(Re)Allocatable Arrays

[Intel_C_Intel]

I've read many posts on this board to do with ALLOCATABLE arrays and doing things like replicating VB's ReDim Preserve by allocating a temporary array, copying the original, deallocating then reallocating the original and copying back again.

Now the clever people always suggest using a POINTER array to reduce the amount of copying required. Try as I might, and read and re-read those posts as I have, I just can't seem to grasp the concept behind these POINTER arrays.

Can anyone please explain it to me in thicko-moron-baby-steps? Preferably with a little example, detailing where things are stored and what points to what?

Thanks a lot.

[jim_dempsey]

A "pointer array" would be an array of pointers. However, I do not believe that this is what you want to use to implement the VBreallocate preserve. I believe what you want to use is a single"pointer to and array". There are many ways to implement this. I will provide a simple, non-optimal, but easy to understand example:

real, pointer :: MyArray(:) => null()

real, target, allocatable :: hidden1MyArray(:)

real, target, allocatable :: hidden2MyArray(:)

! Allocate - first time

! sanity check

if(associated(MyArray)) stop 1

if(allocated(hidden1MyArray)) stop 2

if(allocated(hidden2MyArray)) stop 3

! allocate to hidden allocatable array

allocate(hidden1MyArray(1:Count), STAT=iStat)

if(iStat .ne. 0) stop 4

! point to array

MyArray => hidden1MyArray

...

! Reallocate

! sanity check

if(.not. associated(MyArray)) stop 4

! see which hidden array is current

if(allocated(hidden1MyArray)) then

! 1 is current, 2 must be void

if(allocated(hidden2MyArray)) stop 5

allocate(hidden2MyArray(1:NewCount), STAT=iStat)

if(iStat .ne. 0) stop 6

! make copy (truncate if new array shorter)

hidden2MyArray = hidden1MyArray

! null fill if new array larger

if(NewCount .gt. size(hidden1MyArray)) hidden2MyArray(size(hidden1MyArray)+1:) = 0.

! point to new array

MyArray => hidden2MyArray

! delete old array

deallocate(hidden1MyArray)

else

allocate(hidden1MyArray(1:NewCount), STAT=iStat)

if(iStat .ne. 0) stop 8

hidden1MyArray = hidden2MyArray

if(NewCount .gt. size(hidden2MyArray)) hidden1MyArray(size(hidden2MyArray)+1:) = 0.

MyArray => hidden1MyArray

deallocate(hidden2MyArray)

endif

Notes, the above error tests are not fool proof as MyArray might point to something other than either of the two hidden arrays. I will leave this up to you to fill in the error testing.

Jim Dempsey