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  #1  
Old 12-May-2006, 14:19
earachefl earachefl is offline
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area of n-sided regular polygon

OK, this might be more of a math question, but here goes. According to Wikipedia, the formula for the area of a regular n-sided polygon is

Area = n(t to the 2nd power) / 4tan(PI/n)

where n is the number of sides and t is the length of each side.

Implemented in my code:
CPP / C++ / C Code:
float prism::computeBaseArea()
{
    baseArea = ( (n * pow(t, 2)) / (4 * (tan(PI / n))) );
    return baseArea;
}

When I test it, I get bad values:

Quote:
The attributes for the prism are as follows:
Number of sides: 4
Length of each side: 1
Base area: -1.9998

The attributes for the prism are as follows:
Number of sides: 5
Length of each side: 1
Base area: -1.69474e+38

The attributes for the prism are as follows:
Number of sides: 6
Length of each side: 1
Base area: 0
Any ideas on what I'm doing wrong?
  #2  
Old 12-May-2006, 15:12
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WaltP WaltP is offline
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Re: area of n-sided regular polygon

Check the types of your variables. Don't forget the problems between int and float values during division...

Where did you get this formula? I can't find it, and is doesn't seem to work.
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  #3  
Old 12-May-2006, 15:24
earachefl earachefl is offline
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Re: area of n-sided regular polygon

Quote:
Originally Posted by WaltP

Where did you get this formula? I can't find it, and is doesn't seem to work.
http://en.wikipedia.org/wiki/Regular_polygon#Area
  #4  
Old 12-May-2006, 15:37
earachefl earachefl is offline
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Re: area of n-sided regular polygon

Quote:
Originally Posted by WaltP
Check the types of your variables. Don't forget the problems between int and float values during division...

I figured out the problem. I had multiple constructors (default - no arguments, 2nd - one argument, 3rd - 3 arguments, 4 - 5 arguments). I instantiated an object using the third constructor, which passed number of sides, length of sides, and height as arguments. Where I screwed up was not including a function call (private member function) to compute the area of the base from that constructor.

I'd been meaning to ask about defining multiple constructors - my textbook says to, but Deitel says not to. Unfortunately had to return Deitel to the library as I had too many renewals on it. Is there more than one school of thought on defining multiple constructors?

BTW,

the formula does work, now that I've called it correctly.....
  #5  
Old 12-May-2006, 15:44
davekw7x davekw7x is offline
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Re: area of n-sided regular polygon

Quote:
Originally Posted by earachefl
When I test it, I get bad values:


Any ideas on what I'm doing wrong?

I can think of several ways that you could get bad values

1. You aren't feeding the formula with what you think you are.
2. The formula is incorrect (or you have implemented it incorrectly).
3. The calculation is correct, but, somehow, the correct value is not getting back to where it is used.
4. You aren't calling the function that you think you are.
5. A combination of some or all of the above.

When you have a program that runs but gives you the wrong answer and you can't see how that could happen, then make the program tell you what it is working on. Print out values of variables and intermediate expressions for the steps of the calculation. Run the program for something simple (that you can easily calculate the correct answer by hand).



CPP / C++ / C Code:
float prism::computeBaseArea()
{
    cout << "In computeBaseArea():"<< endl;
    cout << "   n                 = " << n << endl;
    cout << "   t                 = " << t << endl;
    cout << "   pow(t, 2)         = " << pow(t, 2) << endl;
    cout << "   PI                = " << PI << endl;
    cout << "   PI / n            = " << PI / n << endl;
    cout << "   tan(PI / n)       = " << tan(PI / n) << endl;
    cout << "   4 * (tan(PI / n)) = " << 4 * tan(PI / n) << endl;
    cout << "   n * pow(t, 2)     = " << n * pow(t, 2) << endl;

    baseArea = ( (n * pow(t, 2)) / (4 * (tan(PI / n))) );

     cout << "   baseArea          = " << baseArea << endl << endl;

    return baseArea;
}

Regards,

Dave
  #6  
Old 12-May-2006, 15:47
earachefl earachefl is offline
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Re: area of n-sided regular polygon

Quote:
Originally Posted by davekw7x
I can think of several ways that you could get bad values

1. You aren't feeding the formula with what you think you are.
2. The formula is incorrect (or you have implemented it incorrectly).
3. The calculation is correct, but, somehow, the correct value is not getting back to where it is used.
4. You aren't calling the function that you think you are.
5. A combination of some or all of the above.


Dave

See #4 above. I hadn't actually called the function to compute the base area from the constructor, thus was getting garbage values. D'oh!!!
  #7  
Old 12-May-2006, 15:53
davekw7x davekw7x is offline
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Re: area of n-sided regular polygon

Quote:
Originally Posted by earachefl
See #4 above.
Actually, I saw your good news after I posted my suggestion (and I am really, really glad that you found the problem--- congratulations). I thought about deleting my post, as I sometimes do when I see that the advice is redundant. But then I thought I would leave it there in case other people might want to see some clues about debugging things like this.

Regards,

Dave
 
 
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