latticeParseFormula {lattice}  R Documentation 
this function is used by high level Lattice functions like
xyplot
to parse the formula argument and evaluate various
components of the data.
latticeParseFormula(model, data, dimension = 2, subset = TRUE, groups = NULL, multiple, outer, subscripts, drop)
model 
the model/formula to be parsed. This can be in either of two
possible forms, one for 2d and one for 3d formulas, determined by
the dimension argument. The 2d formulas are of the form
y ~ x g1 * ... *gn , and the 3d formulas are of the form
z ~ x * y  g1 * ...* gn . In the first form, y may
be omitted. The conditioning variables g1, ...,gn can be
omitted in either case.

data 
the environment/dataset where the variables in the formula are evaluated. 
dimension 
dimension of the model, see above 
subset 
index for choosing a subset of the data frame 
groups 
the grouping variable, if present 
multiple, outer 
logicals, determining how a ‘+’ in the y and x components of
the formula are processed. See xyplot for details

subscripts 
logical, whether subscripts are to be calculated 
drop 
logical or list, similar to the drop.unused.levels argument
in xyplot , indicating whether unused levels of
conditioning factors and data variables that are factors are to be
dropped.

returns a list with several components, including left,
right, left.name, right.name, condition
for 2D, and left,
right.x, right.y, left.name, right.x.name, right.y.name, condition
for 3D. Other possible components are groups, subscr
Saikat DebRoy, Deepayan Sarkar Deepayan.Sarkar@Rproject.org