PL/SQL :: Write Dynamic SQL To Query Unknown Number Of Columns

I need to write a dynamic SQL in PL SQL to query an unknown number of columns. Let me take a simple example query here:

SELECT FIRST_NAME, LAST_NAME FROM VENDOR_CONTACTS

If I have known the number of columns, e.g. querying two columns: "FIRST_NAME" and "LAST_NAME", I can write a DYNAMIC SQL based on the template in table 8-2 of URL....

DECLARE
stmt_str varchar2(200);
cur_hdl int;
rows_processed int;
FIRST_NAME varchar2(200);
LAST_NAME varchar2(200);
BEGIN
cur_hdl := dbms_sql.open_cursor; -- open cursor
stmt_str := 'SELECT FIRST_NAME, LAST_NAME FROM VENDOR_CONTACTS';
[code]....

However, if I wish to write a dynamical sql to query these two columns for a more general purpose (which should meet the requirement to query different number of columns, e.g. three columns, FIRST_NAME, LAST_NAME, BIRTHDAY instead of two columns FIRST_NAME and LAST_NAME). To do this I first try to query the same two columns but using a different method, following URL.....My code for the same query has error, and I cannot solve it.

DECLARE
stmt_str varchar2(200);
cur_hdl int;
rows_processed int;
l_columns dbms_sql.desc_tab;
l_dummy NUMBER;
l_value NUMBER;
[code]....