denumerate: Transform an Allowable Formula for 'loglm' into one for 'terms'

Description

loglm allows dimension numbers to be used in place of names in the formula. denumerate modifies such a formula into one that terms can process.

Usage

denumerate(x)

Arguments

A formula conforming to the conventions of loglm, that is, it may allow dimension numbers to stand in for names when specifying a log-linear model.

Value

A linear model formula like that presented, except that where dimension numbers, say n, have been used to specify fixed margins these are replaced by names of the form .vn which may be processed by terms.

Details

The model fitting function loglm fits log-linear models to frequency data using iterative proportional scaling. To specify the model the user must nominate the margins in the data that remain fixed under the log-linear model. It is convenient to allow the user to use dimension numbers, 1, 2, 3, … for the first, second, third, …, margins in a similar way to variable names. As the model formula has to be parsed by terms, which treats 1 in a special way and requires parseable variable names, these formulae have to be modified by giving genuine names for these margin, or dimension numbers. denumerate replaces these numbers with names of a special form, namely n is replaced by .vn. This allows terms to parse the formula in the usual way.

Examples

Run this code

# NOT RUN {
denumerate(~(1+2+3)^3 + a/b)
## which gives ~ (.v1 + .v2 + .v3)^3 + a/b
# }