critical impact:

RANDOM_SEED - some behavioral differences (CrCh324)

Problem description

The behavior of RANDOM_SEED is different on Tru64 UNIX and HP-UX.

RANDOM_SEED([SIZE] [, PUT] [, GET]) changes or queries the seed (starting point) for the pseudorandom number generator used by RANDOM_NUMBER. It requires that the size of the array be at least as long as the value N returned by RANDOM_SEED with parameter SIZE.

On Tru64 UNIX this number is 2. The following example executes without error with Compaq Fortran, but will generate an error with HP Fortran.

Identifiers

RANDOM_SEED

Old behavior

PROGRAM TESTRAN
   REAL*4 RSHORT
   REAL*8 RLONG
   INTEGER :: SEED_SIZE, I, J
   INTEGER, DIMENSION(10) :: MYSEED
   SEED_SIZE = 2
   MYSEED(1) = 1641671953
   MYSEED(2) = 407586285
   PRINT *,MYSEED(1),MYSEED(2)
   CALL RANDOM_SEED(PUT = MYSEED(1:SEED_SIZE))
   CALL RANDOM_NUMBER(RSHORT)
   CALL RANDOM_SEED(PUT = MYSEED(1:SEED_SIZE))
   CALL RANDOM_NUMBER(RLONG)
   PRINT *,RSHORT, RLONG
   IF(RSHORT .gt. RLONG) PRINT *,'ERROR'
END PROGRAM TESTRAN

See also

• Background information on Fortran 90 compiler impacts
• RANDOM_SEED(3F)

Solution description

On HP-UX, this number is generally or at least 17. To be safe, programs should not rely upon the seed being any particular size, but should query for the seed size instead.

A runtime error will be generated if SIZE less than 17 is provided to the PUT parameter. In such a case, make changes to the code as suggested in the following example:

New behavior

PROGRAM TESTRAN
   REAL*4 RSHORT
   REAL*8 RLONG
   INTEGER :: SEED_SIZE, I
   INTEGER :: MYSEED1, MYSEED2
   MYSEED1 = 1641671953
   MYSEED2 = 407586285
   PRINT *,MYSEED1,MYSEED2
   CALL RANDOM_SEED(SIZE = I)
   CALL SET_SEED(I,MYSEED1,MYSEED2)
   CALL RANDOM_NUMBER(RSHORT)
   CALL SET_SEED(I,MYSEED1,MYSEED2)
   CALL RANDOM_NUMBER(RLONG)
   PRINT *,RSHORT, RLONG
   IF(RSHORT .gt. RLONG) PRINT *,'ERROR'
END PROGRAM TESTRAN
SUBROUTINE SET_SEED(SEED_SIZE, MYSEED1,MYSEED2)
  INTEGER :: SEED_SIZE
  INTEGER :: MYSEED1,MYSEED2
  INTEGER, DIMENSION(SEED_SIZE) :: RAN_SEED
  RAN_SEED = 0
  RAN_SEED(1) = MYSEED1
  RAN_SEED(2) = MYSEED2
  PRINT *,RAN_SEED
  CALL RANDOM_SEED(PUT=RAN_SEED)
END SUBROUTINE

See also

• HP Fortran Programmer's Reference

Problem summary