ALGOL (short for ALGORITHMIC LANGUAGE) is a family of imperative computer programming languages , originally developed in the mid-1950s, which greatly influenced many other languages and was the standard method for algorithm description used by the ACM in textbooks and academic sources for more than thirty years.
In the sense that the syntax of most modern languages is "Algol-like", it was arguably the most influential of the four high-level programming languages with which it was roughly contemporary: FORTRAN , Lisp , and COBOL . It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages, including PL/I , Simula , BCPL , B , Pascal , and C .
ALGOL introduced code blocks and the begin…end pairs for delimiting them. It was also the first language implementing nested function definitions with lexical scope . Moreover, it was the first programming language which gave detailed attention to formal language definition and through the Algol 60 Report introduced Backus–Naur form , a principal formal grammar notation for language design.
There were three major specifications, named after the year they were first published:
* ALGOL 58 – originally proposed to be called IAL, for International Algebraic Language. * ALGOL 60 – first implemented as X1 ALGOL 60 in mid-1960. Revised 1963. * ALGOL 68 – introduced new elements including flexible arrays, slices, parallelism, operator identification. Revised 1973.
* 1 Important implementations
* 2 History
* 2.1 Algol and programming language research * 2.2 IAL implementations timeline
* 3 Properties
* 4 Examples and portability issues
* 4.1 Code sample comparisons
* 4.2 Timeline: Hello world
* 4.3 Timeline of ALGOL special characters
* 5 See also * 6 References * 7 Further reading * 8 External links
The International Algebraic Language (IAL) was extremely influential and generally considered the ancestor of most of the modern programming languages (the so-called Algol-like languages). Additionally, ALGOL OBJECT CODE was a simple, compact, and stack-based instruction set architecture commonly used in teaching compiler construction and other high order languages (of which Algol is generally considered the first).
ALGOL was developed jointly by a committee of European and American
computer scientists in a meeting in 1958 at
ALGOL was used mostly by research computer scientists in the United States and in Europe. Its use in commercial applications was hindered by the absence of standard input/output facilities in its description and the lack of interest in the language by large computer vendors other than Burroughs Corporation . ALGOL 60 did however become the standard for the publication of algorithms and had a profound effect on future language development.
John Backus developed the Backus normal form method of describing programming languages specifically for ALGOL 58. It was revised and expanded by Peter Naur for ALGOL 60, and at Donald Knuth 's suggestion renamed Backus–Naur form .
Peter Naur: "As editor of the ALGOL Bulletin I was drawn into the international discussions of the language and was selected to be member of the European language design group in November 1959. In this capacity I was the editor of the ALGOL 60 report, produced as the result of the ALGOL 60 meeting in Paris in January 1960."
The following people attended the meeting in Paris (from 1 to 16 January):
* Friedrich L. Bauer , Peter Naur , Heinz Rutishauser , Klaus Samelson , Bernard Vauquois , Adriaan van Wijngaarden , and Michael Woodger (from Europe) * John W. Backus , Julien Green , Charles Katz , John McCarthy , Alan J. Perlis , and Joseph Henry Wegstein (from the USA).
Alan Perlis gave a vivid description of the meeting: "The meetings were exhausting, interminable, and exhilarating. One became aggravated when one's good ideas were discarded along with the bad ones of others. Nevertheless, diligence persisted during the entire period. The chemistry of the 13 was excellent."
ALGOL 60 inspired many languages that followed it. C. A. R. Hoare remarked: "Here is a language so far ahead of its time that it was not only an improvement on its predecessors but also on nearly all its successors." The Scheme programming language, a variant of Lisp that adopted the block structure and lexical scope of ALGOL, also adopted the wording "Revised Report on the Algorithmic Language Scheme" for its standards documents in homage to ALGOL.
ALGOL AND PROGRAMMING LANGUAGE RESEARCH
As Peter Landin noted, the language Algol was the first language to combine seamlessly imperative effects with the (call-by-name) lambda calculus . Perhaps the most elegant formulation of the language is due to John C. Reynolds , and it best exhibits its syntactic and semantic purity. Reynolds's idealized Algol also made a convincing methodological argument regarding the suitability of local effects in the context of call-by-name languages, to be contrasted with the global effects used by call-by-value languages such as ML . The conceptual integrity of the language made it one of the main objects of semantic research, along with PCF and ML.
