Re: Codd's Information Principle

From: paul c <>
Date: Fri, 30 Oct 2009 20:28:40 GMT
Message-ID: <Y%HGm.50892$PH1.14668_at_edtnps82>

paul c wrote:

> wrote:

>> On Oct 30, 11:41 am, paul c <> wrote:
>>> Yes, I mean assignment to a (programming) variable. I don't know of any
>>> logical operator called 'assignment'.
>> You don't seem understand assignment.
>> Assignment has nothing to do with the relationship between relations
>> and logic or the relational model per se. The semantics of assignment
>> in a dbms language is exactly the same as it is in every programming
>> language.
>> You evaluate expressions using the current values of variables. You
>> can assign the value of an expression to a variable. This gives a new
>> overall state. The relationship between the values of one or more
>> variables in the post-(multiple-)assignment state and the expressions
>> that were used to express those values in the pre-assignment state
>> is simply (and tautologically) that the values of the assigned
>> expressions evaluated using the pre-assignment variable values are
>> the same as the values of the expressions consisting of just the
>> variable names evaluated using the post-assignment variable values
>> (ie the post-assignment values of the variables).
>> It doesn't matter what the types of variables are.
>> If a language allows assignment to an expression then the language
>> designer has to say what that means. Presumably the
>> post-assignment values of the variables in the assigned-to expressions
>> must be such that when they are evaluated in the post-assignment
>> state then their values are the same as the values of the assigned-
>> from
>> expressions evaluated in the pre-assignment state. If the language
>> processor cannot determine a single choice for the post-assignment
>> values of variables then the language designer has to give some sort
>> of
>> policy. But again, the particular assigned-from expressions have no
>> other relationship with the new state than that when evaluated in the
>> old state they give the values of the variables in the new state. The
>> particular assigned-from expressions don't matter, just their values.
>> philip
> That's adding more notions, such as 'state' and 'before' and 'after', 
> 'assignment to an expression' et al, all artifacts of certain physical 
> programming attitudes if you ask me.  I'm more interested in subtracting 
> notions as far as possible.

Another example is assigning to a variable twice. Various complications go away if a variable can be assigned to only once. (Is there a name for such a variable? In the old days it might have been called a macro.) Received on Fri Oct 30 2009 - 21:28:40 CET

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