How to get the second sub element of every element in a list

Question

I know I\'ve come across this problem before, but I\'m having a bit of a mental block at the moment. and as I can\'t find it on SO, I\'ll post it here so I can find it next

Answer 1

First of all: if you use str(df) you'll see that df$pre is list. I think you want vector (but I might be wrong).
Return to problem - in this case I will use gsub:

df$pre <- gsub("[0-9]", "", df$lab)
df$suf <- gsub("[A-Z]", "", df$lab)

This guarantee that both columns are vectors, but it fail if your label is not from key (i.e. 'AB01B').

Answer 2

with purrr::map this would be

df$suf %>%  map_chr(c(2)) 

for further info on purrr::map

Answer 3

To select the second element of each list item:

R> sapply(df$suf, "[[", 2)
[1] "00" "01" "02" "00" "01" "02" "21" "01" "03"

An alternative approach using regular expressions:

df$pre <- sub("^([A-Z]+)[0-9]+", "\\1", df$lab)
df$suf <- sub("^[A-Z]+([0-9]+)", "\\1", df$lab)