Concisely adding values in a loop to a column

This is more or less a code review. I am also learning R. There are some parts which you do "by hand", such as dealing with the duplication of topics ("c/d: telefoon"), but I coded a solution where everything is automated. Note however that it makes for a lot more code and you should probably stick to your original method, or @janos's, especially if you only have to build such a table once. But I am posting my solution here nonetheless since you might find something useful.

InvertListMapping <- function(list) {
  # Inverts the mapping relationship of a list: 
  # e.g. { 1 -> "a", 2 -> "a", 3 -> "b" } becomes { "a" -> c(1, 2), "b" -> 3} 
  inverted <- list()
  {
      lapply(names(list), function (key) {
            value <- toString(list[[key]])
            inverted[[value]] <<- c(inverted[[value]], key)
          })
  }
  return(inverted)  
}

ExtractAbbr <- function(groupname) {
  # Extract the source abbreviation: e.g. from "asdf/comp-b/asd" it extracts "b".
  res <- regexpr("/comp-(?<abbr>\\S+)/", groupname, perl=T)
  start <- attr(res, "capture.start")
  if (start < 1)
    stop(paste("Failed to extract the abbreviated newsgroup from:", groupname))
  length <- attr(res, "capture.length")
  return (substr(groupname, start, start + length - 1))
}

ConvertAbbrsToLegend <- function(topic2abbrs) {
  # Generates the map (list) from a set of abbreviations to the corresponding legend;
  # e.g. { a -> "a: spont-conv", c -> "c/d: telefoon", etc.}.
  abbrs2legend <- list()
  lapply(names(topic2abbrs), function (topic) {
        abbrs <- topic2abbrs[[topic]]
        legend <- paste(paste(abbrs, collapse = "/"), ": ", topic, sep = "")
        lapply(abbrs, function (abbr) { abbrs2legend[[abbr]] <<- legend })
      })
  return(abbrs2legend)
}

SourceToLegend <- function(sources, abbrs2legend) {
  # For each source string, it extracts the abbrevation and outputs
  # the corresponding legend.
  GetLegend <- function(source) {
    abbr <- ExtractAbbr(source)
    return(abbrs2legend[[abbr]])
  }
  return(Vectorize(GetLegend)(sources))
}


# Example:

abbr2topic <- list()
abbr2topic$a <- "spont-conv"
abbr2topic$b <- "interv-ler-nl"
abbr2topic$c <- "telefoon"
abbr2topic$d <- "telefoon"
sources <- c("aadf/comp-a/af", "vwef/comp-c/wefw")

topic2abbrs <- InvertListMapping(abbr2topic)
abbrs2legend <- ConvertAbbrsToLegend(topic2abbrs)

legends <- SourceToLegend(sources, abbrs2legend)

print(legends)  # "a: spont-conv"  "c/d: telefoon"

It throws an exception if it fails at some point, but maybe you could change that to have some dummy output value when a source is not recognized.