Substring, Pad and Paste Columns in Dataframe without a Loop

I have this dataframe, it looks like this:

I need to take the first character from the column at, the whole value in an, then put a counter on the end that increments for repeats in column an. This counter has to be always length of three. The end result is this:

So nothing here that dramatic, I was able to do this with the following code (prepare to be impressed):

library(stringr) 
tk <- ""
for (i in 1:nrow(df)){
  if (tk == df$an[i]){
    counter <- counter + 1
  } else {
    tk <- df$an[i]
    counter <- 1
  }
  df$ap[i] <- counter
}

df$ap <- paste0(substr(df$at, 1, 1), df$an, str_pad(df$ap, 3, pad="0"))

I'm so not satisfied with this debacle. It seems not very "R" and I'd like very much never to allow this to see the light of day. How can I make this more "R"?

I appreciate the advice.