How to replace loop by apply function

How to I do this using apply() functions ?

c <- c(df$Station, REF$Secteur, ETAT$Secteur) #three character type columns
for (i in c) {
  i <- toupper(i)
}

sapply(c,toupper)

This provides me with a list, but I would like the modifications to be applied to the original dataframes.

Solution

Here is a way with mapply, test data at the end.
Firs, put the data.frames in a list and the names of the column to be changed in a list or in a vector, below they are in a vector.

df_list <- list(df, REF, ETAT)
cols_vec <- c("Station", "Secteur", "Secteur")

mapply(\(x, col) toupper(x[[col]]), df_list, cols_vec, SIMPLIFY = FALSE)
#> [[1]]
#> [1] "A" "B" "C"
#> 
#> [[2]]
#> [1] "ABC" "DEF" "XYZ" "U"  
#> 
#> [[3]]
#> [1] "ABC" "DEF" "XYZ"

^{Created on 2023-02-09 with reprex v2.0.2}

Now, to assign these results back to the data in the list, change function a bit, make it return the df's.

df <- data.frame(x = 1:3, Station = letters[1:3])
REF <- data.frame(Secteur = c("Abc", "def", "xyz", "u"))
ETAT <- data.frame(Secteur = c("Abc", "def", "xyz"), x = 1:3, y = 4:6)
df_list <- list(df, REF, ETAT)
cols_vec <- c("Station", "Secteur", "Secteur")

mapply(\(x, col) {
  x[[col]] <-  toupper(x[[col]])
  x
}, df_list, cols_vec, SIMPLIFY = FALSE)
#> [[1]]
#>   x Station
#> 1 1       A
#> 2 2       B
#> 3 3       C
#> 
#> [[2]]
#>   Secteur
#> 1     ABC
#> 2     DEF
#> 3     XYZ
#> 4       U
#> 
#> [[3]]
#>   Secteur x y
#> 1     ABC 1 4
#> 2     DEF 2 5
#> 3     XYZ 3 6

^{Created on 2023-02-09 with reprex v2.0.2}

If you want the original data.frames to change, try the following.

df_list <- mget(c("df", "REF", "ETAT"), envir = .GlobalEnv)
df_list <- mapply(\(x, col) {
  x[[col]] <-  toupper(x[[col]])
  x
}, df_list, cols_vec, SIMPLIFY = FALSE)

list2env(df_list, envir = .GlobalEnv)
#> <environment: R_GlobalEnv>

df
#>   x Station
#> 1 1       A
#> 2 2       B
#> 3 3       C
REF
#>   Secteur
#> 1     ABC
#> 2     DEF
#> 3     XYZ
#> 4       U
ETAT
#>   Secteur x y
#> 1     ABC 1 4
#> 2     DEF 2 5
#> 3     XYZ 3 6

^{Created on 2023-02-09 with reprex v2.0.2}

Test data

df <- data.frame(x = 1:3, Station = letters[1:3])
REF <- data.frame(Secteur = c("Abc", "def", "xyz", "u"))
ETAT <- data.frame(Secteur = c("Abc", "def", "xyz"), x = 1:3, y = 4:6)
df_list <- list(df, REF, ETAT)
cols_vec <- c("Station", "Secteur", "Secteur")

^{Created on 2023-02-09 with reprex v2.0.2}