Re: Cubic roots of negative numbers, Math.Pow() - C# Discussion Boards

Cubic roots of negative numbers, Math.Pow()

Brandon Walton19-Jul-09 17:22

19-Jul-09 17:22

Hello everyone,

I ran into a bit of an issue with Math.Pow() today while doing some work with cubic equations. The cubic root of a real negative number, is a real negative number, correct? For some reason, when X is a negative value, Math.Pow(X, (double)1/3) returns NaN instead of a negative real number.

Hopefully I'm not forgetting something...

Any help would be appreciated, thanks.

Re: Cubic roots of negative numbers, Math.Pow()

PIEBALDconsult19-Jul-09 17:28

PIEBALDconsult

19-Jul-09 17:28

Huh, must not realize what you're trying to do. Maybe write your own?

The Algorithm (and Mathematics) forum may be a better place to post this.

Re: Cubic roots of negative numbers, Math.Pow()

Brandon Walton19-Jul-09 17:42

Brandon Walton

19-Jul-09 17:42

Opps, sorry if I posted in the wrong forum, it just seemed like a C# thing.

In case you didn't know, a cubic root is just like a square root (x^(1/2)). As in 2^3 = 8, 8^(1/3) = 2.

Re: Cubic roots of negative numbers, Math.Pow()

PIEBALDconsult19-Jul-09 17:53

PIEBALDconsult

19-Jul-09 17:53

Not the worst choice of forum.

I realize that about power and root, on the other hand, a double can't represent 1/3 precisely.

Re: Cubic roots of negative numbers, Math.Pow()

Brandon Walton19-Jul-09 18:36

Brandon Walton

19-Jul-09 18:36

It gives the correct answer if X is positive in Math.Pow(X, (double)1/3), so something weird is going on...

Re: Cubic roots of negative numbers, Math.Pow()

harold aptroot19-Jul-09 17:53

harold aptroot

19-Jul-09 17:53

Interesting problem here. Math.Pow probably (I'm not sure, but it's what I would do) uses Taylor series to calculate e^(x * ln(y)) (where ^ denotes exponentiation, and the goal is pow(x,y))
And the natural logarithm of a negative real is of course a complex, which it might not be able to handle, causing a NaN somewhere which then propagates through all further calculations to end up in the result.

This is just a theory, and I wouldn't bet anything on it, but I think it's plausible.

Re: Cubic roots of negative numbers, Math.Pow()

Brandon Walton19-Jul-09 18:22

Brandon Walton

19-Jul-09 18:22

Yeah, that sounds right. NaN seems to be the C# default for anything that would be complex.

I made a little work around which seems to work.

static public double Pow(double x, double y)
{
    return y < 1 && (1 / y) % 2 == 1 && x < 0 ? -Math.Pow(Math.Abs(x), y) : Math.Pow(x, y);
}

Though this seems really improper...

Re: Cubic roots of negative numbers, Math.Pow()

harold aptroot19-Jul-09 18:43

harold aptroot

19-Jul-09 18:43

Brandon Walton wrote:
(1 / y) % 2 == 1

This looks dangerous, equality of doubles.. well.. you've heard that before. Are you sure it's not "!= 0"?

But anyway, the default math library that comes with .NET doesn't do complex math. It's a shame, but that's how it is.. Now, I know you probably meant it the right way, so no offense, but just in case (and for completeness) : normal CPU's implement the IEEE 754 kind of math so NaNs just happen, C# can't help it (ok it could, but that would just make it terribly slow at floating point math)

Re: Cubic roots of negative numbers, Math.Pow()

Brandon Walton19-Jul-09 19:09

Brandon Walton

19-Jul-09 19:09

Ohh, I always assumed NaN were native to C#. I need to get out more...

harold aptroot wrote:
This looks dangerous, equality of doubles

Just the way I like it Cool | :cool:

. But yeah, just pretend you didn't see that.

harold aptroot wrote:
Are you sure it's not "!= 0"?

