Values, HACD.2: Types, Variables, Operators
02/10/2008
Values, Types, Variables, Operators
Hugh Darwen
[e-mail] www.dcs.warwick.ac.uk/~hugh
Anatomy of an Imperative
Example: Y := X + 1 ;
Denotes a variable An invocation (of +)
Denotes a value
CS252.HACD: Fundamentals of Relational Databases Section 2: Values, Types, Variables, Operators
Denotes current value of a variable An update operator
A read-only operator
X and 1 are arguments to the invocation of + Y and X+1 are arguments to the invocation of :=
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Important Distinctions
The following very important distinctions emerge from all this and should be firmly taken on board: Value versus variable Variable versus variable reference Update operator versus read-only operator Operator versus invocation Parameter versus argument Parameter subject to update versus parameter not subject to update
A Closer Look at an Operator (+)
Look its a relation! a 1 2 2 The predicate: a + b = c Attributes a and b can be considered as the parameters of +. Its also a function: no two tuples with the same a value also have the same b value, so, given an a and a b, we know the c.
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b 2 3 1
c 3 5 3
and so on (ad infinitum)
An Operator Definition
In Tutorial D: OPERATOR HIGHER_OF ( A INTEGER, B INTEGER ) RETURNS INTEGER ; IF A > B THEN RETURN A ; ELSE RETURN B ; END IF ; END OPERATOR ; So the invocation HIGHER_OF(2,3) = 3 Examples:
What Is a Type?
A type (= domain) is a named set of values.
WEEKDAY: { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday } INTEGER: { , -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, } Monday etc. and -7 etc. are literals. Every value of every type should be able to be denoted by some literal.
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What is a Type For?
It is for constraining the values permitted for some purpose. For example, constraining: the values that can be assigned to a variable the values that can be substituted for a parameter the values that an operator can yield when invoked the values that can appear for a given attribute of a relation The declared type (of the variable, parameter, operator or attribute) constrains its possible values to be of that type.
What is the Type of This?
StudentId S1 S1 S2 S3 S4 Name Anne Anne Boris Cindy Devinder CourseId C1 C2 C1 C3 C1
Perhaps RELATION { StudentId SID, Name NAME, CourseId CID } where SID is the declared type of StudentId , NAME that of Name, and CID that of CourseId.
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How to Write This as a Literal?
StudentId S1 S1 S2 S3 S4 See next slide Name Anne Anne Boris Cindy Devinder CourseId C1 C2 C1 C3 C1
A Relation Literal in Tutorial D
Try: RELATION { TUPLE { StudentId S1, CourseId C1, Name Anne }, TUPLE { StudentId S1, CourseId C2, Name Anne }, TUPLE { StudentId S2, CourseId C1, Name Boris }, TUPLE { StudentId S3, CourseId C3, Name Cindy }, TUPLE { StudentId S4, CourseId C1, Name Devinder } } But this assumes that S1, C1, and Boris are themselves valid literals. They arent in most languages, including Tutorial D.
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Literals for Student Ids, etc
Recall the declared of types the attributes: SID, NAME, CID. Suppose that values of type SID are represented by character strings (values of type CHAR). Suppose that character strings are denoted by text in quotes, like this: S1. Then a literal for the student identifier S1 might be: SID ( S1 ) SID ( S1 ) is an invocation of an operator called SID whose single parameter is of type CHAR. We call SID a selector, because it can be used to select any value of type SID.
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A Tuple Literal
From our first try: TUPLE { StudentId S1, CourseId C1, Name Anne } We can now write this as: TUPLE { StudentId SID ( S1 ) , CourseId CID ( C1 ) , Name NAME( Anne ) } and thats just fine. (See the lecture Notes for the relation literal.)
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Types and Representations
Consider the invocation SID ( S1 ), a literal of type SID. SID is an operator that, when invoked with a suitable character string, returns a value of type SID. Also, every value of type SID can be denoted by an invocation of operator SID. We call such an operator a selector (for values of the type in question). And the parameters (signature) of a selector we call a possible representation (possrep for short).
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A Type Definition for SID
In Tutorial D: TYPE SID POSSREP SID { C CHAR } CONSTRAINT constraint to specify exactly which strings are suitable ; The definition implies operator definitions: OPERATOR SID ( C CHAR ) RETURNS SID ; OPERATOR THE_C ( S SID ) RETURNS CHAR ; whereby, e.g., THE_C ( SID ( S1 ) ) = S1
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Type Constraint for SID
So, we can write the required type constraint something like this: CONSTRAINT LENGTH ( THE_C ( SID ) ) <= 5 AND LEFT ( THE_C ( SID ) , 1) = S AND DIGITS (SUBSTR ( THE_C (SID, 2 ))) Where: LEFT returns a specified leftmost portion of a string, SUBSTR returns the substring specified by a start position and length (defaulting to length of remainder), and DIGITS returns TRUE iff every character of the given string is a digit.
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What Is a Variable?
Here is a variable declaration in Tutorial D: VAR SN SID INIT SID ( S1 ) ;
initial value
imperative key word
variable name
declared type
So a variable has a name, a declared type, and a value. The value can change from time to time. The name and type cannot.
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Updating a Variable
A value is assigned to a variable by invoking an update operator. E.g, assignment (available on variables of all types): SN := SID ( S2 ) ; SN := SID ( LEFT ( THE_C (SN), 1 ) || 5 ) ; Additional update operators might be defined, invoked via CALL: CALL SET_DIGITS ( SN , 23 ) ; Pseudovariable assignment might be supported: THE_C ( SN ) := S2 ; SUBSTR (THE_C ( SN ), 2 ) := 23 ;
Important Distinctions Arising
You should now be able to distinguish clearly between: values and variables values and representations of values types and representations read-only operators and update operators operators and invocations parameters and arguments
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EXERCISES (see Notes)
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