如何在 Fortran 中初始化一个大数组？

Question

我有一个 Fortran 函数，我想在编译时初始化一个大数组。下面是一个简化的工作示例，其中fill_coefficients 中的参数coeff 的大小已大大减小。

当coeff 很大时如何编写类似的代码，而不超过maximum of 255 continuation lines 或maximum of 132 characters per line？这里的fill_coefficients 应该真的是PURE，这可能导致无法在运行时从文件中读取一次coeff，然后存储结果。

文件“main.f03”：

    PROGRAM main
        USE coefficients
        IMPLICIT NONE

        REAL(dp), ALLOCATABLE, DIMENSION(:,:) :: matrix

        CALL fill_coefficients(matrix,2)

        PRINT *, "The first row of 'matrix':"
        PRINT *, matrix(1,:)
    END PROGRAM main

文件“coefficients.f03”：

    MODULE coefficients
        USE iso_fortran_env
        IMPLICIT NONE

        INTEGER, PARAMETER :: dp = REAL64

    CONTAINS
        PURE SUBROUTINE fill_coefficients(my_coefficients, n)
            IMPLICIT NONE
            REAL(dp), ALLOCATABLE, DIMENSION(:,:), INTENT(OUT) :: my_coefficients
            INTEGER, INTENT(IN) :: n

            ! The size of the following array would be roughly 200 x 200 = 40.000.
            REAL(dp), DIMENSION(3,3), PARAMETER :: coeff = &
                RESHAPE ( &
                [ + 10.6770782520313112108115239655957106_dp, &
                - 854.166260162504896864921917247656850_dp, &
                - 85.4166260162504896864921917247656850_dp, &
                + 16250.5130995916556628551394756366716_dp, &
                + 6747.91345528378868523288314625648912_dp, &
                + 106.770782520313112108115239655957106_dp, &
                - 123256.191341449456617608232658836883_dp, &
                - 8328.12103658442274443298869316465429_dp, &
                + 500381.272281447399894682070647642979_dp ], &
                [3,3] )

            IF (ALLOCATED(my_coefficients)) DEALLOCATE(my_coefficients)
            ALLOCATE(my_coefficients(n,n))

            my_coefficients = coeff(1:n,1:n)
        END SUBROUTINE fill_coefficients
    END MODULE coefficients

输出：

The first row of 'matrix': 10.677078252031311 16250.513099591655

Answer 1

从维护的角度来看（也许正如 cmets 中所建议的那样），我会将数据读入 单独的非纯子例程中的模块变量，该子例程在程序启动时调用一次。 fill_coefficients 然后成为该模块变量的简单赋值，并且仍然可以是 PURE。

MODULE coefficients
  IMPLICIT NONE
  ...
  ! Could be PUBLIC, PROTECTED, then you could directly 
  ! assign from it and dispense with fill_coefficients 
  ! altogether.
  REAL(dp), PRIVATE :: coeff(200,200)
CONTAINS
  SUBROUTINE init
    INTEGER :: unit 
    OPEN( NEWUNIT=unit,  &
          FILE='lots-of-numbers.bin',  &
          FORM='UNFORMATTED',  &
!         ACCESS='STREAM',  &    ! Maybe - depending on how you write it.
          STATUS='OLD' )
    READ (unit) coeff
    CLOSE(unit)
  END SUBROUTINE init

  PURE SUBROUTINE fill_coefficients(my_coefficients, n)
    ! implicit none already in force due to the statement in 
    ! the specification part of the host module.
    ! IMPLICIT NONE    
    REAL(dp), ALLOCATABLE, DIMENSION(:,:), INTENT(OUT) :: my_coefficients
    INTEGER, INTENT(IN) :: n

    ! This test is redundant - my_coefficients is INTENT(OUT) so 
    ! it must be not allocated at this point.
    ! IF (ALLOCATED(my_coefficients)) DEALLOCATE(my_coefficients)

    ! This allocate statement is redundant - allocation will 
    ! happen automatically under F2003 with the assignment.
    ! ALLOCATE(my_coefficients(n,n))

    my_coefficients = coeff(1:n,1:n)
  END SUBROUTINE fill_coefficients
END MODULE coefficients

如果您必须将coeff 作为编译时参数，则将其组装成源代码可管理的块——也许是逐列。每个声明的限制是行长 (132) 和续行数 (255)。

REAL(dp), PARAMETER :: column_1(200) = [  &
     + 10.6770782520313112108115239655957106_dp, &
     - 854.166260162504896864921917247656850_dp, &
     - 85.4166260162504896864921917247656850_dp, &
     ... ]
REAL(dp), PARAMETER :: column_2(200) = [ ... ]
...
REAL(dp), PARAMETER :: column_200(200) = [ ... ]

REAL(dp), PARAMETER :: coeff(200,200) = RESHAPE( [  &
       column_1, column_2, ..., column_200 ],  &
     SHAPE=[200,200] )

用 PARAMETER 声明的东西被命名为常量。从概念上讲，这些仅在编译时存在 - 根据您对命名常量所做的操作，编译器可能会或可能不会在可执行映像中为常量留出存储空间。

大的命名常量可能会导致编译器在编译文件时出现问题。