直接给x1,x2赋值和用scanf输入有什么不同吗？

我在学C，刚编的不知道哪错了，下面的程序我把   
        double x1,x2,x;
    scanf("%f %f",&x1,&x2);

换成    double x1=0.0，x2=10.0，x；就对了为什么啊？？？？？


#include<stdio.h>
#include<math.h>
double y(double x);
double x(double x1,double x2);
double root(double x1,double x2);


double y(double x)
{
    double yj;
    yj=x*x*x-5*x*x+16*x-80;
    return yj;
}
double x(double x1,double x2)
{
    double xj;
    xj=(x1*y(x2)-x2*y(x1))/(y(x2)-y(x1));
    return xj;
}
double root(double x1,double x2)
{

    double xx,yy;
    xx=x1;
    while(fabs(y(xx))>0.0001)
    {
        xx=x(x1,x2);
        yy=y(xx);
        if(yy*y(x1)>0)
            x1=xx;
        else
            x2=xx;
    }
    return xx;
}
main()
{
    double x1,x2,x;
    scanf("%f %f",&x1,&x2);
    while(1)
    {
        if(y(x1)*y(x2)<0)
        {x=root(x1,x2);break;}
        else
        {
            printf("重新输入：");
            scanf("%f%f",&x1,&x2);
        }
    }
    printf("%6.3f\n",x);

}