Question

string s = "abcabcabcabcabc";

var foundIndexes = new List<int>();

The question came from the discussion here. I was simply wondering

How can this:

for (int i = s.IndexOf('a'); i > -1; i = s.IndexOf('a', i + 1))  

      foundIndexes.Add(i); 

Be better than this :

for (int i = 0; i < s.Length; i++)

      if (s[i] == 'a') foundIndexes.Add(i);

EDIT : Where all does the performance gain come from?

Answer 1

I did not observe that using IndexOf was any faster than direct looping. Honestly, I don’t see how it could be because each character needs to be checked in both cases. My initial results were this:

Found by loop, 974 ms 
Found by IndexOf 1144 ms

Edit: After running several more times I’ve noticed that you must run release (ie with optimizations) to see my result above. Without optimizations, the for loop is indeed slower.

The benchmark code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Text;
using System.IO;
using System.Diagnostics;

namespace Test
{
    public class Program
    {
        public static void Main(string[] args)
        {
            const string target = "abbbdbsdbsbbdbsabdbsabaababababafhdfhffadfd";

            // Jit methods
            TimeMethod(FoundIndexesLoop, target, 1);
            TimeMethod(FoundIndexesIndexOf, target, 1);            

            Console.WriteLine("Found by loop, {0} ms", TimeMethod(FoundIndexesLoop, target, 2000000));
            Console.WriteLine("Found by IndexOf {0} ms", TimeMethod(FoundIndexesIndexOf, target, 2000000));           
        }

        private static long TimeMethod(Func<string, List<int>> method, string input, int reps)
        {
            var stopwatch = Stopwatch.StartNew();
            List<int> result = null;
            for(int i = 0; i < reps; i++)
            {
                result = method(input);
            }
            stopwatch.Stop();
            TextWriter.Null.Write(result);
            return stopwatch.ElapsedMilliseconds;
        }

        private static List<int> FoundIndexesIndexOf(string s)
        {
            List<int> indexes = new List<int>();

            for (int i = s.IndexOf('a'); i > -1; i = s.IndexOf('a', i + 1))
            {
                 // for loop end when i=-1 ('a' not found)
                indexes.Add(i);
            }

            return indexes;
        }

        private static List<int> FoundIndexesLoop(string s)
        {
            var indexes = new List<int>();
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] == 'a')
                indexes.Add(i);
            }

            return indexes;
        }
    }
}