///////////////////////////////////////////////////
//	§5  方程求根
//
//	程序5.1 二分法求方程根


#include	<stdio.h>
#include	<conio.h>
#include	<math.h>

#include "expressi.cpp"

#define PRECISION	0.000001 

// 二分求根, f(x) = FxString, 區間[a,b]
float RootByBisection(char FxString[], float a, float b )
{
	float x1,x2;

	if( CreateFx(FxString) ) return(0.0); // 初始化表達式, 如果表達式非法則返回
	
	x1 = a;
	x2 = b;

	while( fabs( x2 - x1 ) > PRECISION )
	{
		if( f(x1) * f( (x1 + x2 ) / 2 ) < 0 )
			x2 = ( x1 + x2 ) / 2;
		else
			x1 = ( x1 + x2 ) / 2;
	}

	return( x1 );
}

void main()
{
	float x1,x2;
	char FxString[200];

	printf( "\nInput function with varable x,x1,x2 : " );
	scanf("%s %f %f", FxString, &x1, &x2 );

	printf( "\nRoot x=%f", RootByBisection( FxString, x1, x2 ) );
}

/*
	運行實例：

Input function with varable x,x1,x2 : pow(x,3)-x-1 1 2

Root x=1.324718
*/