How to increase array size on-the-fly in Fortran?

Question

My program is running though 3D array, labelling \'clusters\' that it finds and then doing some checks to see if any neighbouring clusters have a label higher than the curre

Answer 1

If you declare an array allocatable, you use deffered shape in the form real,

allocatable :: labelArray(:,:)

, or

real,dimension(:,:),allocatable :: labelArray

with number of double colons meaning rank (number of your indexes) of your array.

If the array is unallocated you use

 allocate(labelarray(shapeyouwant))

with the correct number of indexes. For example allocate(labelarray(2:3,-1:5)) for array with indexes 2 to 3 in demension 1 and -1 to 5 in dimension 2.

For change of dimension you have to deallocate the array first using

deallocate(labelArray)

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To reallocate an allocated array to a new shape you first need to allocate a new array with the new shape, copy the existing array to the new array and move the reference of the old array to the new array using move_alloc().

  call allocate(tmp(size_old+n_enlarge))
  tmp(1:size_old) = array(1:size_old)
  call move_alloc(tmp, array)

The old array is deallocated automatically when the new array reference is moved by move_alloc().

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Fortran 95 deallocates arrays automatically, if they fall out of scope (end of their subroutine for example).

Fortran 2008 has a nice feature of automatic allocation on assignment. If you say array1=array2 and array1 is not allocated, it is automatically allocated to have the correct shape.

It can also be used for re-allocation (see also Fortran array automatically growing when adding a value and How to add new element to dynamical array in Fortran 90)

 labelArray = [labelArray, new_element]