Separately in the context of those coding in anger with Fortran and coming to this Discourse for information and guidance, what will be useful is to realize
- a need to dynamically grow/shrink an array will be helped by containerizing it suitably, perhaps in a derived type;
- the growth or shrinkage should follow a scheme suitable for the need at hand; a default scheme is usually to double/halve the size of the object in the container but there may be other better schemes depending on the program and the processor.
So then the approach should be to study the options vis-a-vis the needs while paying careful attention to the instrumentation around what is one is measuring and how so. The code in the original post has issues in this aspect as pointed out upthread.
A quick go-by can be as follows where the scheme to double the size each time is considered. To make it somewhat more interesting the option with RESHAPE intrinsic is also thrown in. One can then see the performance variation with the option and the shape and relate that to one’s need in actual code.
Click to see
integer, parameter :: WP = selected_real_kind( p=12 )
character(len=*), parameter :: fmth = "(g0,t10,g0,t30,g0,t50,g0,t70,g0)"
character(len=*), parameter :: fmtg = "(g0,t10,g0,t30,g0.3,t50,g0.3,t70,g0)"
integer, parameter :: nstart = 100000
integer, parameter :: nruns = 12
integer :: i, n(nruns)
real(WP) :: t1, t2
real(WP), allocatable :: a(:),xran(:)
allocate (xran(nstart*(2**nruns)))
call random_number( xran )
n = [( nstart*2**(i-1), i=1,nruns )]
blk1: block
a = [real(WP) :: ]
print *, "Method: Array constructor"
print fmth, "i", "Array Size", "Time (sec)", "Max|a-xran|", "a(n/2)"
do i = 1, nruns
call my_cpu_time(t1)
a = [ a, xran(size(a)+1:n(i)) ]
call my_cpu_time(t2)
print fmtg, i, size(a), (t2-t1), maxval(abs(a-xran)), a(size(a)/2)
end do
end block blk1
blk2: block
a = [real(WP) :: ]
print *, "Method: RESHAPE intrinsic"
print fmth, "i", "Array Size", "Time (sec)", "Max|a-xran|", "a(n/2)"
do i = 1, nruns
call my_cpu_time(t1)
a = reshape( a, shape=[n(i)], pad=xran(size(a)+1:n(i)) )
call my_cpu_time(t2)
print fmtg, i, size(a), (t2-t1), maxval(abs(a-xran)), a(size(a)/2)
end do
end block blk2
blk3: block
a = [real(WP) :: ]
print *, "Method: MOVE_ALLOC"
print fmth, "i", "Array Size", "Time (sec)", "Max|a-xran|", "a(n/2)"
do i = 1, nruns
block
real(WP), allocatable :: tmp(:)
call my_cpu_time(t1)
allocate( tmp(n(i)) )
tmp(1:size(a)) = a
tmp(size(a)+1:) = xran(size(a)+1:size(tmp))
call move_alloc( from=tmp, to=a )
call my_cpu_time(t2)
end block
print fmtg, i, size(a), (t2-t1), maxval(abs(a-xran)), a(size(a)/2)
end do
end block blk3
contains
subroutine my_cpu_time( time )
use, intrinsic :: iso_fortran_env, only : I8 => int64
! Argument list
real(WP), intent(inout) :: time
! Local variables
integer(I8) :: tick
integer(I8) :: rate
call system_clock (tick, rate)
time = real(tick, kind=kind(time) ) / real(rate, kind=kind(time) )
return
end subroutine my_cpu_time
end
- Expected program behavior
C:\Temp>gfortran -O3 p.f90 -o p.exe
C:\Temp>p.exe
Method: Array constructor
i Array Size Time (sec) Max|a-xran| a(n/2)
1 100000 0.157E-2 0.00 0.14632783894497925
2 200000 0.216E-2 0.00 0.93604658738074942
3 400000 0.383E-2 0.00 0.38816074699824510E-2
4 800000 0.780E-2 0.00 0.48353192216229768
5 1600000 0.154E-1 0.00 0.17707306405838441
6 3200000 0.316E-1 0.00 0.34672021796550934
7 6400000 0.619E-1 0.00 0.49127175384201527E-2
8 12800000 0.123 0.00 0.41832146544600479
9 25600000 0.274 0.00 0.34490797395681683
10 51200000 0.499 0.00 0.43429002560932473
11 102400000 1.01 0.00 0.92205452988951198
12 204800000 6.73 0.00 0.86448646582845623
Method: RESHAPE intrinsic
i Array Size Time (sec) Max|a-xran| a(n/2)
1 100000 0.230E-1 0.00 0.14632783894497925
2 200000 0.170E-2 0.00 0.93604658738074942
3 400000 0.343E-2 0.00 0.38816074699824510E-2
4 800000 0.784E-2 0.00 0.48353192216229768
5 1600000 0.143E-1 0.00 0.17707306405838441
6 3200000 0.273E-1 0.00 0.34672021796550934
7 6400000 0.547E-1 0.00 0.49127175384201527E-2
8 12800000 0.107 0.00 0.41832146544600479
9 25600000 0.201 0.00 0.34490797395681683
10 51200000 0.351 0.00 0.43429002560932473
11 102400000 0.646 0.00 0.92205452988951198
12 204800000 1.41 0.00 0.86448646582845623
Method: MOVE_ALLOC
i Array Size Time (sec) Max|a-xran| a(n/2)
1 100000 0.263E-1 0.00 0.14632783894497925
2 200000 0.636E-3 0.00 0.93604658738074942
3 400000 0.187E-2 0.00 0.38816074699824510E-2
4 800000 0.316E-2 0.00 0.48353192216229768
5 1600000 0.552E-2 0.00 0.17707306405838441
6 3200000 0.123E-1 0.00 0.34672021796550934
7 6400000 0.243E-1 0.00 0.49127175384201527E-2
8 12800000 0.472E-1 0.00 0.41832146544600479
9 25600000 0.843E-1 0.00 0.34490797395681683
10 51200000 0.163 0.00 0.43429002560932473
11 102400000 0.225 0.00 0.92205452988951198
12 204800000 0.463 0.00 0.86448646582845623