Fortran 77 Basics |
A Fortran
program is just a sequence of lines of text. The text has to follow a certain
syntax to be a valid Fortran program. We start by looking at a simple
example where we calculate the area of a circle:
A D V E R T I S E M E N T
program circle
real r, area
c This program reads a real number r and prints
c the area of a circle with radius r.
write (*,*) 'Give radius r:'
read (*,*) r
area = 3.14159*r*r
write (*,*) 'Area = ', area
stop
end
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The lines that begin with a "c" are comments and
have no purpose other than to make the program more readable for humans.
Originally, all Fortran programs had to be written in all upper-case letters.
Most people now write lower-case since this is more legible.
Program organization
A Fortran program generally consists of a main program
(or driver) and possibly several subprograms (or procedures or subroutines). For
now we will assume all the statements are in the main program; subprograms will
be treated later. The structure of a main program is:
program name
declarations
statements
stop
end
In this tutorial, words that are
in italics should not be taken as literal text, but rather as a generic
description. The stop statement is optional and may seem
superfluous since the program will stop when it reaches the end anyway but it is
recommended to always terminate a program with the
stop statement to emphasize that the
execution flow stops there.
Column position rules
Fortran 77 is not a free-format language, but has a
very strict set of rules for how the source code should be formatted. The most
important rules are the column position rules:
Col. 1 : Blank, or a "c" or "*" for comments
Col. 2-5 : Statement label (optional)
Col. 6 : Continuation of previous line (optional)
Col. 7-72 : Statements
Col. 73-80: Sequence number (optional, rarely used today)
Most lines in a Fortran 77 program starts with 6 blanks and
ends before column 72, i.e. only the statement field is used. Note that Fortran
90 allows free format.
Comments
A line that begins with the letter "c" or an asterisk in
the first column is a comment. Comments may appear anywhere in the program.
Well-written comments are crucial to program readability. Commercial Fortran
codes often contain about 50% comments. You may also encounter Fortran programs
that use the exclamation mark (!) for comments. This is highly non-standard in
Fortran 77, but is allowed in Fortran 90. The exclamation mark may appear
anywhere on a line (except in positions 2-6).
Continuation
Occasionally, a statement does not fit into one single
line. One can then break the statement into two or more lines, and use the
continuation mark in position 6. Example:
c23456789 (This demonstrates column position!)
c The next statement goes over two physical lines
area = 3.14159265358979
+ * r * r
Any character can be used instead of the plus sign as a
continuation character. It is considered good programming style to use either
the plus sign, an ampersand, or numbers (2 for the second line, 3 for the third,
and so on).
Blank spaces
Blank spaces are ignored in
Fortran 77. So if you remove all blanks in a Fortran 77 program, the program is
still syntactically correct but almost unreadable for humans.
