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3. Program form

3. Program form

FORTRAN 66 was written for the punched card, and its physical form still reflects that: a program is a sequence of 72-column lines, and a few specific columns carry special meaning. This chapter covers the character set a program is written in (§3.1), the column layout of a line (§3.2), how lines group into statements (§3.3), statement labels (§3.4), and the rules for symbolic names (§3.5).

The FORTRAN character set

A program is written using just 49 characters (§3.1): the 26 letters A–Z, the ten digits 0–9, the blank, and twelve special characters:

Char Name Char Name
(blank) blank ( left parenthesis
= equals ) right parenthesis
+ plus , comma
- minus . decimal point
* asterisk $ currency symbol
/ slash

Digits are decimal (§3.1.1), except inside a STOP or PAUSE statement, where a string of digits is read as octal. The blank character has no meaning except in a few specified places (and inside Hollerith data); elsewhere you may use blanks freely to make a program readable (§3.1.4.1). The order in which the characters are listed above does not imply a collating sequence (§3.1) — for that, see Appendix B.

Lines and columns

A line is up to 72 characters from the character set (§3.2). The character positions are columns, numbered 1 to 72 from the left. Each column range has a job:

 column:  1         2 ... 5   6        7 ....................... 72
          +---------+---------+-+------------------------------------+
          | C = comment       | |  the statement text                |
          | or statement label| |  (continuation mark in column 6)   |
          +-------------------+-+------------------------------------+
  • Comment line — a C in column 1 marks the whole line as a comment (§3.2.1). It documents the program and is otherwise ignored.
  • End line — a line blank in columns 1–6 with the letters E, N, D in that order in columns 7–72 (§3.2.2). Every program unit must physically end with one.
  • Initial line — the first line of a statement: not a comment or end line, with a 0 or blank in column 6 (§3.2.3). Columns 1–5 hold the statement label (or are blank).
  • Continuation line — any character other than 0 or blank in column 6 (and not a C in column 1) continues the previous statement (§3.2.4).
C     THIS IS A COMMENT LINE
      X = 1.0 + 2.0 + 3.0 +
     1    4.0 + 5.0
C     -> the "1" in column 6 continues the statement; X = 15.0

Statements

A statement occupies columns 7–72 of an initial line, optionally followed by up to nineteen continuation lines (§3.3) — so a single statement spans at most twenty lines. The text is read by concatenating columns 7–72 of the initial line, then of each continuation line in turn.

Statement labels

A statement may be labeled so other statements can refer to it (§3.4). A label is one to five digits placed anywhere in columns 1–5; its numeric value is not significant but must be greater than zero, and leading zeros do not distinguish labels (010 and 10 are the same label). No two statements in a program unit may carry the same label.

      GO TO 10
      X = 1.0
C     -> skipped
10    X = 2.0
C     -> label 10 is the GO TO target; X = 2.0

Symbolic names

A symbolic name is one to six alphanumeric characters, the first of which must be a letter (§3.5): X, A1, MASS, VALUE2. Names identify variables, arrays, functions, subroutines, and common blocks; the rules for what a name means in each context are in Chapter 5 and Chapter 10.

forterp notes.

  • forterp reads classic fixed-form source: column 1 C comments, the column-6 continuation mark, labels in columns 1–5, and columns past 72 are ignored (the old "identification field" that held card sequence numbers). You can therefore paste real 80-column card images and the sequence numbers in columns 73–80 are harmless.
  • For input typed at a terminal rather than punched, forterp also accepts free CR/LF lines and emulates an 80-column terminal host-side, so you are not forced to pad every line to 72 columns.
  • A name longer than six characters is silently truncated to six — the common historical behavior — so LONGNAME and LONGNAM denote the same variable. forterp does not warn; keep names within six characters to avoid surprise collisions.
  • The standard's only comment marker is C in column 1; forterp also accepts * in column 1 as a comment in every dialect. The ! end-of-line comment is a FORTRAN-10 extension and is rejected under strict F66 — there is no inline comment character in standard FORTRAN 66. See Appendix C.