用C语言写了一个十进制转十六进制的程序，运行不了？？求大神！

#include<stdio.h>
void main()
{
int a,b,c;
scanf("%d",a);
for(a;b!=0;b=c)
{
b=a%16;
c=a/16;
switch(b)
{
case 10: printf("%d",'A');break;
case 11: printf("%d",'B');break;
case 12: printf("%d",'C');break;
case 13: printf("%d",'D');break;
case 14: printf("%d",'E');break;
case 15: printf("%d",'F');break;
default:printf("%d",b);
}
}
}