Don't understand what you're trying to do, sorry. You need the code fields value in another field for searching? What is the field type of the field you're trying to search?

You can usually accomplish that by using CB Auto Actions to update a text field then you can search the text field. Given this is strictly a decimal number I recommend using a Decimal Number field type though. The below auto action can be used to keep the field in sync.

Global
Triggers: onAfterUserRegistration, onAfterNewUser, onAfterUpdateUser, onAfterUserUpdate
Type: Field
User: Automatic
Access: Everybody
Action
Field: DECIMAL_NUMBER_FIELD_HERE
Operator: Set
Value: [FIELD_NAME]

Change FIELD_NAME to whatever the name is for your code field. Now when their profile is updated or when a user registers it'll set the code fields value into your decimal number field.

If you need it to store $index then have your code field return $index. Code fields are just PHP. They have no storage so make as many of them as you need to. So for example create a second one that just returns $index.