Is one of these for loops faster than the other?

Question

for (var keyValue = 0; keyValue < dwhSessionDto.KeyValues.Count; keyValue++)
{...}


var count = dwhSessionDto.KeyValues.Count;
for (var keyValue = 0; keyValue < count; keyValue++)
{...}

I know there's a difference between the two, but is one of them faster than the other? I would think the second is faster.

Answer 1

Yes, the first version is much slower. After all, I'm assuming you're dealing with types like this:

public class SlowCountProvider
{
    public int Count
    {
        get
        {
            Thread.Sleep(1000);
            return 10;
        }
    }
}

public class KeyValuesWithSlowCountProvider
{
    public SlowCountProvider KeyValues
    {
        get { return new SlowCountProvider(); }
    }
}

Here, your first loop will take ~10 seconds, whereas your second loop will take ~1 second.

Of course, you might argue that the assumption that you're using this code is unjustified - but my point is that the right answer will depend on the types involved, and the question doesn't state what those types are.

Now if you're actually dealing with a type where accessing KeyValues and Count is cheap (which is quite likely) I wouldn't expect there to be much difference. Mind you, I'd almost always prefer to use foreach where possible:

foreach (var pair in dwhSessionDto.KeyValues)
{
    // Use pair here
}

That way you never need the count. But then, you haven't said what you're trying to do inside the loop either. (Hint: to get more useful answers, provide more information.)

Answer 2

it depends how difficult it is to compute dwhSessionDto.KeyValues.Count if its just a pointer to an int then the speed of each version will be the same. However, if the Count value needs to be calculated, then it will be calculated every time, and therefore impede perfomance.

EDIT -- heres some code to demonstrate that the condition is always re-evaluated

public class Temp
{
    public int Count { get; set; }
}

static void Main(string[] args)
{
    var t = new Temp() {Count = 5};
    for (int i = 0; i < t.Count; i++)
    {
        Console.WriteLine(i);
        t.Count--;
    }
    Console.ReadLine();
}

The output is 0, 1, 2 - only !