|Developer:||John Backus & IBM|
|Latest Release Version:||Fortran 2008 (ISO/IEC 1539-1:2010)|
|Latest Release Date:||2010|
|Typing:||strong, static, manifest|
|Implementations:||Absoft, Cray, GFortran, G95, IBM, Intel, Lahey/Fujitsu, Open Watcom, Pathscale, PGI, Silverfrost, Oracle, XL Fortran, Visual Fortran, others|
|Influenced:||ALGOL 58, BASIC, C, PL/I, PACT I, MUMPS, Ratfor|
Fortran (previously FORTRAN) is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing. Originally developed by IBM at their campus in south San Jose, California in the 1950s for scientific and engineering applications, Fortran came to dominate this area of programming early on and has been in continual use for over half a century in computationally intensive areas such as numerical weather prediction, finite element analysis, computational fluid dynamics, computational physics and computational chemistry. It is one of the most popular languages in the area of high-performance computing  and is the language used for programs that benchmark and rank the world's fastest supercomputers.
Fortran (a blend derived from The IBM Mathematical Formula Translating System) encompasses a lineage of versions, each of which evolved to add extensions to the language while usually retaining compatibility with previous versions. Successive versions have added support for processing of character-based data (FORTRAN 77), array programming, modular programming and object-oriented programming (Fortran 90 / 95), and object-oriented and generic programming (Fortran 2003).
The names of earlier versions of the language through FORTRAN 77 were conventionally spelled in all-caps (FORTRAN 77 was the version in which the use of lowercase letters in keywords was strictly nonstandard). The capitalization has been dropped in referring to newer versions beginning with Fortran 90. The official language standards now refer to the language as "Fortran". Because the capitalization was never completely consistent in actual usage, this article adopts the convention of using the all-caps FORTRAN in referring to versions of FORTRAN through FORTRAN 77 and the title-caps Fortran in referring to versions of Fortran from Fortran 90 onward. This convention is reflected in the capitalization of FORTRAN in the ANSI X3.9-1966 (FORTRAN 66) and ANSI X3.9-1978 (FORTRAN 77) standards and the title caps Fortran in the ANSI X3.198-1992 (Fortran 90), ISO/IEC 1539-1:1997 (Fortran 95) and ISO/IEC 1539-1:2004 (Fortran 2003) standards.
In late 1953, John W. Backus submitted a proposal to his superiors at IBM to develop a more practical alternative to assembly language for programming their IBM 704 mainframe computer. Backus' historic FORTRAN team consisted of programmers Richard Goldberg, Sheldon F. Best, Harlan Herrick, Peter Sheridan, Roy Nutt, Robert Nelson, Irving Ziller, Lois Haibt, and David Sayre.
A draft specification for The IBM Mathematical Formula Translating System was completed by mid-1954. The first manual for FORTRAN appeared in October 1956, with the first FORTRAN compiler delivered in April 1957. This was the first optimizing compiler, because customers were reluctant to use a high-level programming language unless its compiler could generate code whose performance was comparable to that of hand-coded assembly language.
While the community was skeptical that this new method could possibly outperform hand-coding, it reduced the number of programming statements necessary to operate a machine by a factor of 20, and quickly gained acceptance. John Backus said during a 1979 interview with Think, the IBM employee magazine, "Much of my work has come from being lazy. I didn't like writing programs, and so, when I was working on the IBM 701, writing programs for computing missile trajectories, I started work on a programming system to make it easier to write programs."
The language was widely adopted by scientists for writing numerically intensive programs, which encouraged compiler writers to produce compilers that could generate faster and more efficient code. The inclusion of a complex number data type in the language made Fortran especially suited to technical applications such as electrical engineering.
By 1960, versions of FORTRAN were available for the IBM 709, 650, 1620, and 7090 computers. Significantly, the increasing popularity of FORTRAN spurred competing computer manufacturers to provide FORTRAN compilers for their machines, so that by 1963 over 40 FORTRAN compilers existed. For these reasons, FORTRAN is considered to be the first widely used programming language supported across a variety of computer architectures.
