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The
HP Fortran compiler accepts source files in fixed form
(the standard source form for FORTRAN 77 programs) or free
form. The following sections describe both forms. The
compiler assumes that source files whose names end in the .f90 extension are in free source form and that files
whose names end in the .f or .F extension are in fixed form. You can override
these assumptions by compiling with the +source=free or +source=free option. See the HP Fortran Programmer’s
Guide for more information. Although the two forms are quite different, you can format
a Fortran 90 source file so that the compiler would accept
it as either fixed or free form. This would be necessary, for example, when
preparing a source file containing code that will be inserted through
the INCLUDE line into a file for which the form is not known.
To format a source file to be acceptable as either free or fixed
source form, use the following rules: Put labels in columns1-5. Put statement bodies in columns 7-72. Begin comments with an exclamation mark in anycolumn
except column 6. Indicate all continuations with an ampersand character
(&) in column 73 of the line to be continued and
an ampersand character in column 6 of the continuing line. Do not insert blanks in tokens. Separate adjacent names and keywords with a space.
Free
source form | |
In free source form,
the source line is not divided into fields of predefined width,
as in the fixed form. This makes entering text at an interactive
terminal more convenient. Freeform lines can
contain from 0 to 132 characters. The +extend_source option extends the line to column 254. This is
described in the HP Fortran Programmer’s Guide.
Several statements can appear on a single source line, separated
by semicolons. A single Fortran 90 statement can extend over more
than one source line, as described below in “Statement
continuation”. Multiple statements
may appear on the same line, separated by a semicolon (;). Statement labels are
not required to be in columns 1-5, but must be separated from the statement
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 2-4 “Keywords allowing optional spacing”, however, the space is optional. The
keyword following END can be: BLOCK DATA, DO, FILE, FUNCTION, IF, INTERFACE, MAP, MODULE, PROGRAM, SELECT, SUBROUTINE, STRUCTURE, TYPE, UNION, or WHERE. Table 2-4 Keywords allowing optional spacing BLOCK DATA | GO TO | DOUBLE PRECISION | IN OUT | ELSE IF | SELECT CASE | END keyword | |
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.
Consider the spaces (designated by b) in the following statement: IFbb(TEXT.EQ.'bbbYES') ... ! Valid |
The two spaces after IF are valid and are equivalent to one space. No
spaces are required before or after .EQ., because there is no ambiguity. However, the three
spaces in the character constant are significant. In the next example IF(MbARY.bGE.MIKE) ... ! Faulty in free source form |
the spaces are invalid in free source form but valid in fixed
source form. An exclamation mark
(!) indicates the beginning of a comment in free
source form, except where it appears in a character context. The
compiler considers the rest of the line following the exclamation
mark as part of the comment. Embedding a comment inside program
text within a single source line is not allowed, but it can follow
program text on a source line. A 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 ampersand
character (&) to each source line except the last. The ampersand
must not be within a character constant. A statement can
occupy up to 40 source lines. As an extension, HP Fortran
increases this limit to 100 source lines. The END statement cannot be split by means of a continuation
line. Comments are not statements and cannot be continued. The text of the source statement in a continuation line is
assumed to resume from column 1. However, if the first nonblank
symbol in the line is an ampersand, the text resumes from the first
column after the ampersand. Consider the following two statements: INTEGER marks, total, difference,& ! work variables
mean, average |
INTEGER marks, total, difference, mean_& ! work variables
&value, average |
The second statement declares the integer variable, mean_value. Any spaces appearing in the variable name as
a result of the continuation would be invalid. This is the reason
for the ampersand character in the continuation line. (Alternatively, value could have been positioned at column 1.) Using
the ampersand character to split lexical tokens and character constants
across source lines is permitted, but not recommended. Fixed
source form | |
Statements
or parts of statements must be written between character columns
7 and 72. Any text following column 72 is ignored. The +[no]extend_source option extends the statement to column 254. Columns
1-6 are reserved for special use. | | | |  | NOTE: Programs that depend on the compiler’s ignoring
any characters after column 72 will not compile correctly with the +extend_source option. | | | | |
Multiple statements
may appear on the same line, separated by a semicolon (;). Spaces are not significant
except within a character context. For example, the two statements are equivalent, but are not. There are three types
of lines in fixed source form: The following sections describe each type of source lines. An initial line has
the following form: Columns 1 to 5 may
contain a statement label. Column 6 contains a space or the digit zero. Columns 7 to 72 (optionally, to 254) contain the
statement.
A continuation line
has the following form: Columns 1 to 5 are blank. Column 6 contains any character other than zero
or a space. One practice is to number continuation lines consecutively
from 1. Columns 7 to 72 (optionally, to 254) contain the
continuation of a statement.
The Standard
specifies that a statement must not have more than 19 continuation
lines. As an extension to the Standard, HP Fortran allows
as many as 99 continuation lines. Comment lines may
be included in a program. Comment lines do not affect compilation
in any way, but usually include explanatory notes. The letter C, or c , or an asterisk (*) in column 1 of a line, designates that line as
a comment line; the comment text is written in columns 1 to 72.
The compiler treats a line containing only blank characters in columns
1 to 72 as a comment line. In addition, a line is considered to
be a comment when there is an exclamation mark (!) in column 1 or in any column except column 6. The following are HP
extensions to the comment: A line with D or d in column 1 is by default treated as a comment.
The +dlines option causes the compiler to treat such lines
as statements to be compiled. This extension to the comment—called debugging
lines—is useful for including PRINT statements that are to be compiled during the
debugging stage to display the program state. A line with a pound
sign (#) character in column 1 is treated as a comment.
This extension allows compilation of source files that have been
preprocessed with the C preprocessor (cpp). HP Fortran
allows tab formatting. That is, a tab character may be entered in
the first column of a line to skip past the statement label columns.
If the character following the tab character is a digit, this digit
is assumed to be in column 6, the continuation indicator column.
Any other character following the tab character is assumed to be
in column 7, the start of a new statement. A tab character in any
other column of a line is treated as a space.
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