/in/foo.c: In function 'main':
/in/foo.c:11:2: error: unknown type name 'T'
  T T[100];
  ^
/in/foo.c:11:2: note: use 'struct' keyword to refer to the type
/in/foo.c:15:19: error: request for member 'n' in something not a structure or union
   scanf("%d",&T[i].n);
                   ^
/in/foo.c:16:19: error: request for member 'x' in something not a structure or union
   scanf("%d",&T[i].x);
                   ^
/in/foo.c:17:21: error: request for member 'n' in something not a structure or union
   for(int j=0;j<T[i].n;j++)
                     ^
/in/foo.c:18:20: error: request for member 'a' in something not a structure or union
    scanf("%d",&T[i].a[i]);
                    ^
/in/foo.c:11:4: warning: variable 'T' set but not used [-Wunused-but-set-variable]
  T T[100];
    ^