Increasing the precision of constants

By default, HP Fortran 90 evaluates all floating-point constants as single-precision. For example, the compiler treats following constant

3.14159265358979323846

as though you had specified:

3.1415927

Although the loss of precision might be acceptable when assigning to single-precision variables, it is might be less acceptable when assigning to double-precision variables or when using floating-point constants in expressions where the loss in precision might result in significant round-off differences.

	NOTE: HP Fortran 90 provides two ways to override the default precision of individual constants: the kind parameter and the exponent form. The kind parameter indicates the precision of floating-point constants: 4 for single-precision, 8 for double-precision, and 16 for quad-precision.

In the following example, the kind parameter _8 specifies that the constant is to be evaluated as double-precision:

3.14159265358979323846_8

To change the precision of all floating-point constants (except those having a kind parameter), you can use the +real_constant option. This option takes two forms, +real_constant=double and +real_constant=single, which specify (respectively) double-precision and single-precision for floating-point constants in the files compiled with this option. The +real_constant=single form is the default. Neither form of the option has any affect on constants that have the kind parameter.

To promote all floating-point constants in the source files x.f, y.f, and z.f, compile with the command line:

$ f90
+real_constant=double x.f y.f z.f

The +real_constant=single option specifies that all floating-point constants in a file are to be treated as single-precision (the default). The following command line specifies single-precision for all floating-point constants in the files a.f, b.f, and c.f:

$ f90
+real_constant=single a.f b.f c.f

Note that +real_constant=single does not demote constants that use either the kind parameter or the exponent form (for example, 4.0D0).

For information about increasing the precision of variables, see “Increasing default data sizes”. The HP Fortran 90 Programmer's Reference describes the syntax of the kind parameter and the exponent form and the +real_constant option. For detailed information about how floating-point arithmetic is implemented on HP 9000 computers and how floating-point behavior affects the programmer, refer to the HP-UX Floating-Point Guide.