, 4))

This may look less intuitive, but is much easier to modify when the number of variables is large.

Status

Done (bfd921e)

[sfcheung]

Function drafted. Being tested in the experimental branch experimental_sf