C＃Array.Sort结果不一致

Question

I am trying to sort an array of tab-delimited strings. I call the split function on tab characters on each element to break the strings into string arrays. I then do a comparison on the date element to return the order that they should be in. If the dates are equal I do a secondary comparison on a string field. My implementation of the IComparer interface looks like this:

class CompareLines : IComparer<string>
{
    public int Compare(string x, string y)
    {
        string[] bSplitX = x.Split('\t');
        string[] bSplitY = y.Split('\t');

        int bYearX = Int32.Parse(bSplitX[4].Substring(4, 4));
        int bMonthX = Int32.Parse(bSplitX[4].Substring(0, 2));
        int bDayX = Int32.Parse(bSplitX[4].Substring(2, 2));

        int bYearY = Int32.Parse(bSplitY[4].Substring(4, 4));
        int bMonthY = Int32.Parse(bSplitY[4].Substring(0, 2));
        int bDayY = Int32.Parse(bSplitY[4].Substring(2, 2));

        DateTime bTempDateX = new DateTime(bYearX, bMonthX, bDayX);
        DateTime bTempDateY = new DateTime(bYearY, bMonthY, bDayY);

        if (DateTime.Compare(bTempDateX, bTempDateY) > 0)
        {
            return 1;
        }
        else if (DateTime.Compare(bTempDateX, bTempDateY) < 0)
        {
            return -1;
        }
        else if (DateTime.Compare(bTempDateX, bTempDateY) == 0)
        {
            if (String.Compare(bSplitX[3], bSplitY[3]) > 0)
            {
                return 1;
            }
            else
            {
                return -1;
            }
        }
        else
        {
            Console.WriteLine("ahhh wtf"); //should never be reached. This message has never appeared in my console.
            return 0;
        }
    }
}

My problem is that it will work sometimes and not others. Does anyone have any reasons why the above code would not work 100% of the time?

This has been hurting my brain for a couple days now and I really do not understand why it is not working.

Answer 1

How about replacing

if (String.Compare(bSplitX[3], bSplitY[3]) > 0) {
  return 1;
} else {
  return -1;
}

with

return String.Compare(bSplitX[3], bSplitY[3]);

Your current code in case of equal X and Y strings says that Y is smaller than X. The following is the code with ParseExact method.

class CompareLines : IComparer<string>
{
    public int Compare(string x, string y)
    {
        string[] bSplitX = x.Split('\t');
        string[] bSplitY = y.Split('\t');

        DateTime bTempDateX = DateTime.ParseExact(bSplitX[4], "MMddyyyy", null);
        DateTime bTempDateY = DateTime.ParseExact(bSplitY[4], "MMddyyyy", null);

        if (DateTime.Compare(bTempDateX, bTempDateY) > 0)
            return 1;
        else if (DateTime.Compare(bTempDateX, bTempDateY) < 0)
            return -1;
        else
            return String.Compare(bSplitX[3], bSplitY[3]);
    }
}

Answer 2

The only hole I see is if the date and string are the same then you are not returning 0:

        if (String.Compare(bSplitX[3], bSplitY[3]) > 0) {
            return 1;
        } else {
            return -1;  // if they are equal it will return -1.
        }

That will throw off the sorting routine.

Here's a cleaner version:

   int dateCompare = DateTime.Compare(bTempDateX, bTempDateY);

   if (dateCompare == 0) 
        return String.Compare(bSplitX[3], bSplitY[3]);
   else
       return dateCompare ;

Answer 3

For one, you don't need to rewrite the compare logic built into the framework, it already returns what you're expecting so long as you set up the data properly first-hand. As Jon suggested, simply parse the dates then, return the comparison of the dates. I added a property IncomingFormat which may not be useful considering your input.

Edit: more succinct/clear:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;

namespace ConsoleApplication1
{
    public class CompareLines : IComparer<string>
    {
        public string IncomingFormat { get; set; }

        public int Compare(string first, string second)
        {
            var date1 = DateTime.ParseExact(first, IncomingFormat, CultureInfo.InvariantCulture);
            var date2 = DateTime.ParseExact(second, IncomingFormat, CultureInfo.InvariantCulture);

            return date1.CompareTo(date2);
        }
    }

    internal class Program
    {
        private static void Main()
        {
            const string dataFormat = "MM\tdd\tyyyy";

            var comparer = new CompareLines
            {
                IncomingFormat = dataFormat
            };

            int result;
            string date1, date2;

            date1 = DateTime.Parse("1/1/2000").ToString(dataFormat);
            date2 = DateTime.Parse("1/1/2000").ToString(dataFormat);
            result = comparer.Compare(date1, date2);
            Debug.Assert(result == 0);
            Console.WriteLine("{0} compare {1} = {2}", date1, date2, result);

            date1 = DateTime.Parse("1/1/2000").ToString(dataFormat);
            date2 = DateTime.Parse("1/2/2000").ToString(dataFormat);
            result = comparer.Compare(date1, date2);
            Debug.Assert(result == -1);
            Console.WriteLine("{0} compare {1} = {2}", date1, date2, result);

            date1 = DateTime.Parse("1/2/2000").ToString(dataFormat);
            date2 = DateTime.Parse("1/1/2000").ToString(dataFormat);
            result = comparer.Compare(date1, date2);
            Debug.Assert(result == 1);
            Console.WriteLine("{0} compare {1} = {2}", date1, date2, result);

            Console.ReadLine();
        }
    }
}