LINQ list to sentence format (insert commas & “and

Question

I have a linq query that does something simple like:

var k = people.Select(x=>new{x.ID, x.Name});

I then want a function or linq lambda, or something that will output the names in sentence format using commas and "ands".

{1, John}
{2, Mark}
{3, George}

to

"1:John, 2:Mark and 3:George"

I'm fine with hardcoding the ID + ":" + Name part, but it could be a ToString() depending on the type of the linq query result. I'm just wondering if there is a neat way to do this with linq or String.Format().

Answer 1

StringBuilder Approach

Here's an Aggregate with a StringBuilder. There's some position determinations that are made to clean up the string and insert the "and" but it's all done at the StringBuilder level.

var people = new[]
{
    new { Id = 1, Name = "John" },
    new { Id = 2, Name = "Mark" },
    new { Id = 3, Name = "George" }
};

var sb = people.Aggregate(new StringBuilder(),
             (s, p) => s.AppendFormat("{0}:{1}, ", p.Id, p.Name));
sb.Remove(sb.Length - 2, 2); // remove the trailing comma and space

var last = people.Last();
// index to last comma (-2 accounts for ":" and space prior to last name)
int indexComma = sb.Length - last.Id.ToString().Length - last.Name.Length - 2;

sb.Remove(indexComma - 1, 1); // remove last comma between last 2 names
sb.Insert(indexComma, "and ");

// 1:John, 2:Mark and 3:George
Console.WriteLine(sb.ToString());

A String.Join approach could have been used instead but the "and" insertion and comma removal would generate ~2 new strings.

---

Regex Approach

Here's another approach using regex that is quite understandable (nothing too cryptic).

var people = new[]
{
    new { Id = 1, Name = "John" },
    new { Id = 2, Name = "Mark" },
    new { Id = 3, Name = "George" }
};
var joined = String.Join(", ", people.Select(p => p.Id + ":" + p.Name).ToArray());
Regex rx = new Regex(", ", RegexOptions.RightToLeft);
string result = rx.Replace(joined, " and ", 1); // make 1 replacement only
Console.WriteLine(result);

The pattern is simply ", ". The magic lies in the RegexOptions.RightToLeft which makes the match occur from the right and thereby makes the replacement occur at the last comma occurrence. There is no static Regex method that accepts the number of replacements with the RegexOptions, hence the instance usage.

Answer 2

I have refined my previous answer and I believe this is the most elegant solution yet.
However it would only work on reference types that don't repeat in the collection (or else we'd have to use different means for finding out if item is first/last).

Enjoy!

var firstGuy = guys.First();
var lastGuy = guys.Last();

var getSeparator = (Func<Guy, string>)
    (guy => {
        if (guy == firstGuy) return "";
        if (guy == lastGuy) return " and ";
        return ", ";
    });

var formatGuy = (Func<Guy, string>)
    (g => string.Format("{0}:{1}", g.Id, g.Name));

// 1:John, 2:Mark and 3:George
var summary = guys.Aggregate("",
    (sum, guy) => sum + getSeparator(guy) + formatGuy(guy));

Answer 3

Here's one using a slightly modified version of my answer to Eric Lippert's Challenge which is IMHO the most concise with easy to follow logic (if you're familiar with LINQ).

static string CommaQuibblingMod<T>(IEnumerable<T> items)
{
    int count = items.Count();
    var quibbled = items.Select((Item, index) => new { Item, Group = (count - index - 2) > 0})
                        .GroupBy(item => item.Group, item => item.Item)
                        .Select(g => g.Key
                            ? String.Join(", ", g)
                            : String.Join(" and ", g));
    return String.Join(", ", quibbled);  //removed braces
}

//usage
var items = k.Select(item => String.Format("{0}:{1}", item.ID, item.Name));
string formatted = CommaQuibblingMod(items);

Answer 4

public string ToPrettyCommas<T>(
  List<T> source,
  Func<T, string> stringSelector
)
{
  int count = source.Count;

  Func<int, string> prefixSelector = x => 
    x == 0 ? "" :
    x == count - 1 ? " and " :
    ", ";

  StringBuilder sb = new StringBuilder();

  for(int i = 0; i < count; i++)
  {
    sb.Append(prefixSelector(i));
    sb.Append(stringSelector(source[i]));
  }

  string result = sb.ToString();
  return result;
}

Called with:

string result = ToPrettyCommas(people, p => p.ID.ToString() + ":" + p.Name);

Answer 5

Improving(hopefully) on KeithS's answer:

string nextBit = "";
var sb = new StringBuilder();
foreach(Person person in list)
{
    sb.Append(nextBit);
    sb.Append(", ");
    nextBit = String.Format("{0}:{1}", person.ID, person.Name);
}
sb.Remove(sb.Length - 3, 2);
sb.Append(" and ");
sb.Append(nextBit);