# Building formulae

This article is originally published at https://www.brodrigues.co/

This Stackoverflow question made me think about how to build formulae. For example, you might want to programmatically build linear model formulae and then map these models on data. For example, suppose the following (output suppressed):

```
data(mtcars)
lm(mpg ~ hp, data = mtcars)
lm(mpg ~I(hp^2), data = mtcars)
lm(mpg ~I(hp^3), data = mtcars)
lm(mpg ~I(hp^4), data = mtcars)
lm(mpg ~I(hp^5), data = mtcars)
lm(mpg ~I(hp^6), data = mtcars)
```

To avoid doing this, one can write a function that builds the formulae:

```
create_form = function(power){
rhs = substitute(I(hp^pow), list(pow=power))
rlang::new_formula(quote(mpg), rhs)
}
```

If you are not familiar with `substitute()`

, try the following to understand what it does:

`substitute(y ~ x, list(x = 1))`

`## y ~ 1`

Then using `rlang::new_formula()`

I build a formula by providing the left hand side, which is
`quote(mpg)`

here, and the right hand side, which I built using `substitute()`

. Now I can create a
list of formulae:

```
library(tidyverse)
list_formulae = map(seq(1, 6), create_form)
str(list_formulae)
```

```
## List of 6
## $ :Class 'formula' language mpg ~ I(hp^1L)
## .. ..- attr(*, ".Environment")=<environment: 0x55605f897ca0>
## $ :Class 'formula' language mpg ~ I(hp^2L)
## .. ..- attr(*, ".Environment")=<environment: 0x55605f891418>
## $ :Class 'formula' language mpg ~ I(hp^3L)
## .. ..- attr(*, ".Environment")=<environment: 0x55605da76098>
## $ :Class 'formula' language mpg ~ I(hp^4L)
## .. ..- attr(*, ".Environment")=<environment: 0x55605da6a600>
## $ :Class 'formula' language mpg ~ I(hp^5L)
## .. ..- attr(*, ".Environment")=<environment: 0x55605da68980>
## $ :Class 'formula' language mpg ~ I(hp^6L)
## .. ..- attr(*, ".Environment")=<environment: 0x55605da66d38>
```

As you can see, `power`

got replaced by 1, 2, 3,… and each element of the list is a nice formula.
Exactly what `lm()`

needs. So now it’s easy to map `lm()`

to this list of formulae:

```
data(mtcars)
map(list_formulae, lm, data = mtcars)
```

```
## [[1]]
##
## Call:
## .f(formula = .x[[i]], data = ..1)
##
## Coefficients:
## (Intercept) I(hp^1)
## 30.09886 -0.06823
##
##
## [[2]]
##
## Call:
## .f(formula = .x[[i]], data = ..1)
##
## Coefficients:
## (Intercept) I(hp^2)
## 24.3887252 -0.0001649
##
##
## [[3]]
##
## Call:
## .f(formula = .x[[i]], data = ..1)
##
## Coefficients:
## (Intercept) I(hp^3)
## 2.242e+01 -4.312e-07
##
##
## [[4]]
##
## Call:
## .f(formula = .x[[i]], data = ..1)
##
## Coefficients:
## (Intercept) I(hp^4)
## 2.147e+01 -1.106e-09
##
##
## [[5]]
##
## Call:
## .f(formula = .x[[i]], data = ..1)
##
## Coefficients:
## (Intercept) I(hp^5)
## 2.098e+01 -2.801e-12
##
##
## [[6]]
##
## Call:
## .f(formula = .x[[i]], data = ..1)
##
## Coefficients:
## (Intercept) I(hp^6)
## 2.070e+01 -7.139e-15
```

This is still a new topic for me there might be more elegant ways to do that, using tidyeval to remove
the hardcoding of the columns in `create_form()`

. I might continue exploring this.

Thanks for visiting r-craft.org

This article is originally published at https://www.brodrigues.co/

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