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3. Source form

3. Source form

FORTRAN 77 source is fixed-form: the meaning of a line depends on which column a character sits in. This is the single biggest surprise for newcomers, and the cause of most "why won't it compile" puzzles. Once you know the column rules it is mechanical. (The layout dates from the 80-column punched card.)

The column layout

Each line is up to 72 meaningful characters, divided into four fields (§3.2, §3.3):

 column: 1   2   3   4   5   6   7 ............................... 72 | 73 ... 80
         └────────┬────────┘   │   └──────────────┬──────────────┘   └────┬────┘
          label field (1–5)     │       statement field (7–72)         ignored
                          continuation (6)
Columns Field What goes there
1–5 label an optional statement number (1–5 digits), right- or left-justified
6 continuation blank or 0 → this is a new statement; anything else → continues the previous line
7–72 statement the statement text itself
73–80 (ignored) historically the card sequence number; ignored by the compiler

So an ordinary statement starts in column 7:

C234567 ← column ruler (the digit under each position)
      X = 2.0
      Y = X * X

Comment lines

A line is a comment (§3.2.1) if it has C (or *) in column 1, or if it is entirely blank. The whole line is ignored.

C This whole line is a comment.
*     So is this one.

      X = 1.0          ! (the blank line above is also a comment)

Continuation lines

A long statement is split by putting a non-blank, non-0 character in column 6 of the following line. Columns 1–5 of a continuation must be blank. Any character will do as the continuation mark; a digit or & is conventional.

      TOTAL = A + B + C + D +
     1        E + F + G

The 1 in column 6 of the second line says "this is a continuation of the statement above." A statement may have up to nineteen continuation lines.

Statement labels

A statement label is 1 to 5 digits (at least one non-zero) placed in columns 1–5 (§3.4). It names a statement so other statements can refer to it — a FORMAT referenced by a WRITE, or a target of a GO TO or a DO. Labels need not be in order or contiguous; leading zeros and blanks within the field are not significant (10, 010, and 1 0 are the same label).

      GO TO 100
  100 CONTINUE
   10 FORMAT (I5)

Only labelled statements can be referred to, and a label may name at most one statement in a program unit.

Blanks are not significant

Outside a character constant, blanks have no meaning (§3.1.6) — they exist only to make the source readable. This has two consequences that startle newcomers:

  • Keywords may be spaced (or not): GO TO, GOTO, and G O T O are identical; so are END IF and ENDIF, DOUBLE PRECISION and DOUBLEPRECISION.
  • Names and numbers may contain blanks: N ( 1 ) = 4 2 means N(1) = 42, and I N TEGER X is the declaration INTEGER X.
      I N TEGER N
      N ( 1 ) = 4 2          ! same as  INTEGER N  /  N(1) = 42

You would not normally write code this way, but it explains old listings that do — and it is why a stray space inside a keyword never breaks anything.

The order of statements

Within a program unit, statements must appear in a required order (§3.5). The rules, simply:

  1. The unit's header (PROGRAM, FUNCTION, SUBROUTINE, or BLOCK DATA) comes first, if present.
  2. IMPLICIT statements come before the other specification statements.
  3. All specification statements (type declarations, DIMENSION, COMMON, EQUIVALENCE, PARAMETER, EXTERNAL, INTRINSIC, SAVE) come before the executable statements.
  4. Statement-function definitions come after the specifications and before the executables.
  5. Executable statements come last, then END.
  6. FORMAT and ENTRY may appear anywhere; DATA may appear anywhere after the specifications.

In short: declare first, then do. Putting a declaration after an executable statement is an error.

The END line

Every program unit ends with an END statement on its own initial line (no continuation). END marks the physical end of the unit; in a main program it also stops execution, and in a subprogram it acts like a RETURN (Chapter 15).

forterp notes. forterp reads columns 7–72 as the statement and drops 73–80, exactly as specified, and honors blank-insignificance (the spaced examples above all run). A few latitudes:

  • The order rule (point 3) is enforced under the F77 dialect — a specification statement after an executable is a hard error. F66/FORTRAN10 accept it (lenient). One consequence: reusing a type keyword as a variable name and assigning to it after an executable (REAL = 3.0) is read as a misplaced type-statement; declare such a name up front.
  • forterp also accepts the DEC column-1 comment markers ! $ / D in addition to C/* (a superset; harmless), and does not enforce the nineteen-continuation-line limit.
  • The nineteen-line limit and the 73–80 sequence field are about the punched-card era; forterp keeps the field-dropping behavior but is otherwise relaxed about line counts.