foo.cc: In function 'int main()':
foo.cc:54:6: error: 'i' was not declared in this scope
  for(i=0; i<a; i++)
      ^
foo.cc:73:6: error: 'i' was not declared in this scope
  for(i=0; i<a; i++)
      ^
foo.cc:92:6: error: 'i' was not declared in this scope
  for(i=0; i<a; i++)
      ^
foo.cc:111:6: error: 'i' was not declared in this scope
  for(i=0; i<a; i++)
      ^

自豪的采用 HydroJudge 进行评测(github.com/hydro-dev/HydroJudge)