HP Fortran Programmer's Reference (September 2007)

Expressions and assignment

Expressions

Chapter 492

The Standard allows the compiler to generate code that evaluates an expression by any

sequence that produces a result mathematically equivalent to the sequence implied by the

statement. This laxity permits code optimization, including (for example) the reordering of

expressions and the promotion of common subexpressions.

Because the order of evaluation is not deﬁned by the Standard, a function reference within an

expression may not modify any of the other operands within the same expression. For

example, fun(x)+x is indeterminate if the reference to fun modiﬁes the value of the

argument x.

Special forms of expression

Certain language constructs allow only restricted forms of expressions. For example, the

value speciﬁed for a named constant in a PARAMETER statement may be deﬁned by an

expression, but it must be possible to evaluate the expression at compile-time. This means

that the expression must not contain any operands that depend on program execution for

their value. To take another example, a bound of a dummy array argument may be speciﬁed

as an expression, but it must be possible to evaluate this expression on entry to the

subprogram.

There are special restrictions imposed on operands and operators that may appear in an

expression, depending on whether the expression is one of the following:

• Constant expressions

• Initialization expressions

• Speciﬁcation expressions

The following sections describe the special forms of expression.

Constant expressions

A constant expression is either a constant or an expression containing only intrinsic

operators and constant operands. This restriction also applies to any clearly deﬁned part of a

constant—for example, a substring with constant start and end points, or an array or

structure constructor. A constant expression may include references to intrinsic functions that

can be evaluated at compile-time. A constant expression may appear in any context in which

any expression may appear.

The following are examples of constant expressions:

123 ! an integer literal

”Hello ” // ” World” ! a character constant expression

3.0_single ! a real literal constant where single is