/in/foo.c: In function 'main':
/in/foo.c:9:22: warning: statement with no effect [-Wunused-value]
   for (i = 0; i++; i <= n)
                    ~~^~~~
/in/foo.c:14:7: warning: 'sum' is used uninitialized in this function [-Wuninitialized]
   sum = sum * a;
   ~~~~^~~~~~~~~