Help Files: Fortran77: Converting to Double Precision

     HELP                      June 15, 1985             F77/TO_DOUBLE



                  Converting Programs to Double Precision

     On VAXs, single precision provides about 6-7 digits  of  accuracy
     while  double  precision provides about 15-16 digits of accuracy.
     Many mathematical subroutine libraries  assume  double  precision
     arguments.

     To convert a program from single to double precision:


     1.   Add 'implicit double precision (a-h,o-z)' as the first  line
          of  the  main  program  (second line if you have a 'program'
          statement) and as the second line of all  other  subprograms
          (subroutines, functions, and block data subprograms).


     2.   Change all 'real' or 'real*4' declarations to 'double preci-
          sion'.


     3.   Change ALL single precision floating point constants to dou-
          ble  precision, either by adding 'd0' or changing an 'e' ex-
          ponent term to 'd', e.g.:
                          1.2    becomes 1.2d0
                          3.2e-5 becomes 3.2d-5


     4.   Change 'complex' or 'complex*8' declarations to 'double com-
          plex'.


     5.   Formats do not need to be changed.


     6.   Change single precision intrinsic functions to double.   Be-
          cause  of  the  generic function names in f77, the only ones
          you need to actually change are:

                  single version:         double or generic version:
                     ifix()                       int()
                     sngl()                       dble()
                     real()                       dble()
                     float()                      dble()
                     amax1()                      max()
                     amin1()                      min()
                     amod()                       mod()
                     alog()                       log()
                     alog10()                     log10()


     7.   Change complex intrinsic functions to double complex.

                  complex version:        double complex or generic version:
                     cmplx()                      dcmplx()
                     aimag()                      imag()
                     csqrt()                      sqrt()
                     cexp()                       exp()
                     clog()                       log()
                     ccos()                       cos()
                     csin()                       sin()
                     cabs()                       abs()


     8.   If you are passing functions as arguments to  other  subrou-
          tines,  you  need  to  change all function names to specific
          functions.  For example:

                          x1 = sqrt(x2)
                          z1 = abs(z2)

          works fine when x1 and x2 are double and z1 and z2 are  dou-
          ble complex.  However, if the computation is being done in a
          subroutine,

                          external sqrt, abs
                          call subr( x1, x2, sqrt, z1, z2, abs )

          they must be changed to:

                          external dsqrt, cdabs
                          call subr( x1, x2, dsqrt, z1, z2, cdabs )


     9.   Check equivalences common blocks, and dynamic memory alloca-
          tion.  Often these implicitly assume that floating point and
          integer values use the same amount of storage.  For example,
          if one subroutine declares:

                          common /abc/ key1,vec(5),ival,ans(5)

          and another declares:

                          common /abc/ dummy(6), ival, ans(5)

          then the two instances of 'ival' match.  If now  you  change
          'dummy',  'ans',  and  'vec' from single precision to double
          precision, the  two  instances  of  'ival'  will  no  longer
          correspond.