Formulas in R are hard to manipulate

formulaTools.R

# Kevin Gilbert
# CMU CS '14

# Formulas in R are hard to manipulate, and cannot have string functions applied to them. This fixes that.


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# given two formulas, merge their predictors
# X ~ m + n and X ~ y + z => X ~ m + n + y + z
formula.merge = function(A, B) {
    tmpA = terms(A)
    tmpB = terms(B)
    resp = dimnames(attr(tmpA,"factors"))[[1]][1] # I assume A and B have the same response variable
    pred = c(attr(tmpA, "term.labels"), attr(tmpB, "term.labels"))
    return (as.formula(paste(paste(resp, " ~ "), paste(pred, collapse = "+"))))
}

# by default, wrap all predictors X with s(X), typically for mgcv
# X ~ y + z => X ~ s(y) + s(z)
formula.wrap = function(model, f = function(x) paste(c("s(",x,")"), collapse = ""))  {
    tmp = terms(model)
    resp = dimnames(attr(tmp,"factors"))[[1]][1]
    pred = attr(tmp, "term.labels")
    return(as.formula(paste(paste(resp, " ~ "), paste(sapply(pred, f), collapse = "+"))))
}


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# Please refer to 36-402 lecture on Additive Models
#     ...you can now do MedianHouseValue ∼ s(.)!
look_at_that_formula = formula.wrap(lm(log(MedianHouseValue) ~ ., data=calif))

# log(MedianHouseValue) ~ s(MedianIncome) + s(MedianHouseAge) + 
#     s(TotalRooms) + s(TotalBedrooms) + s(Population) + s(Households) + 
#     s(Latitude) + s(Longitude)

addfit = gam(look_at_that_formula, data=calif)
addfit2 = update(addfit, ~ . - s(Latitude) - s(Longitude) + s(Latitude, Longitude))

# log(MedianHouseValue) ~ s(MedianIncome) + s(MedianHouseAge) + 
#     s(TotalRooms) + s(TotalBedrooms) + s(Population) + s(Households) + 
#     s(Latitude, Longitude)


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# formula.merge is useful for keeping main effects and interactions separate

library(datasets)
main_effects = formula(lm(mpg ~ ., data=mtcars))
interactions = formula(mpg ~ hp:cyl + disp:wt)
formula.merge(main_effects, interactions)

# mpg ~ cyl + disp + hp + drat + wt + qsec + vs + am + gear + carb + 
#     hp:cyl + disp:wt