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DateTime AddHours Interval?

Posted on 2011-03-22
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Last Modified: 2012-05-11
Keep receiving strange datetime (7/1/2336 7:13:47 AM) using RandomInt(.079201111111111) understanding Random returns double.

    private double Randm()
    {          
         Random random = new Random();
         return random.Next(1800000, 36000000); // .5 hour to 10 hours                    
    }

    DateTime dt = DateTime.Now.AddHours(Randm);
    MessageBox.Show(dt.ToString());    
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Question by:pointeman
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15 Comments
 
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Expert Comment

by:Todd Gerbert
Comment Utility
Random.Next returns an int.  You're adding 1.8 million hours.
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by:Todd Gerbert
Comment Utility
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Accepted Solution

by:
eguilherme earned 63 total points
Comment Utility
?/ if you want to add from .5 to 10 hours use

   private double Randm()
    {          
         Random random = new Random();
         return random.Next(30, 600); // .5 hour to 10 hours                    
    }

    DateTime dt = DateTime.Now.AddMinutes(Randm);
    MessageBox.Show(dt.ToString());  
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Assisted Solution

by:philetaylor
philetaylor earned 63 total points
Comment Utility
Hi.

You are assigning a range to Randm of between 1800000 and 36000000

The problem is that you are then using the AddHours method which will be adding between 180000 and 36000000 HOURS to the time!

I think you probably want to use a range of 30 to 600 and use the AddMinutes method instead!

private double Randm()
{          
         Random random = new Random();
         return random.Next(30,600); // .5 hour to 10 hours                    
}

DateTime dt = DateTime.Now.AddMinutes(Randm);
MessageBox.Show(dt.ToString());  

Cheers

Phil
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Assisted Solution

by:Mike Tomlinson
Mike Tomlinson earned 62 total points
Comment Utility
Here's another way to think about it:
private Random random = new Random();

        private TimeSpan RandomHours(double minHour, double maxHour)
        {
            return TimeSpan.FromMilliseconds(TimeSpan.FromHours(minHour).TotalMilliseconds + (random.NextDouble() * TimeSpan.FromHours(maxHour - minHour).TotalMilliseconds));
        }

        private void button1_Click(object sender, EventArgs e)
        {
            DateTime dt = DateTime.Now.Add(RandomHours(.5, 10));
            MessageBox.Show(dt.ToString());  
        }

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Author Comment

by:pointeman
Comment Utility
I'm also using Randm() method to return values to Windows.Forms.Timer:

    return random.Next(1800000, 36000000); // .5 hour to 10 hours

Came up with the idea to monitor the timers 'fire' time as a second thought.
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Author Comment

by:pointeman
Comment Utility
>>The problem is that you are then using the AddHours method which will be adding between 180000 and 36000000 HOURS to the time!

I thought I was using adding approx .079201111111111 or 3/4 hours to the DateTime.AddHours as per original post :)
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LVL 4

Expert Comment

by:philetaylor
Comment Utility
Yes I thought you might!

Here is the definition of the Random.Next Method:

public virtual int Next(
	int minValue,
	int maxValue
)

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Expert Comment

by:Mike Tomlinson
Comment Utility
You could also use my post for your Timer, just use the TotalMilliseconds() property of the returned TimeSpan:

    timer1.Interval = RandomHours(.5, 10).TotalMilliseconds;
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Author Comment

by:pointeman
Comment Utility
Could also:

private double Randm()
{          
         Random random = new Random();
         return random.Next(30,600); // .5 hour to 10 hours                    
}

DateTime dt = DateTime.Now.AddMinutes(Randm);
MessageBox.Show(dt.ToString());  


Timer timer1 = new Timer();
timer1.Interval = Randm() * 60000; //1800000, 36000000 or .5 hour, 10 hours    
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Assisted Solution

by:Todd Gerbert
Todd Gerbert earned 62 total points
Comment Utility
I think I would probably just write - complete with the comment, so I know what the heck it's doing when I read the code later:
public partial class Form1 : Form
{
	static Random rand = new Random();

	public Form1()
	{
		InitializeComponent();

		timer1.Interval = rand.Next(30, 600) * 60000; // 30 - 600 minutes (.5 - 10 hours) times 60000 to get millisecs
	}
}

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Just seems simpler, more straight forward, to me. *shrug*

(Hopefully my math isn't too wrong)
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LVL 85

Expert Comment

by:Mike Tomlinson
Comment Utility
That's why I really like the TimeSpan.FromHours() approach...it's "self documenting":

    timer1.Interval = TimeSpan.FromHours(someHourValue).TotalMilliseconds;

We get a TimeSpan() representing that number of hours (it can accept a double) and then ask it for the total milliseconds that represent that "time span".  No xxx * 60000, or any other value that you have to remember for whatever unit (hours, minutes, seconds) you are looking for.  There are other TimeSpan.FromXXX() methods as well...
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Expert Comment

by:Todd Gerbert
Comment Utility
Agreed, I like that idea better.

Although that other method you posted ealier (http:#a35193935) took me more than a couple scratches of the ole noggin to figure out. ;)
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LVL 85

Expert Comment

by:Mike Tomlinson
Comment Utility
Lolz...that is just a C# version of this very old formula from the VB6 Rnd() function:
http://msdn.microsoft.com/en-us/library/e566zd96(VS.85).aspx

To produce random integers in a given range, use this formula:

    Int((upperbound - lowerbound + 1) * Rnd + lowerbound)

So my code is:

        private TimeSpan RandomHours(double minHour, double maxHour)
        {
            return TimeSpan.FromMilliseconds(TimeSpan.FromHours(minHour).TotalMilliseconds + (random.NextDouble() * TimeSpan.FromHours(maxHour - minHour).TotalMilliseconds));
        }

The "inner part", inside the outer TimeSpan.FromMilliseconds(), is the formula from the link (just the order of the terms has been reversed):

    TimeSpan.FromHours(minHour).TotalMilliseconds + (random.NextDouble() * TimeSpan.FromHours(maxHour - minHour).TotalMilliseconds)
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Author Closing Comment

by:pointeman
Comment Utility
Great Help. Almost ran out of points.
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