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3.5 Double precision real problems

The addresses and the lengths of arrays are given in words, which poses a few problems with real double precision.

3.5.1 Addresses

Suppose that, using the usual notation (appendix A), M, XM, and DM represent the same super-array of type INTEGER, REAL, and DOUBLE PRECISION.

After allocating a dynamic array in double precision, utility READRE always returns an odd index IATAB.

The first element of this array is referenced by M(IATAB), XM(IATAB), or DM((IATAB+1)/2).

Element, I, of this array is referenced by M(IATAB-1+I), XM(IATAB-1+I), or DM((IATAB-1)/2+I).

This last form is rather heavy. Recall that a more elegant solution consists of passing M(IATAB) as a parameter (section 3.4).

3.5.2 Lengths

The programmer must consider allocating an array of length LTAB = NBELEM*NBREMO, where NBELEM is the number of elements in the array (which appears in the Fortran declarations) and NBREMO is the number of words occupied by a real double precision scalar (in general 2).

Furthermore, certain compilers allow us to inhibit type DOUBLE PRECISION by replacing it automatically by type REAL (especially on "super-computers" with 64 bit words).

The instruction:

CALL READRE(5, TAB, IATAB, LTAB, M, ICODE)

must then be replaced by:

CALL READRE(2, TAB, IATAB, LTAB/2, M, ICODE)

The following sequence avoids the modifications:

NTYTAB = IINFO('REEL2')

CALL TYPMOT(NTYTAB, NBREMO)

CALL READRE(NTYTAB, TAB, IATAB, NBELEM*NBREMO, M, ICODE)

where:

IINFO('REEL2')

returns the type actually used: 5 (DOUBLE PRECISION) on a classical machine, 2 ( REAL) on a "super-computer",

TYPMOT

returns NBREMO, number of words occupied by a scalar of given type NTYTAB, and

NBELEM

is the number of elements in the array.

This sequence is longer than the simple instruction seen previously, but more readable and portable!