The development of FORTRAN paralleled the early evolution of compiler technology, and many advances in the theory and design of compilers were specifically motivated by the need to generate efficient code for FORTRAN programs.
The initial release of FORTRAN for the IBM 704 contained 32 statements, including:
IFstatements for checking exceptions (
QUOTIENT OVERFLOW, and
DIVIDE CHECK); and
IFstatements for manipulating sense switches and sense lights
ASSIGN, and assigned
READ INPUT TAPE,
WRITE OUTPUT TAPE,
WRITE TAPE, and
FREQUENCYstatement (for providing optimization hints to the compiler).
IF statement was similar to a three-way branch instruction on the IBM 704. However, the 704 branch instructions all contained only one destination address (e.g., TZE — Transfer AC Zero, TNZ — Transfer AC Not Zero, TPL — Transfer AC Plus, TMI — Transfer AC Minus). The machine (and its successors in the 700/7000 series) did have a three-way skip instruction (CAS — Compare AC with Storage), but using this instruction to implement the
IF would consume 4 instruction words, require the constant Zero in a word of storage, and take 3 machine cycles to execute; using the Transfer instructions to implement the
IF could be done in 1 to 3 instruction words, required no constants in storage, and take 1 to 3 machine cycles to execute. An optimizing compiler like FORTRAN would most likely select the more compact and usually faster Transfers instead of the Compare (use of Transfers also allowed the
FREQUENCY statement to optimize
IFs, which could not be done using the Compare). Also the Compare considered −0 and +0 to be different values while the Transfer Zero and Transfer Not Zero considered them to be the same.
FREQUENCY statement in FORTRAN was used originally and optionally to give branch probabilities for the three branch cases of the Arithmetic IF statement to bias the way code was generated and order of the basic blocks of code generated, in the global optimisation sense, were arranged in memory for optimality. The first FORTRAN compiler used this weighting to do a Monte Carlo simulation of the run-time generated code at compile time. It was very sophisticated for its time. This technique is documented in the original article in 1957 on the first FORTRAN compiler implementation by J. Backus et al. Many years later, the
FREQUENCY statement had no effect on the code, and was treated as a comment statement, since the compilers no longer did this kind of compile-time simulation.
Below is a part of the 1957 paper, "The FORTRAN Automatic Coding System" by Backus et al., with this snippet on the FREQUENCY statement and its use in a compile-time Monte Carlo simulation of the run-time to optimise the code generated. Quoting …
The fundamental unit of program is the basic block; a basic block is a stretch of program which has a single entry point and a single exit point. The purpose of section 4 is to prepare for section 5 a table of predecessors (PRED table) which enumerates the basic blocks and lists for every basic block each of the basic blocks which can be its immediate predecessor in flow, together with the absolute frequency of each such basic block link. This table is obtained by an actual "execution" of the program in Monte-Carlo fashion, in which the outcome of conditional transfers arising out of IF-type statements and computed GO TO'S is determined by a random number generator suitably weighted according to whatever FREQUENCY statements have been provided.
Before the development of disk files, text editors and terminals, programs were most often entered on a keypunch keyboard onto 80 column punched cards, one line to a card. The resulting deck of cards would be fed into a card reader to be compiled. See Computer programming in the punched card era.
Originally Fortran programs were written in a fixed column format. Columns 1 to 5 were the label field: a sequence of digits here was taken as a label for the purpose of a GOTO or a FORMAT reference in a WRITE or READ statement. A letter "C" in column 1 caused the entire card to be treated as a comment and ignored by the compiler. Column 6 was a continuation field: a non-blank character here caused the card to be taken as a continuation of the statement on the previous card. Columns 7 to 72 served as the statement field. Columns 73 to 80 were ignored, so they could be used for identification information. One such use was punching a sequence number which could be used to re-order cards if a stack of cards was dropped, though in practice this was reserved for stable, production programs. An IBM 519 could be used to copy a program deck and add sequence numbers. Some early compilers, e.g. the IBM 650's, had additional restrictions. Later compilers relaxed most fixed format restrictions.
