Understanding Newton's Method in C++

[Sumaya]

Homework Statement

why the loop not looping ?

Homework Equations

fun1=x^3+4^2-x-1
fun2=3x^2+8x-1

The Attempt at a Solution

#include "stdafx.h"
#include<iostream>
using namespace std;
float fun1(float);
float fun2(float);
void main()
{
float a;
cin>>a;

if (fun2(a)>0)

do
{
	a=a-(fun1(a)/fun2(a));
	cout<<a<<endl;

	
}
	while(fun2(a)<0.001);
	


}
		 
float fun1( float a)
{
	float y;
	y=a*a*a+4*a*a-a-1;
	return y;
}
float fun2( float b)
{
	float y;
	y=3*b*b+8*b-1;
	return y;
}

 

[Borek]

Try to trace your program watching values of variable a and values returned by fun1 and fun2. Is it doing what you hoped for, or is it doing something else?

 

[vivekrai]

while(fun2(a)<0.001);

Are you sure about this?

 

[GingerLee]

Keep two variables for iterations.
If the difference between them is < 0.001 then stop loop.

Something like this :

x1 = a - (fun1(a) / fun2(a));

do{
x0 = x1;
x1 = x0 - (fun1(x0) / fun2(x0));
d = x0 - x1;
}while(d > 0.001);

 

[TheoMcCloskey]

Also, check your (math in) function FUN2.

 

[Mark44]

Also, check your (math in) function FUN2.

I don't see anything wrong with it.

 

[TheoMcCloskey]

I don't see anything wrong with it.

You're CORRECT - I misread OP's function F(x)! fun2 code is correct for fun1 code.

 

[Mark44]

You're CORRECT - I misread OP's function F(x)! fun2 code is correct for fun1 code.

?

Maybe you're referring to this line in the OP, in the Relevant equations section:

fun1=x^3+4^2-x-1

There's a missing x. 4^2 should be 4*x^2.
The code has the right formula, though.

 

[Sumaya]

thank u all