Editing Fortran (section)

===FORTRAN 77===
[[File:Ftn-elim-1240x1709.jpg|thumb|upright|FORTRAN-77 program with compiler output, written on a [[Control Data Corporation|CDC]] [[CDC Cyber|175]] at [[RWTH Aachen University]], Germany, in 1987]]
[[File:4.3 BSD UWisc VAX Emulation f77 Manual.png|thumb|[[4.3BSD|4.3 BSD]] for the [[Digital Equipment Corporation]] (DEC) [[VAX]], displaying the [[man page|manual]] for FORTRAN 77 (f77) compiler]]

After the release of the FORTRAN 66 standard, compiler vendors introduced several extensions to ''Standard Fortran'', prompting ANSI committee X3J3 in 1969 to begin work on revising the 1966 standard, under sponsorship of [[CBEMA]], the Computer Business Equipment Manufacturers Association (formerly BEMA).  Final drafts of this revised standard circulated in 1977, leading to formal approval of the new FORTRAN standard in April 1978.  The new standard, called ''FORTRAN 77'' and officially denoted X3.9-1978, added a number of significant features to address many of the shortcomings of FORTRAN 66:

* Block {{code|IF}} and {{code|END IF}} statements, with optional {{code|ELSE IF}} and {{code|ELSE}} clauses, to provide improved language support for [[structured programming]]
* {{code|DO}} loop extensions, including parameter expressions, negative increments, and zero trip counts
* {{code|OPEN}}, {{code|CLOSE}}, and {{code|INQUIRE}} statements for improved I/O capability
* Direct-access file I/O
* {{code|CHARACTER}} data type, replacing Hollerith strings with vastly expanded facilities for character input and output and processing of character-based data
* {{code|PARAMETER}} statement for specifying constants
* {{code|SAVE}} statement for persistent local variables
* Generic names for intrinsic functions (e.g. {{code|SQRT}} also accepts arguments of other types, such as {{code|COMPLEX}} or {{code|REAL*16}}).
* A set of intrinsics ({{code|LGE}}, {{code|LGT}}, {{code|LLE}}, {{code|LLT}}) for ''lexical'' comparison of strings, based upon the [[ASCII]] [[collating sequence]]. (These ASCII functions were demanded by the [[United States Department of Defense|U.S. Department of Defense]], in their conditional approval vote.{{Citation needed|date=October 2011}})
* A maximum of seven dimensions in arrays, rather than three.  Allowed subscript expressions were also generalized.

In this revision of the standard, a number of features were removed or altered in a manner that might invalidate formerly standard-conforming programs.
(Removal was the only allowable alternative to X3J3 at that time, since the concept of "[[deprecation]]" was not yet available for ANSI standards.)
While most of the 24 items in the conflict list (see Appendix A2 of X3.9-1978) addressed loopholes or pathological cases permitted by the prior standard but rarely used, a small number of specific capabilities were deliberately removed, such as:

* [[Hollerith constant]]s and [[Herman Hollerith|Hollerith]] data, such as <syntaxhighlight lang="fortran" inline>      GREET = 12HHELLO THERE!</syntaxhighlight>
* Reading into an H edit (Hollerith field) descriptor in a FORMAT specification
* Overindexing of array bounds by subscripts <syntaxhighlight lang="fortranfixed">
      DIMENSION A(10,5)
      Y = A(11,1)
</syntaxhighlight>
* Transfer of control out of and back into the range of a DO loop (also known as "Extended Range")

A Fortran 77 version of the Heron program requires no modifications to the Fortran 66 version.  However this example demonstrates additional cleanup of the I/O statements, including using list-directed I/O, and replacing the Hollerith edit descriptors in the {{code|FORMAT}} statements with quoted strings.  It also uses structured {{code|IF}} and {{code|END IF}} statements, rather than {{code|GOTO}}/{{code|CONTINUE}}.

<syntaxhighlight lang="fortranfixed">
      PROGRAM HERON
C AREA OF A TRIANGLE WITH A STANDARD SQUARE ROOT FUNCTION
C INPUT - DEFAULT STANDARD INPUT UNIT, INTEGER INPUT
C OUTPUT - DEFAULT STANDARD OUTPUT UNIT, REAL OUTPUT
C INPUT ERROR DISPLAY ERROR OUTPUT CODE 1 IN JOB CONTROL LISTING
      READ (*, *) IA, IB, IC
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 .LE. 0 .OR. IB .LE. 0 .OR. IC .LE. 0) THEN
        WRITE (*, *) 'IA, IB, and IC must be greater than zero.'
        STOP 1
      END IF
C
      IF (IA+IB-IC .LE. 0
     +    .OR. IA+IC-IB .LE. 0
     +    .OR. IB+IC-IA .LE. 0) THEN
        WRITE (*, *) 'Sum of two sides must be greater than third side.'
        STOP 1
      END IF
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 (*, 601) IA, IB, IC, AREA
  601 FORMAT ('A= ', I5, '  B= ', I5, '  C= ', I5, '  AREA= ', F10.2,
     +        ' square units')
      STOP
      END</syntaxhighlight>