Within the statement field, whitespace characters were generally ignored, allowing the programmer to omit space between tokens for brevity, or include spaces within identifiers for clarity (for example,
AVG OF X was a valid identifier, and equivalent to
Early FORTRAN compilers did not support recursion in subroutines. Early computer architectures did not support the concept of a stack, and when they did directly support subroutine calls, the return location was often stored in a single fixed location adjacent to the subroutine code, which does not permit a subroutine to be called again before a previous call of the subroutine has returned. Although not specified in Fortran 77, many F77 compilers supported recursion as an option, while it became a standard in Fortran 90.
IBM's FORTRAN II appeared in 1958. The main enhancement was to support procedural programming by allowing user-written subroutines and functions which returned values, with parameters passed by reference. The COMMON statement provided a way for subroutines to access common (or global) variables. Six new statements were introduced:
Over the next few years, FORTRAN II would also add support for the
DOUBLE PRECISION and
COMPLEX data types.
This program, for Heron's formula, reads one data card containing three 5-digit integers A, B, and C as input. If A, B, and C cannot represent the sides of a triangle in plane geometry, then the program's execution will end with an error code of "STOP 1". Otherwise, an output line will be printed showing the input values for A, B, and C, followed by the computed AREA of the triangle as a floating-point number with 2 digits after the decimal point.
C AREA OF A TRIANGLE WITH A STANDARD SQUARE ROOT FUNCTION C INPUT - CARD 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 INPUT TAPE 5, 501, IA, IB, IC 501 FORMAT (3I5) C IA, IB, AND IC MAY NOT BE NEGATIVE C FURTHERMORE, THE SUM OF TWO SIDES OF A TRIANGLE C IS GREATER THAN THE THIRD SIDE, SO WE CHECK FOR THAT, TOO IF (IA) 777, 777, 701 701 IF (IB) 777, 777, 702 702 IF (IC) 777, 777, 703 703 IF (IA+IB-IC) 777,777,704 704 IF (IA+IC-IB) 777,777,705 705 IF (IB+IC-IA) 777,777,799 777 STOP 1 C USING HERON'S FORMULA WE CALCULATE THE C AREA OF THE TRIANGLE 799 S = FLOATF (IA + IB + IC) / 2.0 AREA = SQRT(S * (S - FLOATF(IA)) * (S - FLOATF(IB)) * + (S - FLOATF(IC))) WRITE OUTPUT TAPE 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
IBM also developed a FORTRAN III in 1958 that allowed for inline assembler code among other features; however, this version was never released as a product. Like the 704 FORTRAN and FORTRAN II, FORTRAN III included machine-dependent features that made code written in it unportable from machine to machine. Early versions of FORTRAN provided by other vendors suffered from the same disadvantage.
FORTRAN was provided for the IBM 1401 by an innovative 63-pass compiler that ran in only 8k of core. It kept the program in memory and loaded overlays that gradually transformed it, in place, into executable form, as described by Haines et al. The executable form was not machine language; rather it was interpreted, anticipating UCSD Pascal P-code by two decades.
Starting in 1961, as a result of customer demands, IBM began development of a FORTRAN IV that removed the machine-dependent features of FORTRAN II (such as
READ INPUT TAPE), while adding new features such as a
LOGICAL data type, logical Boolean expressions and the logical IF statement as an alternative to the arithmetic IF statement. FORTRAN IV was eventually released in 1962, first for the IBM 7030 ("Stretch") computer, followed by versions for the IBM 7090 and IBM 7094.
