insert into new column values generated from trigger

Question

I want to create a trigger which generates an 18 digit code based on the value of several other columns in the table.

R-MU-WEST-RJS-9697 So basically my last 4 digit code is the STORE_CODE based on which I want to compare and insert that 18 digit code against it the column named as CODE_18DIGIT

Below is the select statement that generates the code

SELECT    'R-'
  || CASE
         WHEN INSTR (TRIM (r.state), ' ') > 1
         THEN
                SUBSTR (r.state, 1, 1)
             || SUBSTR (r.state, INSTR (r.state, ' ') + 1, 1)
         ELSE
             UPPER (SUBSTR (r.state, 1, 2))
     END
  || '-'
  || UPPER (SUBSTR (r.zone_name, 1, 4))
  || '-'
  || r.format_code
  || '-'
  || SUBSTR (r.store_code, 1, 4)
FROM tbl_rrsoc_store_info  r
WHERE r.ISACTIVE = 'Y';

and the table description of tbl_rrsoc_store_info is below

Name                        Null     Type           
--------------------------- -------- -------------- 
RRSOC_ID                    NOT NULL NUMBER         
STORE_CODE                  NOT NULL NVARCHAR2(55)  
STATE                                NVARCHAR2(55)  
CITY                                 NVARCHAR2(55)
CODE_18DIGIT                         NVARCHAR2(18)