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Using and Porting GNU Fortran

8.11.5 REAL() and AIMAG() of Complex

The GNU Fortran language disallows REAL(expr) and AIMAG(expr), where expr is any COMPLEX type other than COMPLEX(KIND=1), except when they are used in the following way:

REAL(REAL(expr)) REAL(AIMAG(expr))

The above forms explicitly specify that the desired effect is to convert the real or imaginary part of expr, which might be some REAL type other than REAL(KIND=1), to type REAL(KIND=1), and have that serve as the value of the expression.

The GNU Fortran language offers clearly named intrinsics to extract the real and imaginary parts of a complex entity without any conversion:

REALPART(expr) IMAGPART(expr)

To express the above using typical extended FORTRAN 77, use the following constructs (when expr is COMPLEX(KIND=2)):

DBLE(expr) DIMAG(expr)

The FORTRAN 77 language offers no way to explicitly specify the real and imaginary parts of a complex expression of arbitrary type, apparently as a result of requiring support for only one COMPLEX type (COMPLEX(KIND=1)). The concepts of converting an expression to type REAL(KIND=1) and of extracting the real part of a complex expression were thus "smooshed" by FORTRAN 77 into a single intrinsic, since they happened to have the exact same effect in that language (due to having only one COMPLEX type).

Note: When `-ff90' is in effect, g77 treats `REAL(expr)', where expr is of type COMPLEX, as `REALPART(expr)', whereas with `-fugly-complex -fno-f90' in effect, it is treated as `REAL(REALPART(expr))'.

See section 9.9.3 Ugly Complex Part Extraction, for more information.