Square Root of a number

This is a discussion on Square Root of a number within the C++ Programming forums, part of the General Programming Boards category; Because you are converting your double value to a float, so as soon as you get close enough to 3*3 ...

  1. #16
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    Because you are converting your double value to a float, so as soon as you get close enough to 3*3 is roughly 9, it will retrun 9-9 == 0 from your abs function [which by the way is a bad name, I'd call it absdiff or some such].

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  2. #17
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    Actually he seems to be converting doubles to int's - even worse.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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  3. #18
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    Quote Originally Posted by anon View Post
    Actually he seems to be converting doubles to int's - even worse.
    Yes, that's actually what I meant to type - somehow the brain-to-finger interface obviously is a bit faulty sometimes... ;-)

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  4. #19
    Registered User NeonBlack's Avatar
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    It's a bad idea to use the same names as standard library functions. There are functions named sqrt() and abs() in <cmath>. It was pointed out earlier, but it's worth restating that it's a VERY bad idea to use == or != with floating point numbers as it will produce inconsistent results.
    I copied it from the last program in which I passed a parameter, which would have been pre-1989 I guess. - esbo

  5. #20
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    still has warnings

  6. #21
    C++ Witch laserlight's Avatar
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    What is your current code, and what are the current warnings?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #22
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    Got it working

    Code:
    #include <iostream>
    using namespace std;
    
    
    double absDiff(double a,double b)
    {
    	if ((a-b) < 0)
    	{
    		return (a-b)*-1;
    	}
    	else
    	{
    		return (a-b);
    	}
    }
    double sqrtNum(double x)
    {
    double U = x;
    double L = 0;
    double guess = x/2;
    	if (x == 0 || x == 1)
    	{
    		return x;
    	}
    	else 
    		while (absDiff((guess*guess),x) > .000000000001)
    		if (guess*guess > x)
    		{
    				U = guess;
    				guess = ((U - L)/2) + L;
    		}
    		else 
    		{
    				L = guess;
    				guess = ((U - L)/2) + L;
    		}		
    		return guess;
    }
    int main()
    {
    	cout<<"Root ";
    	cout<<sqrt(5)<<endl;
    }
    Any improvements needed?

  8. #23
    and the Hat of Guessing tabstop's Avatar
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    You may want to do something special, in case someone decides to type sqrtNum(-1). (And notice, that you forgot to change your call in main to your function as opposed to the library function.)

  9. #24
    C++ Witch laserlight's Avatar
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    Any improvements needed?
    You could use abs() from <cmath> instead of implementing abdDiff(), but if your goal is to avoid using stuff from <cmath> entirely then change (a-b)*-1 to (b-a). You could also pull guess = ((U - L)/2) + L; to after the if and else blocks. Oh, and your indentation could be more consistent.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  10. #25
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    What do you mean by pulling guess = ((U - L)/2) + L?

  11. #26
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by freddyvorhees View Post
    What do you mean by pulling guess = ((U - L)/2) + L?
    You do it in the if-part; you do it in the else-part. So why not just do it, outside of the if statement? (Other than "I don't want to fiddle with all the braces".)

  12. #27
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    ahhhhhh

  13. #28
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    Maybe like this?
    Code:
    	if (x == 0 || x == 1)
    	{
    		return x;
    	}
    	else 
    		while (absDiff((guess*guess),x) > .0000001)
    		{
    		if (guess*guess > x)
    			U = guess;
    		else 
    			L = guess;
    		}
    		guess = ((U - L)/2) + L;
    		return guess;

  14. #29
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    Code:
    	if (x == 0 || x == 1)
    as stated several times, don't compare 'double' values with integer values. The condition is not correct.

  15. #30
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    Oh, and your initial guesses implicitly assume that 0 < sqrt(x) < x; this is not true if 0< x < 1. For instance, sqrt(0.25) = 0.5, and you won't find those numbers.

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