HP OpenVMS Systems Documentation

HP Fortran for OpenVMS
Language Reference Manual

When operators are defined for functions, the functions can then be referenced as defined operations.

The operators are defined by using a generic interface block specifying OPERATOR, followed by the defined operator (in parentheses).

A defined operation is not an intrinsic operation. However, you can use a defined operation to extend the meaning of an intrinsic operator.

For defined unary operations, the function must contain one argument. For defined binary operations, the function must contain two arguments.

Interpretation of the operation is provided by the function that defines the operation.

A Fortran 95/90 defined operator can contain up to 31 letters, and is enclosed in periods (.). Its name cannot be the same name as any of the following:

• The intrinsic operators (.NOT., .AND., .OR., .XOR., .EQV., .NEQV., .EQ., .NE., .GT., .GE., .LT., and .LE.)
• The logical literal constants (.TRUE. or .FALSE.).

An intrinsic operator can be followed by a defined unary operator.

The result of a defined operation can have any type. The type of the result (and its value) must be specified by the defining function.

The following examples show expressions containing defined operators:

.COMPLEMENT. A
X .PLUS. Y .PLUS. Z
M * .MINUS. N

For More Information:

• On defining generic operators, see Section 8.9.4.
• On operator precedence, see Section 4.1.6.

Table 4-1 shows the precedence of all intrinsic and defined operators:

A constant expression contains intrinsic operations and parts that are all constants. An initialization expression is a constant expression that is evaluated when a program is compiled. A specification expression is a scalar, integer expression that is restricted to declarations of array bounds and character lengths.

Initialization and specification expressions can appear in specification statements, with some restrictions.

4.1.7.1 Initialization Expressions

An initialization expression must evaluate at compile time to a constant. It is used to specify an initial value for an entity.

In an initialization expression, each operation is intrinsic and each operand is one of the following:

• A constant or subobject of a constant
• An array constructor where each element and the bounds and strides of each implied-do, are expressions whose primaries are initialization expressions
• A structure constructor whose components are initialization expressions
• An elemental intrinsic function reference of type integer or character, whose arguments are initialization expressions of type integer or character
• A reference to one of the following inquiry functions:

  BIT_SIZE	MINEXPONENT
  DIGITS	PRECISION
  EPSILON	RADIX
  HUGE	RANGE
  ILEN	SHAPE
  KIND	SIZE
  LBOUND	TINY
  LEN	UBOUND
  MAXEXPONENT

  
  Each function argument must be one of the following:
  • An initialization expression
  • A variable whose kind type parameter and bounds are not assumed or defined by an ALLOCATE statement, pointer assignment, or an expression that is not an initialization expression
• A reference to one of the following transformational functions (each argument must be an initialization expression):

  REPEAT	SELECTED_REAL_KIND
  RESHAPE	TRANSFER
  SELECTED_INT_KIND	TRIM

• A reference to the transformational function NULL
• An implied-do variable within an array constructor, where the bounds and strides of the corresponding implied-do are initialization expressions
• Another initialization expression enclosed in parentheses

Each subscript, section subscript, and substring starting and ending point must be an initialization expression.

If an initialization expression invokes an inquiry function for a type parameter or an array bound of an object, the type parameter or array bound must be specified in a prior specification statement (or to the left of the inquiry function in the same statement).

In a specification expression, the number of arguments for a function reference is limited to 255.

Examples

The following examples show valid and invalid initialization (constant) expressions:

For More Information:

• On array constructors, see Section 3.5.2.4.
• On structure constructors, see Section 3.3.4.
• On intrinsic functions, see Chapter 9.

A specification expression is a restricted expression that is of type integer and has a scalar value. This type of expression appears only in the declaration of array bounds and character lengths.

In a restricted expression, each operation is intrinsic and each operand is one of the following:

• A constant or subobject of a constant
• A variable that is one of the following:
  • A dummy argument that does not have the OPTIONAL or INTENT (OUT) attribute (or the subobject of such a variable)
  • In a common block (or the subobject of such a variable)
  • Made accessible by use or host association (or the subobject of such a variable)
• A structure constructor whose components are restricted expressions
• An implied-do variable within an array constructor, where the bounds and strides of the corresponding implied-do are restricted expressions
• A reference to one of the following inquiry functions:

  BIT_SIZE	NWORKERS
  DIGITS	PRECISION
  EPSILON	PROCESSORS_SHAPE
  HUGE	RADIX
  ILEN	RANGE
  KIND	SHAPE
  LBOUND	SIZE
  LEN	SIZEOF
  MAXEXPONENT	TINY
  MINEXPONENT	UBOUND
  NUMBER_OF_PROCESSORS

