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Fortran
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Fortran File I/O
Fortran Common blocks
Fortran Data and block data
Fortran Debugging hints
Fortran Sample program

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Fortran 77 Basics


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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

Fortran 77 Basics

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

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.



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Keywords: fortran arrays,fortran data statements,fortran common blocks,fortran file i/o,fortran debugging,fortran format statements,fortran loops,fortran sub programs,fortran example programs

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