Perhaps the most significant development in the early history of FORTRAN was the decision by the American Standards Association (now ANSI) to form a committee 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 became known as FORTRAN 66 (although many continued to refer to it as FORTRAN IV, the language upon which the standard was largely based). FORTRAN 66 effectively became the first "industry-standard" version of FORTRAN. FORTRAN 66 included:
BLOCK DATAprogram units
DATAstatement for specifying initial values
EXTERNAL(e.g., library) functions
GOTO, and computed
IFand arithmetic (three-way)
ENDFILEstatements for sequential I/O
FORMATstatements, and as actual arguments to procedures
After the release of the FORTRAN 66 standard, compiler vendors introduced a number of extensions to "Standard Fortran", prompting ANSI in 1969 to begin work on revising the 1966 standard. Final drafts of this revised standard circulated in 1977, leading to formal approval of the new FORTRAN standard in April 1978. The new standard, known as FORTRAN 77, added a number of significant features to address many of the shortcomings of FORTRAN 66:
END IFstatements, with optional
ELSE IFclauses, to provide improved language support for structured programming
INQUIREstatements for improved I/O capability
CHARACTERdata type, with vastly expanded facilities for character input and output and processing of character-based data
PARAMETERstatement for specifying constants
SAVEstatement for persistent local variables
LGE, LGT, LLE, LLT) for lexical comparison of strings, based upon the ASCII collating sequence. (These ASCII functions were demanded by the U.S. Department of Defense, in their conditional approval vote.)
In this revision of the standard, a number of features were removed or altered in a manner that might invalidate previously 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 previous standard but rarely used, a small number of specific capabilities were deliberately removed, such as:
GREET = 12HHELLO THERE!
Control Data Corporation computers had another version of FORTRAN 77, called Minnesota FORTRAN (MNF), designed especially for student use, with variations in output constructs, special uses of COMMONs and DATA statements, optimizations code levels for compiling, and detailed error listings, extensive warning messages, and debugs.
The development of a revised standard to succeed FORTRAN 77 would be repeatedly delayed as the standardization process struggled to keep up with rapid changes in computing and programming practice. In the meantime, as the "Standard FORTRAN" for nearly fifteen years, FORTRAN 77 would become the historically most important dialect.
An important practical extension to FORTRAN 77 was the release of MIL-STD-1753 in 1978. This specification, developed by the U.S. Department of Defense, standardized a number of features implemented by most FORTRAN 77 compilers but not included in the ANSI FORTRAN 77 standard. These features would eventually be incorporated into the Fortran 90 standard.
IMPLICIT NONEvariant of the
The IEEE 1003.9 POSIX Standard, released in 1991, provided a simple means for FORTRAN 77 programmers to issue POSIX system calls. Over 100 calls were defined in the document — allowing access to POSIX-compatible process control, signal handling, file system control, device control, procedure pointing, and stream I/O in a portable manner.
The much delayed successor to FORTRAN 77, informally known as Fortran 90 (and prior to that, Fortran 8X), was finally released as an ISO standard in 1991 and an ANSI Standard in 1992. This major revision added many new features to reflect the significant changes in programming practice that had evolved since the 1978 standard:
WHEREstatement for selective array assignment
ALLOCATABLEattribute and the
POINTERattribute, pointer assignment, and
NULLIFYstatement to facilitate the creation and manipulation of dynamic data structures
END DOstatement for loop termination, and
CYCLEstatements for "breaking out" of normal
DOloop iterations in an orderly way
SELECT. . .
CASEconstruct for multi-way selection
Unlike the previous revision, Fortran 90 did not delete any features.(Appendix B.1 says, "The list of deleted features in this standard is empty.")Any standard-conforming FORTRAN 77 program is also standard-conforming under Fortran 90,and either standard should be usable to define its behavior.
A small set of features were identified as "obsolescent"and expected to be removed in a future standard.
|Obsolescent feature||Example||Status / Fate in Fortran 95|
|Non-integer DO parameters or control variables||Deleted|
|Shared DO-loop termination or |
termination with a statement
other than END DO or CONTINUE
|Branching to END IF |
from outside a block
|ASSIGN statement |
and assigned GO TO statement
|Assigned FORMAT specifiers||Deleted|
|H edit descriptors||Deleted|
|Computed GO TO statement||(obsolete)|
|DATA statements |
among executable statements
|CHARACTER* form of CHARACTER declaration||(obsolete)|
|Assumed character length functions|
|Fixed form source code|
program helloworld write (*,*) "Hello, world." end program helloworld
Fortran 95 was a minor revision, mostly to resolve some outstanding issues from the Fortran 90 standard. Nevertheless, Fortran 95 also added a number of extensions, notably from the High Performance Fortran specification:
WHEREconstructs to aid vectorization
ALLOCATABLEarrays are automatically deallocated when they go out of scope.
