Editing Fortran (section)

===FORTRAN 66===
Perhaps the most significant development in the early history of FORTRAN was the decision by the ''American Standards Association'' (now [[American National Standards Institute]] (ANSI)) to form a committee sponsored by the [[Business Equipment Manufacturers Association]] (BEMA) to develop an ''American Standard Fortran''.  The resulting two standards, approved in March 1966, defined two languages, ''FORTRAN'' (based on FORTRAN IV, which had served as a de facto standard), and ''Basic FORTRAN'' (based on FORTRAN II, but stripped of its machine-dependent features).  The FORTRAN defined by the first standard, officially denoted X3.9-1966, became known as ''FORTRAN 66'' (although many continued to term it FORTRAN IV, the language on which the standard was largely based).  FORTRAN 66 effectively became the first industry-standard version of FORTRAN. FORTRAN 66 included:
* Main program, {{code|SUBROUTINE}}, {{code|FUNCTION}}, and {{code|BLOCK DATA}} program units
* {{code|INTEGER}}, {{code|REAL}}, {{code|DOUBLE PRECISION}}, {{code|COMPLEX}}, and {{code|LOGICAL}} [[data type]]s
* {{code|COMMON}}, {{code|DIMENSION}}, and {{code|EQUIVALENCE}} statements
* {{code|DATA}} statement for specifying initial values
* [[Intrinsic function|Intrinsic]] and {{code|EXTERNAL}} (e.g., library) functions
* Assignment statement
* {{code|GO TO}}, computed {{code|GO TO}}, assigned {{code|GO TO}}, and {{code|ASSIGN}} statements
* Logical {{code|IF}} and arithmetic (three-way) {{code|IF}} statements
* {{code|DO}} loop statement
* {{code|READ}}, {{code|WRITE}}, {{code|BACKSPACE}}, {{code|REWIND}}, and {{code|ENDFILE}} statements for sequential I/O
* {{code|FORMAT}} statement and assigned format
* {{code|CALL}}, {{code|RETURN}}, {{code|PAUSE}}, and {{code|STOP}} statements
* [[Hollerith constant]]s in {{code|DATA}} and {{code|FORMAT}} statements, and as arguments to procedures
* Identifiers of up to six characters in length
* Comment lines
* {{code|END}} line

The above Fortran II version of the Heron program needs several modifications to compile as a Fortran 66 program.  Modifications include using the more machine independent versions of the {{code|READ}} and {{code|WRITE}} statements, and removal of the unneeded {{code|FLOATF}} type conversion functions.  Though not required, the arithmetic {{code|IF}} statements can be re-written to use logical {{code|IF}} statements and expressions in a more structured fashion.

<syntaxhighlight lang="fortranfixed">
C AREA OF A TRIANGLE WITH A STANDARD SQUARE ROOT FUNCTION
C INPUT - TAPE READER UNIT 5, INTEGER INPUT
C OUTPUT - LINE PRINTER UNIT 6, REAL OUTPUT
C INPUT ERROR DISPLAY ERROR OUTPUT CODE 1 IN JOB CONTROL LISTING
      READ (5, 501) IA, IB, IC
  501 FORMAT (3I5)
C
C IA, IB, AND IC MAY NOT BE NEGATIVE OR ZERO
C FURTHERMORE, THE SUM OF TWO SIDES OF A TRIANGLE
C MUST BE GREATER THAN THE THIRD SIDE, SO WE CHECK FOR THAT, TOO
      IF (IA .GT. 0 .AND. IB .GT. 0 .AND. IC .GT. 0) GOTO 10
        WRITE (6, 602)
  602   FORMAT (42H IA, IB, AND IC MUST BE GREATER THAN ZERO.)
        STOP 1
   10 CONTINUE
C
      IF (IA+IB-IC .GT. 0
     +    .AND. IA+IC-IB .GT. 0
     +    .AND. IB+IC-IA .GT. 0) GOTO 20
        WRITE (6, 603)
  603   FORMAT (50H SUM OF TWO SIDES MUST BE GREATER THAN THIRD SIDE.)
        STOP 1
   20 CONTINUE
C
C USING HERON'S FORMULA WE CALCULATE THE
C AREA OF THE TRIANGLE
      S = (IA + IB + IC) / 2.0
      AREA = SQRT ( S * (S - IA) * (S - IB) * (S - IC))
      WRITE (6, 601) IA, IB, IC, AREA
  601 FORMAT (4H A= ,I5,5H  B= ,I5,5H  C= ,I5,8H  AREA= ,F10.2,
     +        13H SQUARE UNITS)
      STOP
      END
</syntaxhighlight>