And yeah, should be != 0 Dead | X|

Re: Cubic roots of negative numbers, Math.Pow()

PIEBALDconsult19-Jul-09 18:53

PIEBALDconsult

19-Jul-09 18:53

I would try it normally first, then only do weird stuff if the result is NaN (or on Exception).

Re: Cubic roots of negative numbers, Math.Pow()

harold aptroot19-Jul-09 18:59

harold aptroot

19-Jul-09 18:59

Math.Pow([value smaller than 0], [non integral value]) is always NaN, according to the specs[^]
What else do you suggest?

I may be reading it wrong (nearly 7 in the morning, so I make mistakes easily), but to me it looks like you're suggesting that he do what he did already - which already proved to go wrong..

Re: Cubic roots of negative numbers, Math.Pow()

PIEBALDconsult19-Jul-09 20:48

PIEBALDconsult

19-Jul-09 20:48

But if it's to be a general-use routine, why not assume it will be used in the normal way and only try to add special handling as a backup?

Re: Cubic roots of negative numbers, Math.Pow()

Luc Pattyn20-Jul-09 1:05

Luc Pattyn

20-Jul-09 1:05

to summarize what others have said:

1. read the doc, it lists special cases including yours:
x < 0 but not NegativeInfinity; y is not an integer, NegativeInfinity, or PositiveInfinity ==> NaN

2. realize Math.Pow is using exp(y*ln(x)) so x better be positive
(you don't have to know that, but it justifies some of the documented special cases)

3. realize (double)1/3 can't be stored exactly in binary (not really relevant here)

4. make your own cubicroot method, which performs sign(x)*Math.Pow(abs(x),y)

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
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How many times each unique String appears in the collection.

dsrao19-Jul-09 17:14

dsrao

19-Jul-09 17:14

Hi All,

Here are the code...

//

List< String >category=new List < String >();

category.Add("House");
category.Add("Car");
category.Add("Dog");
category.Add("Car");

I want to show report like this...

Name Count
===== =====
Car 2
Dog 1
House 1

Please let me know how to do the above output.

Thanks & Regards
Rao

Re: How many times each unique String appears in the collection.

PIEBALDconsult19-Jul-09 17:16

PIEBALDconsult

19-Jul-09 17:16

You deleted your earlier post[^] and reposted? WTF | :WTF:

WE DON'T DO HOMEWORK HERE!

Re: How many times each unique String appears in the collection.

OriginalGriff19-Jul-09 21:55

OriginalGriff

19-Jul-09 21:55

I refer the honerable gentleman to my earlier answer to his question. I'll do your homework only if you pay me lots.[^]

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Re: How many times each unique String appears in the collection.

Luc Pattyn20-Jul-09 1:07

Luc Pattyn

20-Jul-09 1:07

Sure, however you forgot to mention your price.

Smile | :)

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Re: How many times each unique String appears in the collection.

OriginalGriff20-Jul-09 1:23

OriginalGriff

20-Jul-09 1:23

I feel the word "lots" indicates he would be in the "I don't like the client and so he should be gouged" price band. Laugh | :laugh:

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Re: How many times each unique String appears in the collection.

Luc Pattyn20-Jul-09 1:35

Luc Pattyn

20-Jul-09 1:35

I meant your earlier request for credit card details, that one was very unspecific.
"Lots" has become very precise around here, reminds me of "1, 2, many". Laugh | :laugh:

Luc Pattyn [Forum Guidelines] [My Articles]

The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
Show formatted code inside PRE tags, and give clear symptoms when describing a problem.

Re: How many times each unique String appears in the collection.

OriginalGriff20-Jul-09 1:41

OriginalGriff

20-Jul-09 1:41

Once I have his details, we can see how much he still owes me when I hit his credit limit... Laugh | :laugh:

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C# Windows Forms: How to filter WM_ACTIVATE and WM_NCACTIVATE messages?

