Sorting List in C#

Question

How to sort a List based on the item\'s integer value

The list is like

\"1\"
\"5\"
\"3\"
\"6\"
\"11\"
\"9\"
\"NUM1\"
\"NUM0\"

The r

Answer 1

This is called a "natural sort order", and is usually employed to sort items like those you have, like filenames and such.

Here's a naive (in the sense that there are probably plenty of unicode-problems with it) implementation that seems to do the trick:

You can copy the code below into LINQPad to execute it and test it.

Basically the comparison algorithm will identify numbers inside the strings, and handle those by padding the shortest one with leading zeroes, so for instance the two strings "Test123Abc" and "Test7X" should be compared as though they were "Test123Abc" and "Test007X", which should produce what you want.

However, when I said "naive", I mean that I probably have tons of real unicode problems in here, like handling diacritics and multi-codepoint characters. If anyone can give a better implementation I would love to see it.

Notes:

• The implementation does not actually parse the numbers, so arbitrarily long numbers should work just fine
• Since it doesn't actually parse the numbers as "numbers", floating point numbers will not be handled properly, "123.45" vs. "123.789" will be compared as "123.045" vs. "123.789", which is wrong.

Code:

void Main()
{
    List<string> input = new List<string>
    {
        "1", "5", "3", "6", "11", "9", "A1", "A0"
    };
    var output = input.NaturalSort();
    output.Dump();
}

public static class Extensions
{
    public static IEnumerable<string> NaturalSort(
        this IEnumerable<string> collection)
    {
        return NaturalSort(collection, CultureInfo.CurrentCulture);
    }

    public static IEnumerable<string> NaturalSort(
        this IEnumerable<string> collection, CultureInfo cultureInfo)
    {
        return collection.OrderBy(s => s, new NaturalComparer(cultureInfo));
    }

    private class NaturalComparer : IComparer<string>
    {
        private readonly CultureInfo _CultureInfo;

        public NaturalComparer(CultureInfo cultureInfo)
        {
            _CultureInfo = cultureInfo;
        }

        public int Compare(string x, string y)
        {
            // simple cases
            if (x == y) // also handles null
                return 0;
            if (x == null)
                return -1;
            if (y == null)
                return +1;

            int ix = 0;
            int iy = 0;
            while (ix < x.Length && iy < y.Length)
            {
                if (Char.IsDigit(x[ix]) && Char.IsDigit(y[iy]))
                {
                    // We found numbers, so grab both numbers
                    int ix1 = ix++;
                    int iy1 = iy++;
                    while (ix < x.Length && Char.IsDigit(x[ix]))
                        ix++;
                    while (iy < y.Length && Char.IsDigit(y[iy]))
                        iy++;
                    string numberFromX = x.Substring(ix1, ix - ix1);
                    string numberFromY = y.Substring(iy1, iy - iy1);

                    // Pad them with 0's to have the same length
                    int maxLength = Math.Max(
                        numberFromX.Length,
                        numberFromY.Length);
                    numberFromX = numberFromX.PadLeft(maxLength, '0');
                    numberFromY = numberFromY.PadLeft(maxLength, '0');

                    int comparison = _CultureInfo
                        .CompareInfo.Compare(numberFromX, numberFromY);
                    if (comparison != 0)
                        return comparison;
                }
                else
                {
                    int comparison = _CultureInfo
                        .CompareInfo.Compare(x, ix, 1, y, iy, 1);
                    if (comparison != 0)
                        return comparison;
                    ix++;
                    iy++;
                }
            }

            // we should not be here with no parts left, they're equal
            Debug.Assert(ix < x.Length || iy < y.Length);

            // we still got parts of x left, y comes first
            if (ix < x.Length)
                return +1;

            // we still got parts of y left, x comes first
            return -1;
        }
    }
}

Answer 2

Try writing a small helper class to parse and represent your tokens. For example, without too many checks:

public class NameAndNumber
{
    public NameAndNumber(string s)
    {
        OriginalString = s;
        Match match = Regex.Match(s,@"^(.*?)(\d*)$");
        Name = match.Groups[1].Value;
        int number;
        int.TryParse(match.Groups[2].Value, out number);
        Number = number; //will get default value when blank
    }

    public string OriginalString { get; private set; }
    public string Name { get; private set; }
    public int Number { get; private set; }
}

Now it becomes easy to write a comparer, or sort it manually:

var list = new List<string> { "ABC", "1", "5", "NUM44", "3", 
                              "6", "11", "9", "NUM1", "NUM0" };

var sorted = list.Select(str => new NameAndNumber(str))
    .OrderBy(n => n.Name)
    .ThenBy(n => n.Number);

Gives the result:

1, 3, 5, 6, 9, 11, ABC, NUM0, NUM1, NUM44

Answer 3

I don't think you need anything besides listName.Sort() because sort() method uses default comparer to quick sort nodes. Default comparer does exactly what you are interested in.

