Defines a named constant.
Syntax
A type declaration statement with the PARAMETER
attribute is:
type, attrib-list :: cname1 = cexpr1[, cname2 = cexpr2]... |
- type
is a valid type specification (INTEGER,
REAL, LOGICAL,
CHARACTER, TYPE
(name),
etc.).
- attrib-list
is a comma-separated list of attributes including
PARAMETER and
optionally those attributes compatible with it, namely:
Specifying the SAVE
attribute in a PARAMETER
statement has no effect.
- cname
is the name that will represent the constant.
- cexpr
is an initialization expression that evaluates to
the constant represented by cname. In
the case of an array constant, cexpr
must be an array constructor. In the case of a derived type constant,
cexpr must be a structure constructor.
The syntax of the PARAMETER
statement is:
PARAMETER (cname1 = cexpr1 [, cname2 = cexpr2]...) |
Description
The PARAMETER
statement associates a symbolic name with a constant. A symbolic
name defined in a PARAMETER
statement is known as a named constant. A
named constant must not become defined more than once in a program
unit. Once defined, it can be used only as a named constant. This
means that a named constant cannot be assigned a value like a variable.
When the PARAMETER
attribute is used, the value of the named constant must be provided
by the initialization part of the statement in which the PARAMETER
attribute appears.
The type of a named constant is determined by the implicit
typing rules, unless its type is specified by a type declaration
statement prior to its first appearance in a PARAMETER
statement or by a type declaration statement that includes PARAMETER
as one of its attributes. If a PARAMETER
statement declares and implicitly types a named constant, the named
constant may appear in a subsequent type declaration or IMPLICIT
statement, but only to confirm the type of the named constant.
When the type of the symbolic name and the constant do not
agree, the value of the named constant is assigned in accordance
with assignment statement type-conversion rules, as given in Table 5-5 “Conversion of variable=expression”.
The following rules apply to type agreement between the constant
and the symbolic name:
If cname
is of numeric type, cexpr must be an
arithmetic constant expression.
If cname is of type character,
the corresponding cexpr must be a character
constant expression.
If cname is of type logical,
the corresponding cexpr may be either
an arithmetic or logical constant expression.
Any symbolic name of a constant that appears in cexpr
must have been defined previously in the same or a different PARAMETER
statement in the same program unit. For example, the expression
in the second PARAMETER
statement below is built from the expression in the first PARAMETER
statement, and is legal:
PARAMETER (limit = 1000)
PARAMETER (limit_plus_1 = limit + 1) |
The logical operators (.EQ.,
.NE., .LT.,
.LE., .GT.,
and .GE.), as
well as the following intrinsic functions, can appear in the PARAMETER
statement:
If these intrinsic functions are used in a PARAMETER
statement, their arguments must be constants.
If the named constant is of type character and its length
is not specified, the length must be specified in a type declaration
statement or IMPLICIT
statement prior to the first appearance of the named constant. Its
type and/or length must not be changed by subsequent statements,
including IMPLICIT
statements. If a symbolic name of type CHARACTER*(*)
is defined in a PARAMETER
statement, its length becomes the length of the expression assigned
to it.
If the named constant is an array, the bounds must be explicit
and determined by an initialization expression.
Once such a symbolic name is defined, that name can appear
in any subsequent statement of the defining program unit as a constant
in an expression or DATA
statement.
Examples
! PARAMETER used in a type declaration statement as an attribute
REAL, DIMENSION(4), PARAMETER :: const = &
(/1.2, 1.45, 0.9, 24.3/)
INTEGER year
! PARAMETER used as a statement
PARAMETER year = 1996
! Type declaration statement declaring a derived-type constant
TYPE (postal_info), PARAMETER :: package = &
postal_info (9.5, (/10.0, 5.5, 2.25/) ) |
Related concepts
For information about the type declaration statement, see
“Type declaration for intrinsic types”.
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