Automatic line break in Fortran when writing a very long string

Question

I have a string in fortran that I extend in a loop:

character(:), allocatable :: string
...
do i = 1, n
  string = string // "some stuff depending on i"
end do

This works fine as long as string is smaller than 80 characters. When 80 characters are reached a line break is included into the string.

Does anyone know why this happens and if there is a chance to avoid this?

I have the following full example:

program string

  implicit none

  character(:), allocatable :: long_string
  character(:), allocatable :: line

  integer      :: i
  character(8) :: idx

  line = " This is line number: "

  long_string = "First line" // NEW_LINE('A')

  do i = 1, 3
    write(idx, "(I8)") i
    long_string = long_string // line // adjustl(trim(idx)) // NEW_LINE('A')
  end do

  write(*,*) long_string

end program

If I compile with Intel's Fortran compiler, I get the following output:

 First line
 This is line number: 1
 This is line number: 2
 This
 is line number: 3

Using gfortran gives the expected output:

 First line
 This is line number: 1
 This is line number: 2
 This is line number: 3

Answer 1

As you've noticed, list-directed output will automatically print up to 80 characters before beginning a new line:

character(len=:), allocatable :: a
a = "************************** This is an 84-character string **************************"
print *, a

! This is the output:
 ************************** This is an 84-character string ****************
 *****

---

Notice that the 80-character limit includes all printed and non-printing characters, including the leading space automatically inserted when using list-directed output. I looked, but I couldn't find out whether there are ways you can change this default behavior, or if it is specified by the Fortran standard or whatever.

To be complete, I'll demonstrate what I'm talking about. The best way to deal with this is to define a format, as already suggested in a previous answer. You can do this for character strings very easily:

! print everything in the string:
print "(a)", a
! output:
************************** This is an 84-character string **************************

! or, define the printed field width:
print "(a34)", a
! output:
************************** This is

And so on.

---

UPDATE: I have reviewed the code you've added to your original post. Notice that after you create long_string in the DO loop, the string length exceeds 80 characters. When you then print using list-directed output, the result with ifort makes sense: anything exceeding 80 characters (including leading white spaces and the newline characters) will automatically continue on a new line.

Notice, when you use adjustl(trim(idx)) you still end up with a field 8 characters long. I think you mean to use trim(adjustl(idx)). If you do this, and still use list-directed output to write the long_string, then the output looks like:

First line
This is line number: 1
This is line number: 2
This is line number
: 3

So, you get that extra "unexpected" line at the end of the output again, because the length of long_string exceeds 80 characters. Luckily, the solution is to (again) just use the character format "(a)". Since you've placed the newline characters where you want them, you'll get the output you are hoping for.