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

int main()
{
    int a = 7,b;
    unsigned int c;
    printf("这是一个输出16进制的语句\n");
    printf("请输入一个420000000内的数字：");
    scanf_s("%d",&c);
    printf("0X");
    while (a >= 0);
    {
        b = (int)pow(16, a);
        b = c /b % 16;
        if (b < 10)
        printf("%d", b);
        else if (b == 10)
            printf("a");
        else if (b == 11)
            printf("b");
        else if (b == 12)
            printf("c");
        else if (b == 13)
            printf("d");
        else if (b == 14)
            printf("e");
        else if (b == 15)
            printf("f");
        else
            puts("NO！！！");
        --a;
        
    }
    return 0;
   


   

}

是scanf的安全版本，VS2015新版的，

终于知道了多了一个;号， while (a >= 0); 55555555555555555555555555555555555555555谢谢大神