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Ignoring first X characters from an array when sorting.

Posted on 2010-11-11
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Last Modified: 2013-12-17
I have an array of roughly 5000 elements, each of which is in this format:

"\\\\192.168.5.6\\log\\freeswitch.log.2010-06-20-18-54-29.1"      string
AND
"\\\\192.168.5.8\\log\\freeswitch.log.2010-08-03-20-09-40.1"      string

I need to sort this array by the dates, not by the server address. How would I go about making it so that the Array.Sort() method ignores all characters prior to "log"? From what I can see, I need to implement an IComparer interface, I'm just not too sure how.

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Question by:Kilsol
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Todd Gerbert earned 250 total points
ID: 34115488
Here's a simple example, this skips the first 4 chars
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
	class Program
	{
		static void Main(string[] args)
		{
			string[] strings = { "bbb.log.1", "aaa.log.2", "ddd.log.3", "ccc.log.4" };

			Console.WriteLine("Normal sort:");
			Array.Sort(strings);
			foreach (string s in strings)
				Console.WriteLine(s);

			// Reset array
			strings = new string[] { "bbb.log.1", "aaa.log.2", "ddd.log.3", "ccc.log.4" };
			Console.WriteLine("\r\nCustom sort:");
			Array.Sort(strings, new SkipFirstNCharComparer(3));
			foreach (string s in strings)
				Console.WriteLine(s);

			Console.ReadKey();
		}
	}

	public class SkipFirstNCharComparer : IComparer
	{
		private int _charsToSkip;
		public SkipFirstNCharComparer(int numCharsToSkip)
		{
			_charsToSkip = numCharsToSkip;
		}
		public int Compare(object x, object y)
		{
			int maxLen = ((string)x).Length > ((string)y).Length ? ((string)x).Length : ((string)y).Length;
			return String.CompareOrdinal((string)x, _charsToSkip, (string)y, _charsToSkip, maxLen);
		}
	}
}

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Expert Comment

by:Mike Tomlinson
ID: 34117283
If you want to compare based on the DATES then you need to extract those portions and convert them to Dates so you can do that comparison...

...something like:
private void button1_Click(object sender, EventArgs e)
        {
            string[] data = {"\\\\192.168.5.8\\log\\freeswitch.log.2010-08-03-20-09-40.1", "\\\\192.168.5.6\\log\\freeswitch.log.2010-06-20-18-54-29.1"};
            Array.Sort(data, MyComparer);
        }

        private int MyComparer(string a, string b)
        {
            string[] aVals = a.Split(".".ToCharArray());
            string[] bVals = b.Split(".".ToCharArray());
            int aIndex = Array.IndexOf(aVals, "log");
            int bIndex = Array.IndexOf(bVals, "log");
            if (aIndex != -1 && (aIndex + 1) < aVals.Length && bIndex != -1 && (bIndex + 1) < bVals.Length)
            {
                string dtFormat = "yyyy-MM-dd-HH-mm-ss";
                DateTime aDT, bDT;
                if (DateTime.TryParseExact(aVals[aIndex + 1], dtFormat, null, System.Globalization.DateTimeStyles.None, out aDT))
                {
                    if (DateTime.TryParseExact(bVals[bIndex + 1], dtFormat, null, System.Globalization.DateTimeStyles.None, out bDT))
                    {
                        return aDT.CompareTo(bDT); // sort based on date values
                    }
                }
            }
            return a.CompareTo(b); // something was wrong with formatting; revert to string comparison
        }

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Expert Comment

by:Mike Tomlinson
ID: 34117448
*Note that the code in tgerbert's example works because the dates have been formatted with leading zeroes, military time has been used, and the order of the values lends itself to a straight STRING comparison.  If the order and format of the dates was different it probably wouldn't work...just keep that in mind if the format changes or you need to do a similar thing with a different project.

My code is actually converting the "date/time" strings to DateTime instances using the specified format so they can be compared correctly as DateTimes instead of as strings.
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LVL 33

Expert Comment

by:Todd Gerbert
ID: 34117780
I  should also point out my example also assumes it's safe to just ignore the first n characters, if one of the filenames happens to start with an IP address with more chars that logic breaks.

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