Question

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LINQ list to sentence format (insert commas & “and”)

Imagine these inputs and results:

[] -> ""

["Hello World!"] -> "Hello World!"

["Apples", "bananas"] -> "Apples, and bananas" (put your grammar books away)

["Lions", "Tigers", "Bears"] -> "Lions, Tigers, and Bears" (oh my!)

Now, imagine that the inputs are all of IEnumerable<string>. What is a good (where good may encompass “small and tidy”, “easy to understand”, “uses the full ability of LINQ”, or other as long as it’s justified) to write a function in C# to do this? I would really like to avoid “imperative loop” approaches.

My current approach looks like:

string Commaize (IEnumerable<string> list) {
    if (list.Count() > 1) {
        list = list.Take(list.Count() - 2).Concat(
            new[] { list.Reverse().Take(2).Reverse()
                        .Aggregate((a, b) => a + " and " + b) });
    }
    return String.Join(", ", list.ToArray());
}

But it just doesn’t feel very “good”. It’s for .NET3.5 so the ToArray() bit is required here. If list is null the result is UB.

Answer 1

string Commaize (IEnumerable<string> sequence)
{
    IList<string> list=sequence as IList<string>;
    if(list==null)
      list=sequence.ToList();
    if(list.Count==0)
      return "";
    else if(list.Count==1)
      return list.First();
    else
      return String.Join(", ", list.Take(list.Count-1).ToArray()) + " and " + list.Last();
}

The overhead of this is the allocation of a few additional arrays(one ToList() and one ToArray() call, which probably both use allocation of exponentially growing arrays, so the number of allocated arrays is larger than two).