# Could someone please shed some light on this text example?

I do not understand this example. Could some one please have the patience to describe line for line what is happening?
And what is the meaning of x( : )?

program main
implicit none
integer :: N, err
real, allocatable :: x( : )

print*, ‘Enter an integer N’
allocate(x(N), STAT=err)
if (err /= 0 ) STOP
call random_number( x )

print*, ‘Processing x…’, process()
deallocate(x)
contains
logical function process()
! in this function N and X can be accessed directly
! Please not that this method is not recommended:
! it would be better to pass X as an argument of process
implicit none

``````if (sum(x) > 5.) then
process = .FALSE.
else
process = .TRUE.
endif
``````

end function process
end program main

The program asks the user for an integer, which is set to the size of an allocatable array, which is filled with uniform random variates. If the sum of the uniform variates exceeds 5 it prints F, otherwise T. The line

`real, allocatable :: x( : )`

declares a 1-D allocatable array. A 2-D allocatable array could be declared

`real, allocatable :: x(:,:)`

The answer above was generated without AI. Asking ChatGPT-4 to

Explain what the following Fortran program does, and add inline comments to explain what each line does.

where the “following program” was yours with slight changes, it said the following:

The Fortran program you’ve shared is a simple program that does the following:

1. It asks for an integer input from the user.
2. It then allocates an array of real numbers of size equal to the input number.
3. This array is filled with random numbers.
4. The program prints the array to the console.
5. Then, it checks whether the sum of the elements in the array is greater than 5. If it is, it prints false, otherwise it prints true.
6. Lastly, it deallocates the memory that was previously allocated for the array.

Now let’s go line by line with inline comments:

``````program main                      ! Start of the main program
implicit none                     ! Explicit declaration of variables is required

integer :: N, err                 ! Declare two integer variables: N and err
real, allocatable :: x( : )       ! Declare an allocatable array x of real numbers

print*, "Enter an integer N"      ! Print message to enter an integer
read*, N                          ! Read the user input and store it in N
allocate(x(N), STAT=err)          ! Allocate an array x of size N. If an error occurs during allocation, its status is stored in err
if (err /= 0 ) STOP               ! If an error occurred during the allocation (err is not 0), stop the program

call random_number( x )           ! Fill the array x with random numbers
print*,"x =",x                    ! Print the array x to the console
print*, "sum(x) > 5 ?", process() ! Print the result of the function process(), which checks whether the sum of the array is >5
deallocate(x)                     ! Deallocate the memory occupied by array x

contains                          ! Beginning of the internal functions/subroutines

logical function process()        ! Start of a function named process() that returns a logical value (true or false)
implicit none                     ! Explicit declaration of variables is required within the function as well

if (sum(x) > 5.) then             ! If the sum of the array x is greater than 5
process = .FALSE.              ! Set the result of the function to false
else
process = .TRUE.               ! Otherwise, set the result to true
endif

end function process              ! End of the function process()
end program main                  ! End of the main program
``````

As the comment in the code mentions, accessing the global variable `x` within the function `process()` is not a best practice. It would be better to pass `x` as an argument to the function. This is because using global variables can lead to issues with side-effects and make code harder to understand and debug.

Why not just:

process = sum(x) <= 5.0

?

2 Likes

Beliavski, First of all: thank you for responding at all. secondly: for your superb explanation. I found the text, that you explained so well, likened the final exam of a loony bin, if you understand you get out.