Embarrassing code admission of the day (or why C.S. is good for you)

[Julien Villers]

Yes it is! http://www.codinghorror.com/blog/2011/06/performance-is-a-feature.html[^]

'As programmers go, I'm fairly social. Which still means I'm a borderline sociopath by normal standards.' Jeff Atwood 'I'm French! Why do you think I've got this outrrrrageous accent?' Monty Python and the Holy Grail

[Pete OHanlon]

Basically, one way or the other there's a 0(n) operation - either in finding the element at position n, or removing the element at position n. Removing the element at n where n = 0 would, at first glance, appear to be a good optimisation. Unfortunately, it has side effects.

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"Mind bleach! Send me mind bleach!" - Nagy Vilmos

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[Mladen Jankovic]

Unless the actual implementation of the algorithm starts iterating from the end of the list, from some strange reason.

[Dario Solera]

No, it's O(1) (because there's an array behind): http://msdn.microsoft.com/en-us/library/0ebtbkkc.aspx[^]

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[V 0]

Sorry, can't find anything wrong. The only thing I see on sight is you manipulate the size of a list while looping it. This is potentially dangerous, depending on what you do with it.

		Random r = new Random((int) DateTime.Now.Ticks);			
		List fakelist = new List();
		
		Console.WriteLine("Populating list");
		for(int i = 0; i < 100000; i++){
			fakelist.Add(r.NextDouble() + i);
		}												//end for
		
		Console.WriteLine("looping list, writing to file");
		System.IO.StreamWriter writer = new System.IO.StreamWriter(@"C:\\temp\\fakelist.txt");
		int index = 0;
		while(fakelist.Count > 0){
			double temp = fakelist\[0\];
			writer.Write(temp);
			writer.Write("\\t");
			if(index%10 == 0){
				writer.WriteLine("");
				writer.WriteLine(DateTime.Now.ToString("dd/MM/yyyy HH:mm:ss:ffff"));
				writer.WriteLine("");
			}											//end if
			writer.Flush();
			index++;
			fakelist.RemoveAt(0);
		}												//end while
		Console.WriteLine("Done!");
		writer.Close();			

		Console.WriteLine("Press enter to quit.");
		Console.ReadLine();

V.

[Julien Villers]

Well, in .NET a List is well documented, so list[0] should be O(1), while list[n-1] is also 0(1) (see post below for MSDN link). A .NET List is supposed to be a dynamic array, with the expected performance of an array for single element access. So reversing the loop shown here would not even cause the access speed to be bad, but even if it were bad, it would not be as bad as the constant resizing (RemoveAt(0)) would be. Morality: use a Queue (or not).

'As programmers go, I'm fairly social. Which still means I'm a borderline sociopath by normal standards.' Jeff Atwood 'I'm French! Why do you think I've got this outrrrrageous accent?' Monty Python and the Holy Grail

[Mladen Jankovic]

I'm not familiar with internals of List<> in .Net. I was under assumption that it is implemented as plain vanilla list in which case the removal of either head or tail should consume the same amount of time, but your hint tells me that it isn't usual list.

[GParkings]

IIRC its an extension method so very likely to be the same as it will be using the pre-existing public members unless of course there's some IL magic going on...

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[TheGreatAndPowerfulOz]

Technically, it "works". But, if you also define working as being the most reasonably efficient, then yes it it is a bug.

If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun

[Mladen Jankovic]

Well I was under wrong impession that List<> is implemented as linked list and not array. Blame it on C++ and STL :)

[TheGreatAndPowerfulOz]

Assuming this is C#, removing the first item in a list creates an entirely new list, which I think is a bug in the underlying list code since removing the first item should just rebase the list head at the next item. I think the underlying list code uses an array, believe it or not. It's not a "bug" per se, unless you consider taking a very long time to do a simple operation a bug.

If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun

[Sentenryu]

if you are looking for performance, use a for loop with a control variable and after the loop, call List.Clear(); you'll be saving numerous function calls... remove the first element each time it's not necessary nor benefic.

I'm brazilian and english (well, human languages in general) aren't my best skill, so, sorry by my english. (if you want we can speak in C# or VB.Net =p)

[Mladen Jankovic]

Julien Villers wrote:

A .NET List is supposed to be a dynamic array, with the expected performance of an array for single element access.

Blame it on C++ and STL, but who in his right mind name class for one data structure after another stucture? :)

Julien Villers wrote:

while list[n-1] is also 0(1)

Also, your original post (now edited) suggests that even you was under implession that List<> is in fact a list not an array ;)

[Lost User]

Removing at the beginning is the stupidest thing to do, but not actually a bug is it? A performance regression sure..

[Ennis Ray Lynch Jr]

Trust me when I saw it; I laughed.

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[Andy Brummer]

Just the performance thing? If you want to get into the nitty gritty of performance under dot net, what is the impact of something like List vs. List on garbage collection when we are talking millions of records. Discounting the fact that each string probably takes up more memory than a double.

[Julien Villers]

Yup, it's not a very good choice (ArrayList was nicer, but not generic). On the other hand, consider all the time gained by typing List<> instead of DynamicArray<> :D

'As programmers go, I'm fairly social. Which still means I'm a borderline sociopath by normal standards.' Jeff Atwood 'I'm French! Why do you think I've got this outrrrrageous accent?' Monty Python and the Holy Grail

[Pete OHanlon]

In the examples you get with the Sync Framework, there's a similar issue from the other side. Basically, the code checks to see if it has already found a file and, if not, it adds it to the end. The problem, of course, is that the more files you add, the longer the find takes to complete for files that aren't present in the list. I know of companies who are using this in production code, little realising why their applications are slowing down.

*pre-emptive celebratory nipple tassle jiggle* - Sean Ewington

"Mind bleach! Send me mind bleach!" - Nagy Vilmos

My blog | My articles | MoXAML PowerToys | Mole 2010 - debugging made easier - my favourite utility

[V 0]

Random r = new Random((int) DateTime.Now.Ticks);
List fakelist = new List();

		Console.WriteLine("Populating list");
		for(int i = 0; i < 100000; i++){
			fakelist.Add(r.NextDouble() + i);
		}												//end for
		Console.WriteLine("looping list, remove from front.");
		DateTime start = DateTime.Now;
		while(fakelist.Count > 0){
			double temp = fakelist\[0\];
			fakelist.RemoveAt(0);
		}												//end while

		TimeSpan diff = DateTime.Now - start;
		Console.WriteLine("Time: " + diff.TotalMilliseconds + " milliseconds.");

		Console.WriteLine("Populating list");
		for(int i = 0; i < 100000; i++){
			fakelist.Add(r.NextDouble() + i);
		}												//end for

		Console.WriteLine("looping list,removing from back.");
		int inverseindex = fakelist.Count-1;
		start = DateTime.Now;
		while(fakelist.Count > 0){
		    double temp = fakelist\[inverseindex\];
		    fakelist.RemoveAt(inverseindex);
		    inverseindex--;
		}												//end while

		diff = DateTime.Now - start;
		Console.WriteLine("Time: " + diff.TotalMilliseconds + " milliseconds.");

		Console.WriteLine("Done!");
		Console.WriteLine("Press enter to quit.");
		Console.ReadLine();

output:

Populating list
looping list, remove from front.
Time: 5468,82 milliseconds.
Populating list
looping list,removing from back.
Time: 0 milliseconds.
Done!
Press enter to quit.

So yes, there is a signifant performance gain. Well spotted. :thumbsup:

V.

[Mladen Jankovic]

STL uses vector as name for dynamic arrays, that's just two letters more and much less misleading :)