I have something along the lines of
y ~ x + z
And I would like to transform it to
y ~ x_part1 + x_part2 + z
More generally, I would like to have a function that takes a formula and returns that formula with all terms that match "^x$" replaced by "x_part1" and "x_part2". Here's my current solution, but it just feels so kludgey...
my.formula <- fruit ~ apple + banana
var.to.replace <- 'apple'
my.terms <- labels(terms(my.formula))
new.terms <- paste0('(',
paste0(var.to.replace,
c('_part1', '_part2'),
collapse = '+'),
')')
new.formula <- reformulate(termlabels = gsub(pattern = var.to.replace,
replacement = new.terms,
x = my.terms),
response = my.formula[[2]])
An additional caveat is that the input formula may be specified with interactions.
y ~ b*x + z
should output one of these (equivalent) formulae
y ~ b*(x_part1 + x_part2) + z
y ~ b + (x_part1 + x_part2) + b:(x_part1 + x_part2) + z
y ~ b + x_part1 + x_part2 + b:x_part1 + b:x_part2 + z
MrFlick has advocated the use of
substitute(y ~ b*x + z, list(x=quote(x_part1 + x_part2)))
but when I have stored the formula I want to modify in a variable, as in
my.formula <- fruit ~ x + banana
This approach seems to require a little more massaging:
substitute(my.formula, list(x=quote(apple_part1 + apple_part2)))
# my.formula
The necessary change to that approach was:
do.call(what = 'substitute',
args = list(apple, list(x=quote(x_part1 + x_part2))))
But I can't figure out how to use this approach when both 'x' and c('x_part', 'x_part2') are stored in variables with names, e.g. var.to.replace and new.terms above.
How about working with the formula as a string? Many base R models like lm() accept a string formulas (and you can always use formula() otherwise). In this case, you can use something like gsub():
f1 <- "y ~ x + z"
f2 <- "y ~ b*x + z"
gsub("x", "(x_part1 + x_part2)", f1)
#> [1] "y ~ (x_part1 + x_part2) + z"
gsub("x", "(x_part1 + x_part2)", f2)
#> [1] "y ~ b*(x_part1 + x_part2) + z"
For example, with mtcars data set, and say we want to replace mpg (x) with disp + hp (x_part1 + x_part2):
f1 <- "qsec ~ mpg + cyl"
f2 <- "qsec ~ wt*mpg + cyl"
f1 <- gsub("mpg", "(disp + hp)", f1)
f2 <- gsub("mpg", "(disp + hp)", f2)
lm(f1, data = mtcars)
#>
#> Call:
#> lm(formula = f1, data = mtcars)
#>
#> Coefficients:
#> (Intercept) disp hp cyl
#> 22.04376 0.01017 -0.02074 -0.56571
lm(f2, data = mtcars)
#>
#> Call:
#> lm(formula = f2, data = mtcars)
#>
#> Coefficients:
#> (Intercept) wt disp hp cyl
#> 20.421318 1.554904 0.026837 -0.056141 -0.876182
#> wt:disp wt:hp
#> -0.006895 0.011126