Do "set serveroutput on" first.
SQL> declare
2 val pls_integer := 2;
3 res pls_integer := 1;
4 pwr pls_integer := 5;
5 begin
6 for i in 1 .. pwr loop
7 res := val * res;
8 end loop;
9 dbms_output.put_line(res);
10 end;
11 /
PL/SQL procedure successfully completed.
SQL> set serveroutput on
SQL> declare
2 val pls_integer := 2;
3 res pls_integer := 1;
4 pwr pls_integer := 5;
5 begin
6 for i in 1 .. pwr loop
7 res := val * res;
8 end loop;
9 dbms_output.put_line(res);
10 end;
11 /
32
PL/SQL procedure successfully completed.