foo.cc: In function 'int main()':
foo.cc:17:6: error: 'i' was not declared in this scope
   17 |  for(i=100;i<=999;i++)
      |      ^
foo.cc:16:6: warning: unused variable 'a' [-Wunused-variable]
   16 |  int a,x,y,z,b;
      |      ^

[Hydro](https://hydro.ac)提供评测服务