Arrays of pointers

arrays pointers fortran fortran90
milancurcic picture milancurcic · Jan 17, 2012 · Viewed 7.2k times · Source

I am trying to implement an array of pointers, so that I can loop over the elements. However I am not sure how to do this correctly:

TYPE(domain),POINTER              :: d01,d02,d03
TYPE(domain),DIMENSION(:),POINTER :: dom
...

dom(1) => d01
dom(2) => d02
dom(3) => d03
...

and then:

...
IF(ASSOCIATED(dom(2),d02))THEN
...

The compiler (pgf90 10.6-0 64-bit target on x86-64 Linux -tp istanbul-64) gives me this error message:

 PGF90-S-0074-Illegal number or type of arguments to associated - keyword argument pointer (test_ptr.f90: 10)
  0 inform,   0 warnings,   1 severes, 0 fatal for MAIN

As far as I understand, there is something wrong about how I subset an array of pointers. Both dom(2) and d02 are rank 0 (scalar pointers). What is the correct way to implement this?

Thanks.

Answer

Jonathan Dursi picture Jonathan Dursi · Jan 17, 2012

Yeah, pointer arrays are funny in Fortran.

The problem is that this:

TYPE(domain),DIMENSION(:),POINTER :: dom

does not define an array of pointers, as you might think, but a pointer to an array. There's a number of cool things you can do with these things in Fortran - pointing to slices of large arrays, even with strides - but it is definitely a pointer to an array, not an array of pointers.

The only way to get arrays of pointers in Fortran is to define a type:

type domainptr
  type(domain), pointer :: p
end type domainptr

type(domainptr), dimension(3) :: dom

dom(1)%p => d01
dom(2)%p => d02
dom(3)%p => d03

etc. As far as I can tell, the only real reason you have to do this in Fortran is syntax. I'd love to see this fixed in some later version of the standard.

Related questions

Return array in a function

I have an array int arr[5] that is passed to a function fillarr(int arr[]): int fillarr(int arr[]) { for(...); return arr; } How can I return that array? How will I use it, say I returned a pointer how am …

Read More
Passing a 2D array to a C++ function

I have a function which I want to take, as a parameter, a 2D array of variable size. So far I have this: void myFunction(double** myArray){ myArray[x][y] = 5; etc... } And I have declared an array elsewhere in my …

Read More
How to find the 'sizeof' (a pointer pointing to an array)?

First off, here is some code: int main() { int days[] = {1,2,3,4,5}; int *ptr = days; printf("%u\n", sizeof(days)); printf("%u\n", sizeof(ptr)); return 0; } Is there a way to find out the size of the array that ptr is pointing …

Read More