How to split a string into an array of two letter substrings with C#

your problem was that you incremented your index by 1 instead of 2 every time

 var res = new List<string>();
 for (int i = 0; i < x.Length - 1; i += 2)
 {
     res.Add(x.Substring(i, 2));
 }

should work

EDIT: because you ask for a default _ suffix in case of odd characters amount, this should be the change:

  var testString = "odd";
  string workOn = testString.Length % 2 != 0
     ? testString + "_"
     : testString;
  var res = new List<string>();
  for (int i = 0; i < workOn.Length - 1; i += 2)
  {
      res.Add(workOn.Substring(i, 2));
  }

two notes to notice:

• in .NET 6 Chunk() is available so you can use this as suggested in other answers
• this solution might not be the best in case of a very long input so it really depends on what are your inputs and expectations

If you are using latest .NET version i.e (.NET 6.0 RC 1), then you can try Chunk() method,

var strChunks = "abcdef".Chunk(2); //[['a', 'b'], ['c', 'd'], ['e', 'f']]

var result = strChunks.Select(x => string.Join('', x)).ToArray(); //["ab", "cd", "ef"]

Note: I am unable to test this on fiddle or my local machine due to latest version of .NET

.net 6 has an IEnumerable.Chunk() method that you can use to do this, as follows:

public static void Main()
{
    string[] result = 
       "abcdef"
       .Chunk(2)
       .Select(chunk => new string(chunk)).ToArray();

    Console.WriteLine(string.Join(", ", result)); // Prints "ab, cd, ef"
}

Before .net 6, you can use MoreLinq.Batch() to do the same thing.

[EDIT] In response the the request below:

MoreLinq is a set of Linq utilities originally written by Jon Skeet. You can find an implementation by going to Project | Manage NuGet Packages and then browsing for MoreLinq and installing it.

After installing it, add using MoreLinq.Extensions; and then you'll be able to use the MoreLinq.Batch extension like so:

public static void Main()
{
    string[] result = "abcdef"
       .Batch(2)
       .Select(chunk => new string(chunk.ToArray())).ToArray();

    Console.WriteLine(string.Join(", ", result)); // Prints "ab, cd, ef"
}

Note that there is no string constructor that accepts an IEnumerable<char>, hence the need for the chunk.ToArray() above.

I would say, though, that including the whole of MoreLinq just for one extension method is perhaps overkill. You could just write your own extension method for Enumerable.Chunk():

public static class MyBatch
{
    public static IEnumerable<T[]> Chunk<T>(this IEnumerable<T> self, int size)
    {
        T[] bucket = null;
        int count  = 0;

        foreach (var item in self)
        {
            if (bucket == null)
                bucket = new T[size];

            bucket[count++] = item;

            if (count != size)
                continue;

            yield return bucket;

            bucket = null;
            count  = 0;
        }

        if (bucket != null && count > 0)
            yield return bucket.Take(count).ToArray();
    }
}

With linq you can achieve it with the following way:

char[] word = "abcdefg".ToCharArray();
var evenCharacters = word.Where((_, idx) => idx % 2 == 0);
var oddCharacters = word.Where((_, idx) => idx % 2 == 1);
var twoCharacterLongSplits = evenCharacters
    .Zip(oddCharacters)
    .Select((pair) => new char[] { pair.First, pair.Second });

The trick is the following, we create two collections:

• one where we have only those characters where the original index was even (% 2 == 0)
• one where we have only those characters where the original index was odd (% 2 == 1)

Then we zip them. So, we create a tuple by taking one item from the even and one item from the odd collection. Then we create a new tuple by taking one item from the even and ...

And last we convert the tuples to arrays to have the desired output format.

You are on the right track but you need to increment by 2 not by one. You also need to check if the array has not ended before taking the second character else you risk running into an index out of bounds exception. Try this code I've written below. I've tried it and it works. Best!

public static List<string> splitstring(string str)
        {
            List<string> result = new List<string>();
            int strlen = str.Length;
            for(int i = 0; i<strlen; i+=2)
            {
                string currentstr = str[i].ToString();
                if (i + 1 <= strlen-1)
                { currentstr += str[i + 1].ToString(); }
                result.Add(currentstr);
            }
            return result;
        }