Converting a character into a variable name

Question

I need to convert a data frame into a matrix using the model.matrix function. The name of the original data frame is train, and the outcome variable of interest is called adequacy_ratio_total_percent. The below R code works.

X_train_matrix <- model.matrix(adequacy_ratio_total_percent ~ ., train)[, -1]

However, since my outcome variables may vary and I hope to simplify the changing of the outcome variables using the below code, which does not work.

list_outcome <- c("adequacy_ratio_total_percent")
X_train_matrix <- model.matrix(list_outcome ~ ., train)[, -1]

Error in model.frame.default(object, data, xlev = xlev) : variable lengths differ (found for 'adequacy_ratio_total_percent')

I also tried the following, which does not work either.

list_outcome <- c("adequacy_ratio_total_percent")
X_train_matrix <- model.matrix(train$list_outcome ~ ., train)[, -1]

Error in model.frame.default(object, data, xlev = xlev) : invalid type (NULL) for variable 'train$list_outcome'

Or the following:

list_outcome <- c("adequacy_ratio_total_percent")
X_train_matrix <- model.matrix(list_outcome[1] ~ ., train)[, -1]

Error in model.frame.default(object, data, xlev = xlev) : variable lengths differ (found for 'adequacy_ratio_total_percent')

How can I extract the variable name from list_outcome and apply it to the model.matrix function? Thank you in advance for any advice!

Answer 1

Here's an answer that uses the same idea as @user20650, but with multiple possibilities for outcomes:

data(mtcars)
list_outcomes = c("qsec", "mpg")
Xmats <- lapply(list_outcomes, function(l){
  model.matrix(reformulate(".", response=l), data=mtcars)
})

lapply(Xmats, head)
#> [[1]]
#>                   (Intercept)  mpg cyl disp  hp drat    wt vs am gear carb
#> Mazda RX4                   1 21.0   6  160 110 3.90 2.620  0  1    4    4
#> Mazda RX4 Wag               1 21.0   6  160 110 3.90 2.875  0  1    4    4
#> Datsun 710                  1 22.8   4  108  93 3.85 2.320  1  1    4    1
#> Hornet 4 Drive              1 21.4   6  258 110 3.08 3.215  1  0    3    1
#> Hornet Sportabout           1 18.7   8  360 175 3.15 3.440  0  0    3    2
#> Valiant                     1 18.1   6  225 105 2.76 3.460  1  0    3    1
#> 
#> [[2]]
#>                   (Intercept) cyl disp  hp drat    wt  qsec vs am gear carb
#> Mazda RX4                   1   6  160 110 3.90 2.620 16.46  0  1    4    4
#> Mazda RX4 Wag               1   6  160 110 3.90 2.875 17.02  0  1    4    4
#> Datsun 710                  1   4  108  93 3.85 2.320 18.61  1  1    4    1
#> Hornet 4 Drive              1   6  258 110 3.08 3.215 19.44  1  0    3    1
#> Hornet Sportabout           1   8  360 175 3.15 3.440 17.02  0  0    3    2
#> Valiant                     1   6  225 105 2.76 3.460 20.22  1  0    3    1

^{Created on 2022-06-28 by the reprex package (v2.0.1)}