Basic Program Structure

Source format

Fortran's heritage from old (and plain outdated) computer systems is clearly visible in the rules for the source code format. Back in the old days, source code was kept on punched cards. Each card corresponds to one statement in the program.

Since a punched card has 80 columns, lines in a Fortran program are limited to 80 characters, and in fact, some columns have special meanings.

• Column 1 can contain a "C" or "*" to indicate that this statement is a comment (in fact, it's often called a `comment card'), that is, a line that contains a plain English comment about the program, rather than a Fortran statement. It is a good idea to supply such comments as a way of documenting your code.
• Columns 1-5 can contain a statement label. These serve as targets for jump statements, the end of DO loops, or formatted I/O. Labels can have up to 5 digits. Leading zeros are ignored, and a label has to be unique to a program unit.
• Column 6 contains a blank (space), or a continuation character. Any non-blank character in column 6 indicates that this line is a continuation of the last line, which presumable contains the start of an expression too long to be written on one line. The number of continuation lines is limited to 19.
• Columns 7-72 contain the actual Fortran statements.
• Columns 73-80 are ignored. They can contain labeling information.

Because of Fortran's one-line-one-statement paradigm, there is no semcolon or other statement terminator such as in C or Pascal. On the other hand, it is not possible to write a number of short statements on one line.

F90 extension. In Fortran 90 the columns have lost their special meanings. Statements can begin in any column, and the number of columns is at most 132. Inline comments start with an "!" and extend to the end of the line. Continuations are indicated by an "&" at the end of an unfinished line (but before any inline comment); there can be 39 continuation lines. To group statements on one line, a ";" separator is used.

Program unit structure

Each Fortran program (and subprrogram) has a structure of a header line, a specification part of non-executable statements, the execution part, and an ending part. To be precise (and note that all of the following parts are optional):

• First comes the PROGRAM or other subprogram header;
• Then IMPLICIT statements;
• Then other specification statements;
• Then statement function definitions;
• Then executable statements;
• And finally an END statement.

Program unit header

The main program is supposed to start with a line

      PROGRAM myprog

where "myprog" is any name you want to give your program. This name is usually not used anywhere. Some systems let you add arguments to this line, eg to specify input/output channels or to pass command line arguments.

Other possible header keywords are FUNCTION or SUBROUTINE for subprograms, or BLOCK DATA for a common block initialisation.

Specification part

After the unit header there can be specification statements. These are not executable, though they can contain arithmetic expressions that can be evaluated at compile time.

• Type statements to declare the type of parameters and local variables.
• Dimension statements to declare the size of arrays.
• Data statements to initialise variables and arrays.
• Parameter statements introducting named constants.
• Common blocks.
• EXTERNAL and INTRINSIC declarations.
• Equivalence and Save statements.

Execution part

After any specification part follows the execution part. This is where the work gets done.

Program unit ending

Any type of program unit ends with an END line. Subprograms (with the exception of BLOCK DATA) additionally need a RETURN statement. Its recommended location is right before the END.