Fortran increase dynamic array size in function

Question

I need a variable size array in Fortran. In C++ I would use vector. So I have a function like

integer function append(n, array, value)
  integer, pointer, d         


        

Answer 1

The answer by IRO-bot is the correct approach for Fortran 90. If you can limit yourself to compilers that support the Fortran 2003 MOVE_ALLOC intrinsic (included in gfortran since the 4.2 release), you can avoid one of the copies. That is, increasing the size of an array by a factor of 2 can be written as

allocate(tmp_arr(2*size(array)))
tmp_arr(1:size(array)) = array
deallocate(array)
move_alloc(tmp_arr, array)
! tmp_arr is now deallocated

Answer 2

Thank you a lot janneb.It was very helpful your comment.

Only a few change I have made was to omit the deallocate(array) . There was'nt any erro omitting this line in my code. This change is specially helpful if you need to put it into a loop and you don't allocated array before the loop. My specific case follows below (look that I don't allocate x_all before or into the loop):

begin program test 
  integer,allocatable::x_all(:),tmp_arr(:)
  integer,allocatable::x_tmp(:)
  integer::N
  allocate(x_tmp(2*N))
  (...)

  i=1
  do while(logical test)
    ...
    x_tmp(i)=some calculus
    i=i+1
    ...
  end do
  i=i-1
  allocate( tmp_arr( 1:(i+size(x_all) ) ) )
  tmp_arr(1:size(x_all))=x_all
  tmp_arr(size(x_all)+1:)=xtemp
  call MOVE_ALLOC(tmp_arr,x_all)  
  ...
end program

Answer 3

OK, the problem is that you cannot deallocate and re-allocate the array that you are assigning a function value to. You are correct about the cause of your problem (arguments passed by reference and not by value as it is in C). Since you deallocate the array inside the function body, the assignment to that array becomes invalid, leading to segfault. This is not a gfortran issue, tried it with ifort and pgf90, all of them report the same problem. This works for me:

PROGRAM dynamic_size
INTEGER,DIMENSION(:),ALLOCATABLE :: array

ALLOCATE(array(10))

array=(/1,2,5,7,4,3,6,5,6,7/)

WRITE(*,*)SIZE(array)
CALL resize_array
WRITE(*,*)size(array)

CONTAINS

SUBROUTINE resize_array
INTEGER,DIMENSION(:),ALLOCATABLE :: tmp_arr

ALLOCATE(tmp_arr(2*SIZE(array)))
tmp_arr(1:SIZE(array))=array
DEALLOCATE(array)
ALLOCATE(array(size(tmp_arr)))
array=tmp_arr

ENDSUBROUTINE resize_array

ENDPROGRAM dynamic_size