A number of intrinsic functions were extended (for example a
dim argument was added to the
Several features noted in Fortran 90 to be deprecated were removed from Fortran 95:
END IFstatement from outside its block
GOTOstatement, and assigned format specifiers
An important supplement to Fortran 95 was the ISO technical report TR-15581: Enhanced Data Type Facilities, informally known as the Allocatable TR. This specification defined enhanced use of
ALLOCATABLE arrays, prior to the availability of fully Fortran 2003-compliant Fortran compilers. Such uses include
ALLOCATABLE arrays as derived type components, in procedure dummy argument lists, and as function return values. (
ALLOCATABLE arrays are preferable to
POINTER-based arrays because
ALLOCATABLE arrays are guaranteed by Fortran 95 to be deallocated automatically when they go out of scope, eliminating the possibility of memory leakage. In addition, aliasing is not an issue for optimization of array references, allowing compilers to generate faster code than in the case of pointers.)
Another important supplement to Fortran 95 was the ISO technical report TR-15580: Floating-point exception handling, informally known as the IEEE TR. This specification defined support for IEEE floating-point arithmetic and floating point exception handling.
In addition to the mandatory "Base language"(defined in ISO/IEC 1539-1 : 1997),the Fortran 95 language also includes two optional modules:
which, together, comprise the multi-part International Standard (ISO/IEC 1539).
According to the standards developers, "the optional parts describe self-contained features which have been requested by a substantial body of users and/or implementors, but which are not deemed to be of sufficient generality for them to be required in all standard-conforming Fortran compilers." Nevertheless, if a standard-conforming Fortran does provide such options, then they "must be provided in accordance with the description of those facilities in the appropriate Part of the Standard."
Fortran 2003 is a major revision introducing many new features. A comprehensive summary of the new features of Fortran 2003 is available at the Fortran Working Group (WG5) official Web site.
From that article, the major enhancements for this revision include:
VOLATILEattribute, explicit type specification in array constructors and allocate statements, pointer enhancements, extended initialization expressions, and enhanced intrinsic procedures.
FLUSHstatement, regularization of keywords, and access to error messages.
An important supplement to Fortran 2003 was the ISO technical report TR-19767: Enhanced module facilities in Fortran. This report provided submodules, which make Fortran modules more similar to Modula-2 modules. They are similar to Ada private child subunits. This allows the specification and implementation of a module to be expressed in separate program units, which improves packaging of large libraries, allows preservation of trade secrets while publishing definitive interfaces, and prevents compilation cascades.
The most recent standard, ISO/IEC 1539-1:2010, informally known as Fortran 2008, was approved in September 2010. As with Fortran 95, this is a minor upgrade, incorporating clarifications and corrections to Fortran 2003, as well as introducing a select few new capabilities. The new capabilities include:
The Final Draft international Standard (FDIS) is available as document N1830.
Since Fortran has been in use for more than fifty years, there is a vast body of Fortran in daily use throughout the scientific and engineering communities. It is the primary language for some of the most intensive supercomputing tasks, such as weather and climate modeling, computational fluid dynamics, computational chemistry, computational economics, animal breeding, plant breeding and computational physics. Even today, half a century later, many of the floating-point benchmarks to gauge the performance of new computer processors are still written in Fortran (e.g., CFP2006, the floating-point component of the SPEC CPU2006 benchmarks).
See main article: Fortran language features.
Portability was a problem in the early days because there was no agreed standard—not even IBM's reference manual—and computer companies vied to differentiate their offerings from others by providing incompatible features. Standards have improved portability. The 1966 standard provided a reference syntax and semantics, but vendors continued to provide incompatible extensions. Although careful programmers were coming to realize that use of incompatible extensions caused expensive portability problems, and were therefore using programs such as The PFORT Verifier, it was not until after the 1977 standard, when the National Bureau of Standards (now NIST) published FIPS PUB 69, that processors purchased by the U.S. Government were required to diagnose extensions of the standard. Rather than offer two processors, essentially every compiler eventually had at least an option to diagnose extensions.
