Sort number strings

Data files sometimes contain a leading integer on each line. When we want to sort these lines, we cannot use a textual sort on the strings.

Parsing values

To sort lines with numbers in them, we must take into account the numbers. We can treat numbers as numbers (not strings) by parsing them with C# code.

Required input, output

Consider this file—each line has two main parts, separated by a hyphen and some spaces. We can parse this so the lines are sorted by the left-side number.

2 - pa
2 - zo
23 - zo
3 - ad
3 - za
2 - pa
2 - zo
3 - ad
3 - za
23 - zo

Example code

Each line, as it is read from there source file, is also parsed and stored into an object. The leading integer on each line is stored as an int field in the object.

using System;
using System.Collections.Generic;
using System.IO;

class Line : IComparable<Line>
{
    int _number;
    string _afterNumber;
    string _line;

    public Line(string line)
    {
        // Get leading integer.
        int firstSpace = line.IndexOf(' ');
        string integer = line.Substring(0, firstSpace);
        this._number = int.Parse(integer);

        // Store string.
        this._afterNumber = line.Substring(firstSpace);
        this._line = line;
    }

    public int CompareTo(Line other)
    {
        // First compare number.
        int result1 = _number.CompareTo(other._number);
        if (result1 != 0)
        {
            return result1;
        }
        // Second compare part after number.
        return _afterNumber.CompareTo(other._afterNumber);
    }

    public override string ToString()
    {
        return this._line;
    }
}

class Program
{
    static void Main()
    {
        List<Line> list = new List<Line>();
        using (StreamReader reader = new StreamReader("c:\\p.txt"))
        {
            while (true)
            {
                string line = reader.ReadLine();
                if (line == null)
                {
                    break;
                }
                list.Add(new Line(line));
            }
        }
        list.Sort();
        foreach (Line value in list)
        {
            Console.WriteLine(value);
        }
    }
}
2 - pa
2 - zo
3 - ad
3 - za
23 - zo

Alphanumeric sort

A more general-purpose alphanumeric sorting algorithm would also work in this case. The alphanumeric sort would eliminate the need to parse lines on your own.

A summary

Using an object that stores the integer as a field allows better sorting. In C# programs, implementing the IComparable interface and using a custom constructor is a solution.