Editing Fortran (section)

===FORTRAN IV===
IBM began development of '''FORTRAN IV''' in 1961 as a result of customer demands. FORTRAN IV removed the machine-dependent features of FORTRAN II (such as {{code|READ INPUT TAPE}}), while adding new features such as a [[Boolean data type|{{code|LOGICAL}} data type]], logical [[Boolean expression]]s, and the ''logical IF statement'' as an alternative to the ''arithmetic IF statement.''  Type declarations were added, along with an {{code|IMPLICIT}} statement to override earlier conventions that variables are {{code|INTEGER}} if their name begins with {{code|I}}, {{code|J}}, {{code|K}}, {{code|L}}, {{code|M}}, or {{code|N}}; and {{code|REAL}} otherwise.<ref name=s360fortran4>{{cite manual|title=IBM System/360 FORTRAN IV Language, Form C28-6515-6|url=http://www.bitsavers.org/pdf/ibm/360/fortran/C28-6515-6_FORTRAN_IV_Language_1966.pdf|publisher=IBM|date=April 1966}}</ref>{{rp|pp.70,71}}<ref name=":0" />{{rp|p.6-9}}

FORTRAN IV was eventually released in 1962, first for the [[IBM 7030 Stretch|IBM 7030]] ("Stretch") computer, followed by versions for the [[IBM 7090]], [[IBM 7090|IBM 7094]], and later for the [[IBM 1401]] in 1966.<ref>{{cite manual|title=Fortran IV Language Specifications, Program Specifications, and Operating Procedures, IBM 1401, 1440, and 1460|url=http://bitsavers.org/pdf/ibm/1401/C24-3322-2_Fortran_IV_Language_Specifications_IBM_1401_1440_1460_Apr66.pdf |archive-url=https://web.archive.org/web/20170923222805/http://www.bitsavers.org/pdf/ibm/1401/C24-3322-2_Fortran_IV_Language_Specifications_IBM_1401_1440_1460_Apr66.pdf |archive-date=September 23, 2017 |url-status=live|date=April 1966|publisher=IBM|id=C24-3322-2}}</ref>

By 1965, FORTRAN IV was supposed to be compliant with the ''standard'' being developed by the [[American National Standards Institute|American Standards Association]] X3.4.3 FORTRAN Working Group.<ref name="McCracken">{{cite book|last=McCracken|first=Daniel D.|title=A Guide to FORTRAN IV Programming|chapter-url=https://archive.org/details/guidetofortraniv00mccr|chapter-url-access=registration|year=1965|publisher=Wiley|location=New York|isbn=978-0-471-58281-6|page=v|chapter=Preface}}</ref>

Between 1966 and 1968, IBM offered several FORTRAN IV compilers for its [[IBM System/360|System/360]], each named by letters that indicated the minimum amount of memory the compiler needed to run.
<ref>{{cite web
| url = http://www.fortran.bcs.org/2007/jubilee/implementations.php
|website= BCS Fortran Specialist Group 
| title = List of FORTRAN Implementations 1957 – 1967
| publisher = IEEE Annals
| year = 2017
| access-date = October 17, 2017
}}</ref>
The letters (F, G, H) matched the codes used with System/360 model numbers to indicate memory size, each letter increment being a factor of two larger:<ref>{{cite manual|url=http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/A22-6898-1_360-50_funcChar_1967.pdf |via=bitsavers |archive-url=https://web.archive.org/web/20211029211006/http://bitsavers.org/pdf/ibm/360/functional_characteristics/A22-6898-1_360-50_funcChar_1967.pdf |archive-date=October 29, 2021 |url-status=live|title=IBM System/360 Model 50 Functional Characteristics|publisher=IBM|year=1967|id=A22-6898-1}}</ref>{{rp|p. 5}}
* 1966 : FORTRAN IV F for DOS/360 (64K bytes)
* 1966 : FORTRAN IV G for OS/360 (128K bytes)
* 1968 : FORTRAN IV H for OS/360 (256K bytes)

[[Digital Equipment Corporation]] maintained DECSYSTEM-10 Fortran IV (F40) for [[PDP-10]] from 1967 to 1975.<ref name=":0">{{cite web |title=DECSYSTEM-10 FORTRAN IV (F40) Programmers Reference Manual |url=https://github.com/PDP-10/f40/blob/master/doc/DEC-10-LFLMA-B-D%20FORTRAN%20IV%20(F40)%20Programmer's%20Reference%20Manual.pdf |website=Github |publisher=Digital Equipment Corporation |access-date=April 15, 2022}}</ref> Compilers were also available for the [[UNIVAC 1100/2200 series|UNIVAC 1100 series]] and the [[Control Data Corporation|Control Data]] [[CDC 6000 series|6000 series]] and [[CDC 7600|7000 series]] systems.<ref name=":1">{{Cite web |title=FORTRAN IV  |url=https://search.worldcat.org/title/20673993 |access-date=2023-12-10 |website=WorldCat.org |language=en}}</ref>

At about this time FORTRAN IV had started to become an important educational tool and implementations such as the University of Waterloo's WATFOR and [[WATFIV]] were created to simplify the complex compile and link processes of earlier compilers.

In the FORTRAN IV programming environment of the era, except for that used on Control Data Corporation (CDC) systems, only one instruction was placed per line. The CDC version allowed for multiple instructions per line if separated by a {{char|$}} (dollar) character. The FORTRAN [[Punched card|sheet]] was divided into four fields, as described above.

Two compilers of the time, IBM "G" and UNIVAC, allowed comments to be written on the same line as instructions, separated by a special character: "master space": V (perforations 7 and 8) for UNIVAC and perforations 12/11/0/7/8/9 (hexadecimal FF) for IBM. These comments were not to be inserted in the middle of continuation cards.<ref name=":0" /><ref name=":1" />