Fortran subroutine returning wrong values

Question

Hey I\'m working on a fortran program and have ran across an odd problem. When I try to output some values of an array directly before calling a specific subroutine I get th

Answer 1

I figured it out. VP was allocated as

VP(0:DIM,0:NPTMAX) 

in the main program and

VP(0:DIM-1,0:NPTMAX) 

in the subroutine! This caused the error.

Answer 2

Manually, setting explicit-shape arrays a(0:N-1,0:M) is extremely error prone. I suggest using assumed-shape arrays a(:,:) to pass arguments. Also, when you pass arrays as arguments to subroutines, the upper and lower bounds of the actual argument are not maintained in the call unless the lower bound is 1. However, when passing pointers, the lower and upper bounds are maintained. For instance,

program main

  use, intrinsic :: iso_fortran_env, only: &
       stdout => OUTPUT_UNIT, &
       compiler_version, &
       compiler_options

  ! Explicit typing only
  implicit none

  real               :: foo(10,10)
  real, target       :: bar(0:9,0:9)
  real, pointer      :: ptr(:,:) => null()

  call peek_assumed_shape(foo, 'peek_assumed_shape: foo(10,10)')
  call peek_assumed_shape(bar, 'peek_assumed_shape: bar(0:9,0:9)')

  ptr => bar
  call peek_pointer(ptr, 'peek_pointer: ptr => bar')

  ptr(42:,42:) => bar
  call peek_pointer(ptr, 'peek_pointer: ptr(42:,42:) => bar')

  nullify( ptr )

  write (stdout, '(/4a/)') &
       'This file was compiled using compiler version ', compiler_version(), &
       ' and compiler options ', compiler_options()

contains


  subroutine peek_assumed_shape(array, description)
    ! Calling arguments
    real,              intent (in) :: array(:,:)
    character (len=*), intent (in) :: description

    write (stdout, '(/a)') description
    write (stdout, *) 'dim=1 ', lbound(array, dim=1), ubound(array,dim=1)
    write (stdout, *) 'dim=2 ', lbound(array, dim=2), ubound(array,dim=2)


  end subroutine peek_assumed_shape

  subroutine peek_pointer(array, description)
    ! Calling arguments
    real, pointer,     intent (in) :: array(:,:)
    character (len=*), intent (in) :: description

    if (associated(array)) then
       write (stdout, '(/a)')  description
       write (stdout, *) 'dim=1 ', lbound(array, dim=1), ubound(array,dim=1)
       write (stdout, *) 'dim=2 ', lbound(array, dim=2), ubound(array,dim=2)
    end if

  end subroutine peek_pointer

end program main

returns the following

peek_assumed_shape: foo(10,10)
 dim=1            1          10
 dim=2            1          10

peek_assumed_shape: bar(0:9,0:9)
 dim=1            1          10
 dim=2            1          10

peek_pointer: ptr => bar
 dim=1            0           9
 dim=2            0           9

peek_pointer: ptr(42:,42:) => bar
 dim=1           42          51
 dim=2           42          51

This file was compiled using compiler version GCC version 5.4.0 20160609 and compiler options -mtune=generic -march=x86-64 -O3 -Wall -std=f2008ts