Magnat7519-Jul-09 15:18

Magnat75

19-Jul-09 15:18

Hey all,

I want to intercept windows message that is sent to my program when form is deactivated (or activated, no matter). I can intercept mouse clicks etc. using regular message filter, but I can't intercept WM_NCACTIVATE or WM_ACTIVATE messages, why is that?

I'm trying something like that:

[SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
public class MessageFilter : IMessageFilter
{
    private const int WM_ACTIVATE = 0x006;
    private const int WM_NCACTIVATE = 0x086;

    public bool PreFilterMessage(ref Message m)
    {
        if(m.Msg == WM_NCACTIVATE)
            return true;
        return false;
    }
}

and then in Main:

MessageFilter mf = new MessageFilter();
Application.AddMessageFilter(mf);

I can see using debugger that it doesn't react on activation and deactivation of my form. How to do it correclty?

Re: C# Windows Forms: How to filter WM_ACTIVATE and WM_NCACTIVATE messages?

N a v a n e e t h19-Jul-09 16:44

N a v a n e e t h

19-Jul-09 16:44

Magnat75 wrote:
but I can't intercept WM_NCACTIVATE or WM_ACTIVATE messages, why is that?

Because they are sent to the control directly rather than posting into message queue. I believe, PreFilterMessage will only be called for messages sent to message queue.

Here[^] is a similar discussion.

Smile | :)

Navaneeth

How to use google | Ask smart questions

How get a content of field in html table? [modified]

MassimoPallara19-Jul-09 12:36

MassimoPallara

19-Jul-09 12:36

Hi ,

I'm very beginner in c#, i try to develope a application to retrieve data from html page , but
i have a confused idea on mode to retrive data from field on table on HTML page, anyone can help me? D'Oh! | :doh:

Thanks
Massimo

modified on Sunday, July 19, 2009 6:58 PM

Re: How get a content of field in html table?

Christian Graus19-Jul-09 14:49

Christian Graus

19-Jul-09 14:49

I assume you're teaching yourself for fun, because if you were in a class, you'd talk to your teacher, and if you were being paid for this, that would be immoral and borderline criminal.

This is a complex task for a 'very beginner'. Instead, buy a C# book and learn some basics first.

Christian Graus

Driven to the arms of OSX by Vista.

Read my blog to find out how I've worked around bugs in Microsoft tools and frameworks.

Image Brightness and Contrast change together

lexx_zone19-Jul-09 9:32

lexx_zone

19-Jul-09 9:32

Guys, I need help. Sorry for my English now,my brain just refused to work. I have an Image in PictureBox and have some filters to its process: contrast and brightness. Two trackbars controls filters changing values. When trackBar1_ValueChanged or trackBar2_ValueChanged events raises, function ApplyFilter() starts to work in different thread. When I using the brightness trackbar it changes the brightness of basic image. When I using the contrast trackbar it changes the contrast of image, But I don't know how to change the brightness and contrast together and show it in pictureBox component. For example I want to increase a brightness of an image. After that I want to increase a contrast but not of basic image, and what has turned out at brightness increase. After that I want to decrease a brightness to its basic level or lower. I don't know How to explain it clearly... If anybody have Vista you can see the same example in Vista PhotoGallery fix component. There are a lot of filters which you can apply to your image and change it back as you need. Here is a logic of my code...

Bitmap previewimage = null;
int brightness_value = 0;
int contrast_value = 0;

public PicPreviewForm(string filename) 
{
	previewimage = new Bitmap(Image.FromFile(filename));
	pictureBox1.Image = previewimage;
}
public void ApplyFilter()
{
      Bitmap bit;
      bit = new Bitmap(previewimage);
	
if(brightness had been changed)
{
	BrightnessFilter(bit, brightness_value);
}
If(contrast had been changed)
{
	ContrastFilter(bit, contrast_value);            
}
PictureBox.Image = bit;               
                
        
}

I could apply all filters every time when I changing some trackbar but it really decrease program performance..

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.