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, eg var.to.replace
and new.terms
above.
You can use the substitute
function for this
substitute(y ~ b*x + z, list(x=quote(x_part1 + x_part2)))
# y ~ b * (x_part1 + x_part2) + z
Here we use the named list to tell R to replace the variable x
with the expression x_part1 + x_part2
You can write a recursive function to modify the expression tree of the formula:
replace_term <- function(f, old, new){
n <- length(f)
if(n > 1) {
for(i in 1:n) f[[i]] <- Recall(f[[i]], old, new)
return(f)
}
if(f == old) new else f
}
Which you can use to modify eg interactions:
> replace_term(y~x*a+z - x, quote(x), quote(x1 + x2))
y ~ (x1 + x2) * a + z - (x1 + x2)
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
If you just want to modify main effects, you can subtract x, and add in the two new variables.
> f <- y ~ x + z
> update(f, .~.-x+x_part1 + x_part2)
y ~ z + x_part1 + x_part2
As requested by rcorty
, storing 'x' and c('x_part', 'x_part2') in var.to.replace
and new.terms
, respectively, and adotping MrFlick
's suggestion to use setNames
, we could perhaps do the following:
my.formula <- fruit ~ x + banana
var.to.replace <- "x"
new.terms <- c('x_part', 'x_part2')
new.terms1 <- paste(new.terms, collapse="+")
do.call("substitute", list(my.formula, setNames(list(str2lang(new.terms1)), var.to.replace)))
> fruit ~ x_part + x_part2 + banana
As an aside, I have found Paul Johnson's Rchaeology (Section 2.1) relevant, educational, and entertaining.
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