Answer 4

How about:

    list.Sort((x, y) =>
    {
        int ix, iy;
        return int.TryParse(x, out ix) && int.TryParse(y, out iy)
              ? ix.CompareTo(iy) : string.Compare(x, y);
    });

Answer 5

Jeff Atwood has a blog post about natural sorting where he links to some available implementations of the desired algorithm.

One of Jeffs links points to Dave Koelle how has a C# implementation:

/*
 * The Alphanum Algorithm is an improved sorting algorithm for strings
 * containing numbers.  Instead of sorting numbers in ASCII order like
 * a standard sort, this algorithm sorts numbers in numeric order.
 *
 * The Alphanum Algorithm is discussed at http://www.DaveKoelle.com
 *
 * Based on the Java implementation of Dave Koelle's Alphanum algorithm.
 * Contributed by Jonathan Ruckwood <jonathan.ruckwood@gmail.com>
 *
 * Adapted by Dominik Hurnaus <dominik.hurnaus@gmail.com> to
 *   - correctly sort words where one word starts with another word
 *   - have slightly better performance
 *
 * Released under the MIT License - https://opensource.org/licenses/MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 * USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */
using System;
using System.Collections;
using System.Text;

/*
 * Please compare against the latest Java version at http://www.DaveKoelle.com
 * to see the most recent modifications
 */
namespace AlphanumComparator
{
    public class AlphanumComparator : IComparer
    {
        private enum ChunkType {Alphanumeric, Numeric};
        private bool InChunk(char ch, char otherCh)
        {
            ChunkType type = ChunkType.Alphanumeric;

            if (char.IsDigit(otherCh))
            {
                type = ChunkType.Numeric;
            }

            if ((type == ChunkType.Alphanumeric && char.IsDigit(ch))
                || (type == ChunkType.Numeric && !char.IsDigit(ch)))
            {
                return false;
            }

            return true;
        }

        public int Compare(object x, object y)
        {
            String s1 = x as string;
            String s2 = y as string;
            if (s1 == null || s2 == null)
            {
                return 0;
            }

            int thisMarker = 0, thisNumericChunk = 0;
            int thatMarker = 0, thatNumericChunk = 0;

            while ((thisMarker < s1.Length) || (thatMarker < s2.Length))
            {
                if (thisMarker >= s1.Length)
                {
                    return -1;
                }
                else if (thatMarker >= s2.Length)
                {
                    return 1;
                }
                char thisCh = s1[thisMarker];
                char thatCh = s2[thatMarker];

                StringBuilder thisChunk = new StringBuilder();
                StringBuilder thatChunk = new StringBuilder();

                while ((thisMarker < s1.Length) && (thisChunk.Length==0 ||InChunk(thisCh, thisChunk[0])))
                {
                    thisChunk.Append(thisCh);
                    thisMarker++;

                    if (thisMarker < s1.Length)
                    {
                        thisCh = s1[thisMarker];
                    }
                }

                while ((thatMarker < s2.Length) && (thatChunk.Length==0 ||InChunk(thatCh, thatChunk[0])))
                {
                    thatChunk.Append(thatCh);
                    thatMarker++;

                    if (thatMarker < s2.Length)
                    {
                        thatCh = s2[thatMarker];
                    }
                }

                int result = 0;
                // If both chunks contain numeric characters, sort them numerically
                if (char.IsDigit(thisChunk[0]) && char.IsDigit(thatChunk[0]))
                {
                    thisNumericChunk = Convert.ToInt32(thisChunk.ToString());
                    thatNumericChunk = Convert.ToInt32(thatChunk.ToString());

                    if (thisNumericChunk < thatNumericChunk)
                    {
                        result = -1;
                    }

                    if (thisNumericChunk > thatNumericChunk)
                    {
                        result = 1;
                    }
                }
                else
                {
                    result = thisChunk.ToString().CompareTo(thatChunk.ToString());
                }

                if (result != 0)
                {
                    return result;
                }
            }

            return 0;
        }
    }
}

Answer 6

Here is a C# 7 solution (assuming the list has the name a):

    var numericList = a.Where(i => int.TryParse(i, out _)).OrderBy(j => int.Parse(j)).ToList();
    var nonNumericList = a.Where(i => !int.TryParse(i, out _)).OrderBy(j => j).ToList();
    a.Clear();
    a.AddRange(numericList);
    a.AddRange(nonNumericList);