Incompatible extensions were not the only portability problem. For numerical calculations, it is important to take account of the characteristics of the arithmetic. This was addressed by Fox et al. in the context of the 1966 standard by the PORT library. The ideas therein became widely used, and were eventually incorporated into the 1990 standard by way of intrinsic inquiry functions. The widespread (now almost universal) adoption of the IEEE 754 standard for binary floating-point arithmetic has essentially removed this problem.
Access to the computing environment (e.g. the program's command line, environment variables, textual explanation of error conditions) remained a problem until it was addressed by the 2003 standard.
Large collections of "library" software that could be described as being loosely related to engineering and scientific calculations, such as graphics libraries, have been written in C, and therefore access to them presented a portability problem. This has been addressed by incorporation of C interoperability into the 2003 standard.
It is now possible (and relatively easy) to write an entirely portable program in Fortran, even without recourse to a preprocessor.
Fortran 5 was a programming language marketed by Data General Corp in the late 1970s and early 80s, for the Nova, Eclipse, and MV line of computers. It had an optimizing compiler that was quite good for minicomputers of its time. The language most closely resembles Fortran 66. The name is a pun on the earlier Fortran IV.
Univac also offered a compiler for the 1100 series known as Fortran V. A spinoff of Univac Fortran V was Athena Fortran.
Fortran 6 or Visual Fortran 2001 was licensed to Compaq by Microsoft. They have licensed Compaq Visual Fortran and have provided the Visual Studio 5 environment interface for Compaq v6 up to v6.1.
Vendors of high-performance scientific computers (e.g., Burroughs, CDC, Cray, Honeywell, IBM, Texas Instruments, and UNIVAC) added extensions to Fortran to take advantage of special hardware features such as instruction cache, CPU pipelines, and vector arrays. For example, one of IBM's FORTRAN compilers (H Extended IUP) had a level of optimization which reordered the machine code instructions to keep multiple internal arithmetic units busy simultaneously. Another example is CFD, a special variant of Fortran designed specifically for the ILLIAC IV supercomputer, running at NASA's Ames Research Center.IBM Research Labs also developed an extended FORTRAN-based language called "VECTRAN"for processing of vectors and matrices.
Object-Oriented Fortran was an object-oriented extension of Fortran, in which data items can be grouped into objects, which can be instantiated and executed in parallel. It was available for Sun, Iris, iPSC, and nCUBE, but is no longer supported.
Such machine-specific extensions have either disappeared over time or have had elements incorporated into the main standards; the major remaining extension is OpenMP, which is a cross-platform extension for shared memory programming. One new extension, Co-array Fortran, is intended to support parallel programming.
"FOR TRANSIT" was the name of a reduced version of the IBM 704 FORTRAN language,which was implemented for the IBM 650, using a translator program developedat Carnegie in the late 1950s.The following comment appears in the IBMReference Manual ("FOR TRANSIT Automatic Coding System" C28-4038, Copyright 1957, 1959 by IBM):
The FORTRAN system was designed for a more complex machine than the 650, and consequently some of the 32 statements found in the FORTRAN Programmer's Reference Manual are not acceptable to the FOR TRANSIT system. In addition, certain restrictions to the FORTRAN language have been added. However, none of these restrictions make a source program written for FOR TRANSIT incompatible with the FORTRAN system for the 704.
The permissible statements were:
Arithmetic assignment statements, e.g. a = b
GO to n
GO TO (n1, n2, ..., nm), i
IF (a) n1, n2, n3
DO n i = m1, m2
READ n, list
PUNCH n, list
DIMENSION V, V, V, ...
EQUIVALENCE (a,b,c), (d,c), ...Up to ten subroutines could be used in one program.
FOR TRANSIT statements were limited to columns 7 thru 56, only.Punched cards were used for input and output on the IBM 650. Three passes were required to translate source code to the "IT" language, then to compile the IT statements into SOAP assembly language, and finally to produce the object program, which could then be loaded into the machine to run the program (using punched cards for data input, and outputting results onto punched cards).