  
  Each function argument must be one of the following:
  • A restricted expression
  • A variable whose properties inquired about are not dependent on the upper bound of the last dimension of an assumed-size array, are not defined by an expression that is a restricted expression, or are not definable by an ALLOCATE or pointer assignment statement.
• A reference to any other intrinsic function where each argument is a restricted expression.
• A reference to a specification function (see below) where each argument is a restricted expression
• An array constructor where each element and the bounds and strides of each implied-do, are expressions whose primaries are restricted expressions
• Another restricted expression enclosed in parentheses

Each subscript, section subscript, and substring starting and ending point must be a restricted expression.

Specification functions can be used in specification expressions to indicate the attributes of data objects. A specification function is a pure function. It cannot have a dummy procedure argument or be any of the following:

• An intrinsic function
• An internal function
• A statement function
• Defined as RECURSIVE

A variable in a specification expression must have its type and type parameters (if any) specified in one of the following ways:

• By a previous declaration in the same scoping unit
• By the implicit typing rules currently in effect for the scoping unit
• By host or use association

If a variable in a specification expression is typed by the implicit typing rules, its appearance in any subsequent type declaration statement must confirm the implied type and type parameters.

If a specification expression invokes an inquiry function for a type parameter or an array bound of an object, the type parameter or array bound must be specified in a prior specification statement (or to the left of the inquiry function in the same statement).

Examples

The following shows valid specification expressions:

MAX(I) + J          ! I and J are scalar integer variables
UBOUND(ARRAY_B,20)  ! ARRAY_B is an assumed-shape dummy array

For More Information:

• On array constructors, see Section 3.5.2.4.
• On implicit typing rules, see Section 3.5.1.2.
• On structure constructors, see Section 3.3.4.
• On intrinsic functions, see Chapter 9.
• On use and host association, see Section 15.5.1.2.
• On pure procedures, see Section 8.5.1.2.

An assignment statement causes variables to be defined or redefined. This section describes the following kinds of assignment statements: intrinsic, defined, pointer, masked array (WHERE), and element array (FORALL).

The ASSIGN statement assigns a label to an integer variable. It is discussed in Section 7.2.3.

4.2.1 Intrinsic Assignments

Intrinsic assignment is used to assign a value to a nonpointer variable. In the case of pointers, intrinsic assignment is used to assign a value to the target associated with the pointer variable. The value assigned to the variable (or target) is determined by evaluation of the expression to the right of the equal sign.

An intrinsic assignment statement takes the following form:

variable

Is the name of a scalar or array of intrinsic or derived type (with no defined assignment). The array cannot be an assumed-size array, and neither the scalar nor the array can be declared with the PARAMETER or INTENT(IN) attribute.

expression

Is of intrinsic type or the same derived type as variable. Its shape must conform with variable. If necessary, it is converted to the same type and kind as variable.

Rules and Behavior

Before a value is assigned to the variable, the expression part of the assignment statement and any expressions within the variable are evaluated. No definition of expressions in the variable can affect or be affected by the evaluation of the expression part of the assignment statement.

If the variable is a pointer, it must be associated with a definable target. The shape of the target and expression must conform and their type and kind parameters must match.

The following sections discuss numeric, logical, character, derived-type, and array intrinsic assignment.

For More Information:

• On subroutine subprograms that define assignment, see Section 8.9.5.
• On arrays, see Section 3.5.2.
• On pointers, see Section 5.15.
• On derived data types, see Section 3.3.

For numeric assignment statements, the variable and expression must be numeric type.

The expression must yield a value that conforms to the range requirements of the variable. For example, a real expression that produces a value greater than 32767 is invalid if the entity on the left of the equal sign is an INTEGER(2) variable.

Significance can be lost if an INTEGER(4) value, which can exactly represent values of approximately the range --2*10**9 to +2*10**9, is converted to REAL(4) (including the real part of a complex constant), which is accurate to only about seven digits.

If the variable has the same data type as that of the expression on the right, the statement assigns the value directly. If the data types are different, the value of the expression is converted to the data type of the variable before it is assigned.

Table 4-2 summarizes the data conversion rules for numeric assignment statements.

For more information on the referenced intrinsic functions, see Chapter 9.

Examples

The following examples demonstrate valid and invalid numeric assignment statements: