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Fortran 90 has a free source form, but also accepts FORTRAN
77's fixed source form (as it must, since FORTRAN 77 is
a subset of Fortran 90). Although the two forms are quite different,
it is possible to use an "intersection" form that
satisfies both. This would be necessary, for example, when preparing
Fortran text that was intended to be INCLUDEd
(see “INCLUDE line”) in a Fortran
program whose source form, fixed or free, was not known. The fixed source form, the free source form, and the intersection
form are described below. The
HP Fortran 90 compilation system assumes that source files are in
the fixed format form, unless the source file being compiled has
the suffix .f90
or the free-format compile-line option, the +source={fixed|free}option,
is specified when the compiler is invoked (see Chapter 13 for further
details on the option). Fixed source form | |
Statements
or parts of statements must be written between character positions
7 and 72. Any text following position 72 is ignored. The +[no]extend_source
option (see Chapter 13) extends the statement to position 254. Positions
1-6 are reserved for special use. Blanks are not significant except
within a character context. For example: are equivalent, but: are not equivalent. Multiple statements may appear on one line separated by a
semicolon (;). There are three classes of lines in Fortran 90 fixed source
form: Initial line Continuation line Comment line
An
initial line has the following form: Positions 1 to 5 may contain a statement
label. Position 6 contains a space or the digit 0. Positions 7 to 72 (optionally, to 254) can contain
the statement.
A continuation line has the following
form: Positions 1 to 5 are blank. Position 6 contains any character other than 0 or
a space. One practice is to number continuation lines consecutively
from 1. Positions 7 to 72 (optionally, to 254) contain the
continuation of a statement.
The Fortran 90 Standard specifies that a statement must not
have more than 19 continuation lines. In HP Fortran 90, a statement
consists of an initial line and up to 99 continuation lines. Comment
lines may be included in a program. Such lines do not affect the
program in any way but can be used by the programmer to include
explanatory notes. The letter C,
or c , or an
asterisk (*)
in position 1 of a line, designates that line as a comment line;
the comment text is written in positions 1 to 72. A line containing
only blank characters in positions 1 to 72 is also treated as a
comment line. An exclamation mark (!)
in position 1 or in any position except position 6, causes the rest
of the line to be treated as a comment. In
HP Fortran 90 a line with D or
d in position
1 is by default treated as a comment . A compile-line option, the
+[no]dlines option, treats lines with D
or d in position
1 as statements to be compiled. This facility is useful in program
debugging. See Chapter 13 for more information about the +dlines
option. In HP Fortran 90, a line with #
in position 1 is treated as a comment. This allows source files
which have been preprocessed with cpp
to be compiled. In
HP Fortran 90 a tab character in the first position of a line can
be used to skip past the statement label positions. If the character
following the tab character is a digit, this is assumed to be in
position 6, the continuation indicator position. Any other character
following the tab character is assumed to be in position 7, the
start of a new statement. A tab character in any other position
of a line is treated as a space. Free source form | |
In
this form the source line is not divided into fields of predefined
width, as in the fixed form. This makes it more convenient for input
of text at an interactive terminal. The details of the free source
form are as follows. Lines
can contain from 0 to 132 characters. A compile-line option, the
+extend_source
option (see Chapter 13), extends the statement to position 254.
Several Fortran 90 statements can be placed on a single source line,
separated by ";" characters, and a single Fortran
90 statement can extend over more than one source line, as described
below in “Statement continuation”. Statement
labels are not required to be in columns 1-5, but must be separated
from the statement proper by at least one space. Spaces are significant: They may not appear within a lexical
token, such as a name or an operator. In general one or more spaces are required to separate
adjacent statement keywords, names, constants, or labels. Within
the keyword pairs listed in Table 2-2 “Keywords allowing optional spacing”,
however, the space is optional. Table 2-2 Keywords allowing optional spacing BLOCK DATA | GO TO | DOUBLE PRECISION | IN OUT | ELSE IF | SELECT CASE | END keyword | |
The keyword after END
can be any allowed by the Fortran 90 syntax, including the following:
BLOCK DATA, DO,
FILE, FUNCTION,
IF, INTERFACE,
MAP, MODULE,
PROGRAM, SELECT,
SUBROUTINE, STRUCTURE,
TYPE, UNION,
or WHERE. Spaces are not required between a
name and an operator because the latter begins and ends with special
symbols that cannot be part of a name. Multiple spaces, unless in
a character context, are equivalent to a single space.
Spaces are denoted here (and throughout this manual where
it is necessary to stress their presence) by b. IFbb(TEXT.EQ.'bbbYES') ... ! Valid |
Valid: the two spaces after IF
are equivalent to one space. No spaces are required before or after
.EQ., because
there is no ambiguity. Note that the three spaces in the character
constant are significant. IF(MbARY.bGE.MIKE) ... ! Faulty |
Faulty: the space is invalid in MbARY,
and the space is invalid in .bGE..
(This example would be valid in the fixed form.) In
free source form, the only way of indicating a comment is by use
of the "!"
symbol. Unless it appears in a character context, the occurrence
of a "!"
symbol defines the start of a comment, which always continues to
the end of the source line. It is thus not possible to embed a comment
inside program text within a single source line, but it can follow
program text on a source line. Furthermore, a Fortran 90 statement
on a line with a trailing comment can be continued on subsequent
lines. A
statement can be split over two or more source lines by appending
an "&"
symbol to each source line involved except the last. The ampersand
must not be within a character constant. In this way, in Standard Fortran 90, a statement can occupy
up to 40 source lines. As an extension, HP Fortran 90 increases
this limit to 100 source lines. The END statement cannot be split
by means of a continuation line. The text of the source statement in a continuation line is
assumed to resume from column 1, unless the first nonblank symbol
in the line is an "&",
in which case the text resumes from the first position after the
"&". Example of statement continuationConsider the following two statements: INTEGER marks, total, difference,& ! work variables |
INTEGER marks, total, difference, mean_& ! work variables |
The second of these statements declares an integer variable
called mean_value.
Any spaces appearing in the variable name as a result of the continuation
would have been invalid, which is why another "&"
was used in the continuation line. (Alternatively "value"
could have been positioned at column 1.) Splitting lexical tokens,
including character constants, across source lines in this way,
is permitted but should be avoided if possible. Comments cannot be continued. Intersection source form | |
It is possible to write programs in
a way that is acceptable as both free source form and fixed source
form, unless in extended fixed format source mode. The rules are: Put labels in positions 1-5. Put statement bodies in positions 7-72. Begin comments with an exclamation mark (!)
in any position except 6. Indicate all continuations with an ampersand in
position 73 of the line to be continued and an ampersand in position
6 of the continuing line. Do not insert blanks in tokens. Separate adjacent names and keywords with a space.
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