Two versions existed for the 650s with a 2000 word memory drum: FOR TRANSIT I (S) and FOR TRANSIT II, the latter for machines equipped with indexing registers and automatic floating point decimal (bi-quinary) arithmetic. Appendix A of the manual included wiring diagrams for the IBM 533 card reader/punch control panel.
Prior to FORTRAN 77, a number of preprocessors were commonly used to provide a friendlier language, with the advantage that the preprocessed code could be compiled on any machine with a standard FORTRAN compiler. Popular preprocessors included FLECS, MORTRAN, SFtran, S-Fortran, Ratfor, and Ratfiv. Ratfor and Ratfiv, for example, implemented a C-like language, outputting preprocessed code in standard FORTRAN 66.
LRLTRAN was developed at the Lawrence Radiation Laboratory to provide support for vector arithmetic and dynamic storage, among other extensions to support systems programming. The distribution included the LTSS operating system.
The Fortran-95 Standard includes an optional Part 3 which defines an optional conditional compilation capability. This capability is often referred to as "CoCo".
Many Fortran compilers have integrated subsets of the C preprocessor into their systems.
SIMSCRIPT is an application specific Fortran preprocessor for modeling and simulating large discrete systems.
The F programming language was designed to be a clean subset of Fortran 95 that attempted to remove the redundant, unstructured, and deprecated features of Fortran, such as the
EQUIVALENCE statement. F retains the array features added in Fortran 90, and removes control statements that were made obsolete by structured programming constructs added to both Fortran 77 and Fortran 90. F is described by its creators as "a compiled, structured, array programming language especially well suited to education and scientific computing."
The following program illustrates dynamic memory allocation and array-based operations, two features introduced with Fortran 90. Particularly noteworthy is the absence of
DO loops and
THEN statements in manipulating the array; mathematical operations are applied to the array as a whole. Also apparent is the use of descriptive variable names and general code formatting that conform with contemporary programming style. This example computes an average over data entered interactively.
program average ! Read in some numbers and take the average ! As written, if there are no data points, an average of zero is returned ! While this may not be desired behavior, it keeps this example simple implicit none real, dimension(:), allocatable :: points integer :: number_of_points real :: average_points=0., positive_average=0., negative_average=0. write (*,*) "Input number of points to average:" read (*,*) number_of_points allocate (points(number_of_points)) write (*,*) "Enter the points to average:" read (*,*) points ! Take the average by summing points and dividing by number_of_points if (number_of_points > 0) average_points = sum(points) / number_of_points ! Now form average over positive and negative points only if (count(points > 0.) > 0) then positive_average = sum(points, points > 0.) / count(points > 0.) end if if (count(points < 0.) > 0) then negative_average = sum(points, points < 0.) / count(points < 0.) end if deallocate (points) ! Print result to terminal write (*,'(a,g12.4)') 'Average = ', average_points write (*,'(a,g12.4)') 'Average of positive points = ', positive_average write (*,'(a,g12.4)') 'Average of negative points = ', negative_average end program average
During the same Fortran Standards Committee meeting at which the name "FORTRAN 77" was chosen, a technical proposal was incorporated into the official distribution, bearing the title, "Letter O considered harmful". This proposal purported to address the confusion that sometimes arises between the letter "O" and the numeral zero, by eliminating the letter from allowable variable names. However, the method proposed was to eliminate the letter from the character set entirely (thereby retaining 48 as the number of lexical characters, which the colon had increased to 49). This was considered beneficial in that it would promote structured programming, by making it impossible to use the notorious
GO TO statement as before. (Troublesome
FORMAT statements would also be eliminated.) It was noted that this "might invalidate some existing programs" but that most of these "probably were non-conforming, anyway". 
During the standards committee battle over whether the "minimum trip count" for the FORTRAN 77 "DO" statement should be zero (allowing no execution of the block) or one (the "plunge-ahead" DO), another facetious alternative was proposed (by Loren Meissner) to have the minimum trip be two—since there is no need for a loop if it is only executed once.