进制问题16进制转换为10进制

Question

include<stdio.h>
int main()
{
int n,m,r;
scanf("%d",&n);
while(n!=0)
{r=n%16;
n=n/16;
m=m*16+r;}
while(m!=0)
{ r=m%16;
m=m/16;
if(r>=0&&r<=9)
printf("%d",r);
else
{switch(r)
{
case 10:printf("A");break;
case 11:printf("B");break;
case 13:printf("C");break;
case 14:printf("D");break;
case 15:printf("E");break;

  }
}

}
return 0;
}
请问各位大神前面的那个两个循环我看不懂，有谁能教我一下