Eric Lippert's challenge “comma-quibbling”, best answer?

Question

I wanted to bring this challenge to the attention of the stackoverflow community. The original problem and answers are here. BTW, if you did not follow it before, you should

Answer 1

Another variant - separating punctuation and iteration logic for the sake of code clarity. And still thinking about perfomrance.

Works as requested with pure IEnumerable/string/ and strings in the list cannot be null.

public static string Concat(IEnumerable<string> strings)
{
    return "{" + strings.reduce("", (acc, prev, cur, next) => 
               acc.Append(punctuation(prev, cur, next)).Append(cur)) + "}";
}
private static string punctuation(string prev, string cur, string next)
{
    if (null == prev || null == cur)
        return "";
    if (null == next)
        return " and ";
    return ", ";
}

private static string reduce(this IEnumerable<string> strings, 
    string acc, Func<StringBuilder, string, string, string, StringBuilder> func)
{
    if (null == strings) return "";

    var accumulatorBuilder = new StringBuilder(acc);
    string cur = null;
    string prev = null;
    foreach (var next in strings)
    {
        func(accumulatorBuilder, prev, cur, next);
        prev = cur;
        cur = next;
    }
    func(accumulatorBuilder, prev, cur, null);

    return accumulatorBuilder.ToString();
}

F# surely looks much better:

let rec reduce list =
    match list with
    | []          -> ""
    | head::curr::[]  -> head + " and " + curr
    | head::curr::tail  -> head + ", " + curr :: tail |> reduce
    | head::[] -> head

let concat list = "{" + (list |> reduce )  + "}"

Answer 2

Just for fun, using the new Zip extension method from C# 4.0:

private static string CommaQuibbling(IEnumerable<string> list)
{
    IEnumerable<string> separators = GetSeparators(list.Count());
    var finalList = list.Zip(separators, (w, s) => w + s);
    return string.Concat("{", string.Join(string.Empty, finalList), "}");
}

private static IEnumerable<string> GetSeparators(int itemCount)
{
    while (itemCount-- > 2)
        yield return ", ";

    if (itemCount == 1)
        yield return " and ";

    yield return string.Empty;
}

Answer 3

If I was doing a lot with streams which required first/last information, I'd have thid extension:

[Flags]
public enum StreamPosition
{
   First = 1, Last = 2
}

public static IEnumerable<R> MapWithPositions<T, R> (this IEnumerable<T> stream, 
    Func<StreamPosition, T, R> map)
{
    using (var enumerator = stream.GetEnumerator ())
    {
        if (!enumerator.MoveNext ()) yield break ;

        var cur   = enumerator.Current   ;
        var flags = StreamPosition.First ;
        while (true)
        {
            if (!enumerator.MoveNext ()) flags |= StreamPosition.Last ;
            yield return map (flags, cur) ;
            if ((flags & StreamPosition.Last) != 0) yield break ;
            cur   = enumerator.Current ;
            flags = 0 ;
        }
    }
}

Then the simplest (not the quickest, that would need a couple more handy extension methods) solution will be:

public static string Quibble (IEnumerable<string> strings)
{
    return "{" + String.Join ("", strings.MapWithPositions ((pos, item) => (
       (pos &  StreamPosition.First) != 0      ? "" : 
        pos == StreamPosition.Last   ? " and " : ", ") + item)) + "}" ;
}

Answer 4

Here's a simple F# solution, that only does one forward iteration:

let CommaQuibble items =
    let sb = System.Text.StringBuilder("{")
    // pp is 2 previous, p is previous
    let pp,p = items |> Seq.fold (fun (pp:string option,p) s -> 
        if pp <> None then
            sb.Append(pp.Value).Append(", ") |> ignore
        (p, Some(s))) (None,None)
    if pp <> None then
        sb.Append(pp.Value).Append(" and ") |> ignore
    if p <> None then
        sb.Append(p.Value) |> ignore
    sb.Append("}").ToString()

(EDIT: Turns out this is very similar to Skeet's.)

The test code:

let Test l =
    printfn "%s" (CommaQuibble l)

Test []
Test ["ABC"]        
Test ["ABC";"DEF"]        
Test ["ABC";"DEF";"G"]        
Test ["ABC";"DEF";"G";"H"]        
Test ["ABC";null;"G";"H"]        

Answer 5

public static string CommaQuibbling(IEnumerable<string> items)
{
  int count = items.Count();
  string answer = string.Empty;
  return "{" + 
      (count==0)  ?  ""  :  
         (  items[0] + 
             (count == 1 ? "" :  
                 items.Range(1,count-1).
                     Aggregate(answer, (s,a)=> s += ", " + a) +
                 items.Range(count-1,1).
                     Aggregate(answer, (s,a)=> s += " AND " + a) ))+ "}";
}

It is implemented as,

if count == 0 , then return empty,
if count == 1 , then return only element,
if count > 1 , then take two ranges, 
   first 2nd element to 2nd last element
   last element

Answer 6

It hasn't quite been a decade since the last post so here's my variation:

    public static string CommaQuibbling(IEnumerable<string> items)
    {
        var text = new StringBuilder();
        string sep = null;
        int last_pos = items.Count();
        int next_pos = 1;

        foreach(string item in items)
        {
            text.Append($"{sep}{item}");
            sep = ++next_pos < last_pos ? ", " : " and ";
        }

        return $"{{{text}}}";
    }