Pull out only two variables from a column - r

Question

I have a dataframe in R for which one column has multiple variables. The variables either start with ABC, DEF, GHI. Those variables are followed by a series of 6 numbers (ie ABC052689, ABC062895, DEF045158).
For each row, i would like to pull one instance of ABC (the one with the largest number).
If the row has ABC052689, ABC062895, DEF045158, I would like it to pull out ABC062895 because it is greater than ABC052689.
I would then want to do the same for the variable that starts with DEF######.
I have managed to filter the data to have rows where ABC is there and either DEF or GHI is there:
library(tidyverse)
data_with_ABC <- test %>%
filter(str_detect(car,"ABC"))
data_with_ABC_and_DEF_or_GHI <- data_with_ABC %>%
filter(str_detect(car, "DEF") | str_detect(car, "GHI"))
I don't know how to pull out let's say ABC with the greatest number
ABC052689, ABC062895, DEF045158 -> ABC062895

Answer 1

For a base R solution, we can try using lapply along with strsplit to identify the greatest ABC plate in each CSV string, in each row.
df <- data.frame(car=c("ABC052689,ABC062895,DEF045158"), id=c(1),
stringsAsFactors=FALSE)
df$largest <- lapply(df$car, function(x) {
cars <- strsplit(x, ",", fixed=TRUE)[[1]]
cars <- cars[substr(cars, 1, 3) == "ABC"]
max <- cars[which.max(substr(cars, 4, 9))]
return(max)
})
df
car id largest
1 ABC052689,ABC062895,DEF045158 1 ABC062895
Note that we don't need to worry about casting the substring of the plate number, because it is fixed width text. This means that it should sort properly even as text.

Answer 2

Besides Tim's answer, if you want to do all ABC/DEF at one time, following code may help with library(tidyverse):
> df <- data.frame(car=c("ABC052689", "ABC062895", "DEF045158", "DEF192345"), stringsAsFactors=FALSE)
>
> df2 = df %>%
+ mutate(state = str_sub(car, 1, 3), plate = str_sub(car, 4, 9))
>
> df2
car state plate
1 ABC052689 ABC 052689
2 ABC062895 ABC 062895
3 DEF045158 DEF 045158
4 DEF192345 DEF 192345
>
> df2 %>%
+ group_by(state) %>%
+ summarise(maxplate = max(plate)) %>%
+ mutate(full = str_c(state, maxplate))
# A tibble: 2 x 3
state maxplate full
<chr> <chr> <chr>
1 ABC 062895 ABC062895
2 DEF 192345 DEF192345