Fortran: initializing of and assigning value to arrays

Question

To initialize and assign value to arrays in Fortran we do as the following:

Initializing:

real(kind=8):: r(3,4)
...
r(:,:) = 0.0_8

what if we use only

real(kind=8):: r(3,4)
...
r = 0.0_8

and what if we do as:

real(kind=8):: r(3,4)
...
r = 0

also for situation such as:

real(kind=8):: r(3,4), q(3,4), p(30,40)
...
q = 0
r = q
r = p(1:3,21:24)

we prefer to do as:

real(kind=8):: r(3,4), q(3,4), p(30,40)
...
q = 0.0_8
r(:,:) = q(:,:)
r(:,:) = p(1:3,21:24)

we are not sure so hope you provide us some reasons for each one you prefer.

max · Accepted Answer · 2012-01-12 23:17:21Z

13

For general considerations (including declaration and assignation) about efficiently using arrays in Fortran, I would suggest to read this.

For more precise answer to your question, I did some tests some months ago that may interest you. Here are the results. This is a test on my personnal laptop on Linux Archlinux x86-64, with GNU Fortran (GCC) 4.6.1 20110819 (prerelease) without optimization options.

do i = 1 , 100
  do j = 1 , 100
    do k = 1 , 100 ! innest is fastest
      array ( i , j , k ) = 0.0d0
    end do
  end do
end do
! reference time : 1.00

to

do i = 1 , 100
  do j = 1 , 100
    do k = 1 , 100
      array ( k , j , i ) = 0.0d0
    end do
  end do
end do
! time : 0.499

to

array = 0.0d0
! time : 0.250

to

array ( : , : , : ) = 0.0d0
! time : 0.250

answered Jan 12, 2012 at 23:17

max

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2 Comments

Eli S Over a year ago

Hi Max, the link to your work is broken. Moreover, I'm not sure what a comparison with optimization turned off would reveal. The inner loop here would optimize trivially. Often your examples 2,3,4 will be the same in optimized mode.

Kevin Powell Over a year ago

I guess the time difference is due to the way array is stored in memory. In Fortran, take 2 dimensional matrix for example, column vectors are stored continuously instead of row vectors which is the case of C++.

M. S. B. · Accepted Answer · 2012-01-12 15:49:07Z

6

Do you think that "kind=8" means 8 bytes? That isn't true for all compilers. The maximum portability is achieved using the selected_real_kind intrinsic to define a kind value:

integer, parameter :: DRK = selected_real_kind (14)

Of some of the options that you list, I prefer r = 0.0_8, or better r=0.0_DRK. This defines the entire array. There is no need to designate array sections in this case, since you are calling out the entire array: r (:, :). Steve Lionel has a discussion of why trying to make arrays obvious with ":" isn't a good idea -- there are differences between array and array (:). As an argument, the first has it declared dimensions, while the second always begins at 1 -- see http://software.intel.com/en-us/blogs/2008/03/31/doctor-it-hurts-when-i-do-this/

Since r is real, r=0 implies a conversion. For the value zero this almost certainly doesn't matter, but it could make an important difference with other values. e.g.,

r = 3.1415927654

and

r = 3.141592654_8

will give different values to r because the constants are different, the first being converted to single precision before the assignment.

edited Jan 12, 2012 at 15:49

answered Jan 12, 2012 at 4:28

M. S. B.

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1 Comment

Rook Over a year ago

The part about kind=8 not being equivalent to 8 bytes has been discussed a lot in the past. It is somewhat of a general opinion that kind types should't have been designated with integers to begin with. It causes a lot of confusion.

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