谢谢 大家
这样学习真的感觉很容易啊

# include <stdio.h>
int main(int argc, char* argv[])
{
     int num;
     printf("Please input a integer:");
     scanf("%d",&num);
     for(int i=1; num>=10||num<=-10;i++) num/=10;
     printf("The length of integer is: %d\n ",i);
     return 0;
}