Is there a String.IndexOf that takes a predicate?

Question

I need to be able to say something like myString.IndexOf(c => !Char.IsDigit(c)), but I can't find any such method in the .NET framework. Did I miss something?

The following works, but rolling my own seems a little tedious here:

using System;

class Program
{
    static void Main()
    {
        string text = "555ttt555";
        int nonDigitIndex = text.IndexOf(c => !Char.IsDigit(c));
        Console.WriteLine(nonDigitIndex);
    }
}

static class StringExtensions
{
    public static int IndexOf(this string self, Predicate<char> predicate)
    {
        for (int index = 0; index < self.Length; ++index) {
            if (predicate(self[index])) {
                return index;
            }
        }
        return -1;
    }
}

Answer 1

Your current solution is better but could do something like ...

int c = myString.TakeWhile(c => Char.IsDigit(c)).Count();
return (c == myString.Length) ? -1 : c;

Slightly shorter and somewhat fewer exception cases than the other proposals here, edited to show how to handle -1 case. Note that the predicate is the inverse of the predicate in the question because we want to count how many characters ARE digits before we find a non-digit.

Answer 2

You're not missing anything. There is no IndexOf in the manner you're searching for in the framework. The closest thing you could do without rolling your own would be

text
  .Select((c, index) => new { Char = c, Index = index })
  .Where(pair => !Char.IsDigit(pair.Char))
  .Select(pair => pair.Index)
  .FirstOrDefault(-1);

However that is not easy to follow and causes senseless allocations. I'd much prefer to roll my own IndexOf in this case.

EDIT Whoops. Forgot that FirstOrDefault is a function I have hand rolled in my apps and is not a part of the standard LINQ libraries with this overload.

public static T FirstOrDefault<T>(this IEnumerable<T> enumerable, T defaultValue) {
  using (var enumerator = enmumerable.GetEnumerator()) {
    if (enumerator.MoveNext()) {
      return enumerator.Current;
    }
    return defaultValue;
}

Here is a version that works without any custom extensions. Note this is for example only, please don't put this in your app ;)

text
  .Select((c, index) => new { Char = c, Index = index })
  .Where(pair => !Char.IsDigit(pair.Char))
  .Select(pair => pair.Index)
  .Concat(Enumerable.Repeat(-1, 1))
  .First();