foo.cc: In function ‘int main()’:
foo.cc:8:9: warning: statement has no effect [-Wunused-value]
    8 |     for(y;y<=xx;y++)
      |         ^
foo.cc:23:16: error: ‘o’ was not declared in this scope
   23 |         return o
      |                ^
foo.cc:23:17: error: expected ‘;’ before ‘}’ token
   23 |         return o
      |                 ^
      |                 ;
   24 | }
      | ~                
foo.cc:5:15: warning: unused variable ‘num’ [-Wunused-variable]
    5 |     int x,y=2,num=0,xx,a,b;
      |               ^~~
foo.cc:5:24: warning: unused variable ‘a’ [-Wunused-variable]
    5 |     int x,y=2,num=0,xx,a,b;
      |                        ^
foo.cc:5:26: warning: unused variable ‘b’ [-Wunused-variable]
    5 |     int x,y=2,num=0,xx,a,b;
      |                          ^