Program Layout

Character Set

FORTRAN requires only a limited character set. For truly portable software, you should use only the following:

The letters A-Z and digits 0-9 are often grouped together and referred to as alpha-numeric characters. Although this character set seems somewhat limited, it should be available on any computer. The blank character is ignored except within character constants and the currency symbol is not actually used. Of course, most computer keyboards provide many other characters, not least of which are the lower-case letters. Any printable characters may be used within comments and character constants.

Symbolic Names

Symbolic names can be used to identify constants, variables, arrays, functions, subroutines and common blocks. Each symbolic name must begin with a letter and may be followed by up to five letters or digits.

Be careful using O (oh) and 0 (zero) in symbolic names as they are easily confused. Another troublesome twosome is I (eye) and 1 (one), not to mention S (ess) and 5 (five). It is not illegal to use these characters in symbolic names but you must take extra care not to mix them up.

Statements

A leftover from the bad old days of punch cards is the statement layout in FORTRAN. Each line is divided into four fields:

columns 1-5

label field

column 6

continuation field

columns 7-72

statement field

columns 73-80

Danger! Stay out! (card sequence field)

A statement label is a number from 1 to 99999 that precedes a statement. Any executable statement (and some nonexecutable ones) may have a label.

The continuation field is used only when a statement is too long to fit on one line (within columns 7 to 72). A statement may be continued on the next line by placing any symbol (like the currency symbol) in the sixth column preceding the continuation. Up to 19 continuations are permitted for a single statement; hence, a single statement can cover up to 20 lines. Finally, the statement itself is placed within columns 7 to 72. Any characters that stray past the last allowed column will be ignored by the compiler and will almost certainly cause an error in the program.

It is imperative to document your programs. A piece of code that is clear today will be difficult if not impossible to understand six months down the road so you should include comments in your program explaining what it does. To place a comment line in a program, place a C or an asterisk * in the first column. The text of the comment follows. Each comment line must be preceded by a C or an asterisk *. Like statements, comments should not extend past column 72 but this rule is rarely enforced. A comment may appear anywhere in a program except after the END statement. Completely blank lines are also permitted.

Program Units

A complete program consists of one or more program units. There is exactly one main program which begins with a PROGRAM statement:

PROGRAM main-name

This statement appears only at the beginning of the main program and the symbolic name main-name must be unique (not used elsewhere in any of the program units).

There may also be any number of external procedures (subroutines and functions) and BLOCKDATA subprograms. Every program unit must end with an END statement.

END

The END statement is executable and may have a label. When an END statement is encountered in an external procedure or BLOCKDATA subprogram, control is returned to the calling program unit. When an END statement is encountered in the main program, any open files are closed and execution stops. At this point control is returned to the operating system.

Statement Order

FORTRAN statements must appear in a certain order within a program unit.

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Last modified on 1 April 2007