Pointer to multiple non-contiguous array slices

Question

Given a 1D array, I want to create a pointer to the array boundaries, say the first 2 and last 2 elements of the array. I have investigated a bit about this but the only non-contiguous pointer examples I have come across are those defined as strided access.

Given the following test case

program test
    implicit none

    integer, target :: a(10)
    integer, pointer :: p(:)
    integer :: i

    do i=1,10
        a(i)=i
    end do

    p => a(:)   ! Point all
    p => a(::2) ! Point every 2 elments

    print*, p
end program test

how can I point to separated array slices of the original array using a single pointer? So to be clear, in this example when printing p I would like to get 1 2 9 10.

Vladimir F Героям слава · Accepted Answer · 2017-11-06T12:00:57.297

0

You can't do a completely general pointer. It is the point of arrays, that they are well structured and can be accessed in a sequenced manner. That's why they have to be strided.

In your case you still have some structure, so you could point to those pairs using a 2D array.

~~Here was an example.with equivalence. However equivalence cannot be used with target for an unknown reason in standard Fortran.~~

One could use a 2D pointer p as:

integer, pointer :: p(:,:), b(:,:)

b(1:2,1:5) => a

p => b(:,1::4)

but, again, 2D pointer p is required here as far as I can see.

You could also use a C-style cast for the first re-map, but is is probably useless:

type(c_ptr) ptr

ptr = c_loc(a)
call c_f_pointer(ptr, b, [2,5])

p =>  b(:,1::4)

I can't find a way with a 1D pointer.

edited Nov 06 '17 at 12:00

answered Nov 06 '17 at 07:27

Vladimir F Героям слава

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What is the `equivalence` statement doing here if you don't mind explaining? – b-fg Nov 06 '17 at 07:42
Accessing `a` as two dimensional array. It will be also possible with pointers. – Vladimir F Героям слава Nov 06 '17 at 07:43
So basically I can reshape the original array in a way that when the access is strided it gives me the slices I want? – b-fg Nov 06 '17 at 07:45
Yes, that is the idea. – Vladimir F Героям слава Nov 06 '17 at 07:49
Ok. By the way, the `equivalence` statement is throwing me an error. Could you please show something I can compile? – b-fg Nov 06 '17 at 07:52
I am not testing it for the real code, just for the MCVE I posted. Please avoid using bold font when making assumptions, it does not help the discussion. Still, thanks for your help. – b-fg Nov 06 '17 at 07:56
`EQUIVALENCE attribute conflicts with TARGET attribute in ‘a’ at (1)`. I have solved it using `reshape` and an strided access. However `b(2,5) => a` is no right, I had to use `c = reshape(a,[2,5]); b => c(:,1::4)`. – b-fg Nov 06 '17 at 08:07
OK, didn't have an idea about this conflict, don't know why it exists. Reshape looks suspicious. You are copying the array to `c`, not making a pointer. – Vladimir F Героям слава Nov 06 '17 at 08:08
The easiest thing is to start with a 2D array. – Vladimir F Героям слава Nov 06 '17 at 08:09
@b-fg Check the last edit for another way. I don't believe reshape will help here. – Vladimir F Героям слава Nov 06 '17 at 08:28
The last example works, however you have a typo in `loc`, I used `c_loc` instead. Reshape also did work for me before anyways. Why would not help? – b-fg Nov 06 '17 at 08:55
Because reshape creates a copy, it does not let you modify the original array. It is not a pointer. – Vladimir F Героям слава Nov 06 '17 at 08:56
I updated the answer as I finally got to a computer with a compiler. – Vladimir F Героям слава Nov 06 '17 at 12:02

Pointer to multiple non-contiguous array slices

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