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4.5.1 Logical Operators and Short-circuit Control Forms
Name Resolution Rules
1
An
expression consisting of two
relations
connected by
and then or
or else (a
short-circuit control
form) shall resolve to be of some boolean type;
the
expected type for both
relations
is that same boolean type.
Static Semantics
2
The
following logical operators are predefined for every boolean type
T,
for every modular type
T, and for every one-dimensional array
type
T whose component type is a boolean type:
3
function "and"(Left, Right : T) return T
function "or" (Left, Right : T) return T
function "xor"(Left, Right : T) return T
4
For boolean types, the predefined logical operators
and, or, and xor perform the conventional operations
of conjunction, inclusive disjunction, and exclusive disjunction, respectively.
5
For modular types, the predefined logical operators
are defined on a bit-by-bit basis, using the binary representation of
the value of the operands to yield a binary representation for the result,
where zero represents False and one represents True. If this result is
outside the base range of the type, a final subtraction by the modulus
is performed to bring the result into the base range of the type.
6
The logical operators on arrays are performed
on a component-by-component basis on matching components (as for equality
-- see
4.5.2), using the predefined logical
operator for the component type. The bounds of the resulting array are
those of the left operand.
Dynamic Semantics
7
The short-circuit control
forms
and then and
or else deliver the same result as the
corresponding predefined
and and
or operators for boolean
types, except that the left operand is always evaluated first, and the
right operand is not evaluated if the value of the left operand determines
the result.
8
For the
logical operators on arrays, a check is made that for each component
of the left operand there is a matching component of the right operand,
and vice versa.
Also, a check
is made that each component of the result belongs to the component subtype.
The exception Constraint_Error is raised if either
of the above checks fails.
9
12 The
conventional meaning of the logical operators is given by the following
truth table:
10
A B (A and B) (A or B) (A xor B)
True True True True False
True False False True True
False True False True True
False False False False False
Examples
11
Examples of
logical operators:
12
Sunny or Warm
Filter(1 .. 10) and Filter(15 .. 24) -- see 3.6.1
13
Examples of short-circuit
control forms:
14
Next_Car.Owner /= null and then Next_Car.Owner.Age > 25 -- see 3.10.1
N = 0 or else A(N) = Hit_Value
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