PL/SQL numeric value or error: confusion with functions and procedures (Oracle PL/SQL) - oracle

Question

I previously asked a question in stackoverflow regarding a certain function that I was having a hard time with and I found the answer when I saw the RETURN statement missing.
Now, I am dealing with a procedure that calls for the input text letters to be converted to UPPER if in the LOWER case and vice versa.
e.g IF I put in 'AbC' it should return 'aBc'
So far this is my code
CREATE OR REPLACE PROCEDURE Opposite_Case (p_string IN VARCHAR2)
IS
var_string VARCHAR2(20);
var_contain VARCHAR2(20);
i NUMBER;
BEGIN
var_string:=substr(Opposite_Case.p_string,i,1);
var_contain:= var_string || var_contain;
FOR i in 1.. length(var_string)
LOOP
BEGIN
IF var_string IN ('ABCDEFGHIJKLMNOPQRSTUVWXYZ') THEN
SELECT LOWER(var_string) INTO var_contain FROM dual;
ELSE
SELECT UPPER(var_string) INTO var_contain FROM dual;
END IF;
END;
END LOOP;
END;
/
BUT the following error is returned:
EXECUTE Opposite_Case('AbC')
begin Opposite_Case('AbC'); end;
ORA-06502: PL/SQL: numeric or value error
ORA-06512: at "SYS.OPPOSITE_CASE", line 10
ORA-06512: at line 1
Thanks in advance.
PS. This is just my 3rd day with a PL/SQL language so please bear with me.
EDIT: I got it to work finally thanks to #Satya's help. Now I get to convert them like it should but how do I output my selects in one line though?
I'm almost there. Appreciate the help a lot from this community!!
What I have so far:
SQL> EXECUTE Opposite_Case('AbC')
A
b
C
PL/SQL procedure successfully completed

Answer 1

You should investigate the TRANSLATE function. To use it to switch the case of the characters in your string you'd do something like the following:
SELECT TRANSLATE('AbC',
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz',
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
FROM DUAL;
which should return 'aBc'.
SQLFiddle here
Share and enjoy.

Answer 2

Finally found the answer! Thanks guys! (without TRANSLATE though)!
CREATE OR REPLACE PROCEDURE Opposite_Case (p_string IN VARCHAR2)
IS
var_string VARCHAR2(20);
var_contain VARCHAR2(20);
i NUMBER;
BEGIN
FOR i in 1.. length(p_string)
LOOP
BEGIN
var_string:=substr(p_string,i,1);
var_contain:= var_string;
IF var_string IN ('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z') THEN
SELECT LOWER(var_string) INTO var_contain FROM dual;
DBMS_OUTPUT.PUT(var_contain);
ELSE
SELECT UPPER(var_string) INTO var_contain FROM dual;
DBMS_OUTPUT.PUT(var_contain);
END IF;
END;
END LOOP;
dbms_output.new_line;
END;
/