Manual Reference Pages - comment (5fortran)

NAME

COMMENT(5) - [SYNTAX] code annotation

SYNOPSIS

Fixed and Free comments

C fixed-format comment ! free-format comment ! free-format comment CODE_LINE ! free-format comment

DESCRIPTION

In free-format files The character "!" initiates a comment except when it appears in a character context as part of a literal string.
The comment extends to the end of the line.
If the first nonblank character on a line is an "!", the line is a "comment line". Lines containing only blanks or containing no characters are also comment lines.
"comment lines" may appear anywhere. There are a few restrictions on comments trailing statements or continued statements though.
Comments may appear anywhere in a program unit and may precede the first statement of a program unit or follow the last statement of a program unit.
Comments have no effect on the interpretation of the program unit.
A continued line ending in an ampersand can additionally be followed by an exclamation and remarks unless a literal string is being continued.
Comment lines cannot be continued with an ampersand. An ampersand appearing in a comment has no special effect and is merely a regular character.
The standard does not restrict the number of consecutive comment lines.
FIXED-FORMAT SOURCE FILES
Additionally, in fixed-format source files a "C" in column 1 indicates the remainder of the line is a comment. An asterisk "*" in column 1 beginning a comment is a common extension as well.
There is a conflict in fixed-format files that can occur with the otherwise universal rule that at exclamation outside of a literal string begins a comment -- If the first non-blank character in a line is in column 6 in a fixed-format file it is a continuation line, not a comment. This rule includes an exclamation character as well.

FREE FORM COMMENTARY

A comment is explanatory text embedded in program source intended to help human readers understand it.
Code completely without comments is often hard to read, but code with too many comments is also bad, especially if the comments are not kept up-to-date with changes to the code.
Too much commenting may mean that the code is over-complicated.
A good rule is to comment everything that needs it but write code that doesn’t need much of it.
Comments that explain WHY something is done and how the code relates to its environment are useful.
A particularly irksome form of over-commenting explains exactly what each statement does, even when it is obvious to any reasonably competant programmer.

EXAMPLES

Sample program:

    program demo_comment
    integer :: values(8)
    character(len=:),allocatable :: string
    character(len=1),parameter   :: dash=’-’,colon=’:’,dot=’.’
    real :: x=3.0, y=4.0
       ! comments may appear on a continued line
       ! blank lines are comment lines
       call date_and_time(values=values)
       associate( &

        ! DATE
        YR=>values(1),      & ! The year
        MO=>values(2),      & ! The month
        DY=>values(3),      & ! The day of the month

        ! TIME
        UTC=>values(4),     & ! Time difference with UTC in minutes
        HR=>values(5),      & ! The hour of the day
        MIN=>values(6),     & ! The minutes of the hour
        SEC=>values(7),     & ! The seconds of the minute
        MILLI=>values(8) )    ! The milliseconds of the second

        write(*,’(*(g0))’)YR,dash,MO,dash,DY,’T’, &
        & HR,colon,MIN,colon,SEC,dot,MILLI
       end associate

       string=’no comment allowed &
          &on the end of a continued string &
          ! keep going ...
          & but comment lines are allowed between ’ ! but can go on the end

       ! the next exclamation is part of a literal string, and so has
       ! nothing to do with comments
       print *, ’Hello World! X=’,x,’Y=’,y

    end program demo_comment

Results:

  2024-10-13T0:7:25.283 Hello World! X= 4.59107416E-41 Y= 2.76724564E-36

Manual Reference Pages - comment (5fortran)

NAME

SYNOPSIS

DESCRIPTION

FREE FORM COMMENTARY

EXAMPLES

SEE ALSO