ParseFormula: Parse a Formula and Create a Model Frame

Description

Create a model frame for a formula object, by handling the left hand side the same way the right hand side is handled in model.frame. Especially variables separated by + are interpreted as separate variables.

Usage

ParseFormula(formula, data = parent.frame(), drop = TRUE)

Arguments

formula

an object of class "formula" (or one that can be coerced to that class): a symbolic description for the variables to be described.

data

an optional data frame, list or environment (or object coercible by as.data.frame to a data frame) containing the variables in the model. If not found in data, the variables are taken from environment(formula), typically the environment from which lm is called.

drop

if drop is TRUE, unused factor levels are dropped from the result when creating interaction terms. The default is to drop all unused factor levels.

Value

Details

This is used by Desc.formula for describing data by groups while remaining flexible for using I(...) constructions, functions or interaction terms.

Examples

Run this code

set.seed(17)
piz <- d.pizza[sample(nrow(d.pizza),10), c("temperature","price","driver","weekday")]

f1 <- formula(. ~ driver)
f2 <- formula(temperature ~ .)
f3 <- formula(temperature + price ~ .)
f4 <- formula(temperature ~ . - driver)
f5 <- formula(temperature + price ~ driver)
f6 <- formula(temperature + price ~ driver * weekday)
f7 <- formula(I(temperature^2) + sqrt(price) ~ driver + weekday)
f8 <- formula(temperature + price ~ 1)
f9 <- formula(temperature + price ~ driver * weekday - price)

ParseFormula(f1, data=piz)  
ParseFormula(f2, data=piz)  
ParseFormula(f3, data=piz)
ParseFormula(f4, data=piz)
ParseFormula(f5, data=piz)
ParseFormula(f6, data=piz)
ParseFormula(f7, data=piz)
ParseFormula(f8, data=piz)

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