请牛人帮忙分析此函数中弦截法的逻辑和运行程序时为什么printf("Please input x1, x2:\n");会出两遍

Question

请牛人帮忙分析此函数中弦截法的逻辑和运行程序时为什么主函数中的printf("Please input x1, x2:\n");会出两遍!!!

/* 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);
getch();
}

Answer 1

你的printf放在循环里了当然会循环出现了，我没用过弦接法，不过我感觉你可能应该把
printf("Please input x1, x2:\n");
scanf("%f, %f", &x1, &x2);
放在do的前面