#include <stdio.h>
int main (void)
{
    float a,b = 0;
    for (;;)
    {
        printf ("Please Enter a number");
        scanf ("%f",&a);
        
        if (a == 0 || a < 0)
        {
            break;
        }
        else
        {
            if (b > a)
            {
                b = a;
            }
            else
            {
                b = b;
            }
        }
        
    }
    printf ("The max number is : %f\n",b);
   
    return 0;
}

if (b > a)
            {
                b = a;
            }
改为
if( b < a )
    b = a;

谢谢呢，我的逻辑错了