thrice.c

《C语言精彩编程百例》一书源代码,很不错的说

/* 用弦截法求解方程的根 */
# include <stdio.h>
# include <math.h>

float f(float x)    
{
	float y;
	y = ((x-8.0)*x+12.0)*x - 30.0;
	return y;
}

float xpoint(float x1, float x2)    /* 定义函数xpoint，求出弦与x轴的焦点 */    
{
	float y;
	y = (x1*f(x2)-x2*f(x1)) / (f(x2)-f(x1));
	return y;
}

float root(float x1, float x2)    /* 定义函数root，求近似根 */
{
	float x, y, y1;
	y1 = f(x1);
	do
	{
		x = xpoint(x1, x2);
		y = f(x);
		if(y*y1 > 0)    /* f(x)和f(x1)同符号 */
		{
			y1 = y;
			x1 = x;
		}
		else
			x2 = x;
	} while(fabs(y) >= 0.0001);
	return x;
}

void main()    /* 主函数 */
{
	float x1, x2, f1, f2, x;
	do
	{
		printf("Please input x1, x2:\n");
		scanf("%f, %f", &x1, &x2);
		f1 = f(x1);
		f2 = f(x2);
	} while(f1*f2 > 0);
	x = root(x1, x2);
	printf("A root of equation is %9.6f\n", x);
}