Square Root of a number

**matsp** · 07-30-2008

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].

--
Mats

**anon** · 07-30-2008

Actually he seems to be converting doubles to int's - even worse.

**matsp** · 07-30-2008

Originally Posted by anon

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... ;-)

--
Mats

**NeonBlack** · 07-30-2008

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.

**freddyvorhees** · 07-30-2008

still has warnings

**laserlight** · 07-30-2008

What is your current code, and what are the current warnings?

**freddyvorhees** · 07-30-2008

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?

**tabstop** · 07-30-2008

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.)

**laserlight** · 07-30-2008

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.

**freddyvorhees** · 07-30-2008

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

**tabstop** · 07-30-2008

Originally Posted by freddyvorhees

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".)

**freddyvorhees** · 07-30-2008

ahhhhhh

**freddyvorhees** · 07-30-2008

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;

**MarkZWEERS** · 07-30-2008

Code:

	if (x == 0 || x == 1)

as stated several times, don't compare 'double' values with integer values. The condition is not correct.

**tabstop** · 07-30-2008

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.