IAL IMPLEMENTATIONS TIMELINE
To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.
NAME YEAR AUTHOR COUNTRY DESCRIPTION TARGET CPU
JOVIAL 1960 Jules Schwarz USA A DOD HOL prior to Ada Various (see article)
(Several variants) 1961
Burroughs Corporation (with participation by Hoare, Dijkstra , and
Basis of the Burroughs (and now
For ODIN time-sharing system
USS 90 Algol 1962 L. Petrone Italy
Algol Translator 1962 G. van der Mey and W.L. van der Poel Netherlands Staatsbedrijf der Posterijen, Telegrafie en Telefonie ZEBRA
Kidsgrove Algol 1963 F. G. Duncan UK
English Electric Company KDF9
Brian Randell and L. J. Russell
Atomic Power Division of
English Electric Company. Precursor to
NU ALGOL 1965
USSR Minsk-22 АЛГЭК, based on ALGOL-60 and COBOL support, for economical tasks
M ALGOL 1966 publ. A. Viil, M Kotli implementations defined their own in ways that were rarely compatible with each other. In contrast, ALGOL 68 offered an extensive library of transput (input/output) facilities.
ALGOL 60 allowed for two evaluation strategies for parameter passing: the common call-by-value , and call-by-name . Call-by-name has certain effects in contrast to call-by-reference . For example, without specifying the parameters as value or reference, it is impossible to develop a procedure that will swap the values of two parameters if the actual parameters that are passed in are an integer variable and an array that is indexed by that same integer variable. Think of passing a pointer to swap(i, A) in to a function. Now that every time swap is referenced, it is reevaluated. Say i := 1 and A := 2, so every time swap is referenced it'll return the other combination of the values (, , and so on). A similar situation occurs with a random function passed as actual argument.
Call-by-name is known by many compiler designers for the interesting "thunks " that are used to implement it. Donald Knuth devised the "man or boy test " to separate compilers that correctly implemented "recursion and non-local references." This test contains an example of call-by-name.
ALGOL 68 was defined using a two-level grammar formalism invented by Adriaan van Wijngaarden and which bears his name. Van Wijngaarden grammars use a context-free grammar to generate an infinite set of productions that will recognize a particular ALGOL 68 program; notably, they are able to express the kind of requirements that in many other programming language standards are labelled "semantics" and have to be expressed in ambiguity-prone natural language prose, and then implemented in compilers as ad hoc code attached to the formal language parser.
EXAMPLES AND PORTABILITY ISSUES
CODE SAMPLE COMPARISONS
(The way the bold text has to be written depends on the implementation, e.g. 'INTEGER' -- quotation marks included -- for INTEGER. This is known as stropping .)
PROCEDURE Absmax(a) Size:(n, m) Result:(y) Subscripts:(i, k); VALUE n, m; ARRAY a; INTEGER n, m, i, k; REAL y; COMMENT The absolute greatest element of the matrix a, of size n by m is transferred to y, and the subscripts of this element to i and k; BEGIN INTEGER p, q; y := 0; i := k := 1; FOR p := 1 STEP 1 UNTIL n DO FOR q := 1 STEP 1 UNTIL m DO IF abs(a) > y THEN BEGIN y := abs(a); i := p; k := q END END Absmax
Here's an example of how to produce a table using Elliott 803 ALGOL.
FLOATING POINT ALGOL TEST' BEGIN REAL A,B,C,D' READ D' FOR A:= 0.0 STEP D UNTIL 6.3 DO BEGIN PRINT PUNCH(3),££L??' B := SIN(A)' C := COS(A)' PRINT PUNCH(3),SAMELINE,ALIGNED(1,6),A,B,C' END' END'
PUNCH(3) sends output to the teleprinter rather than the tape punch. SAMELINE suppresses the carriage return + line feed normally printed between arguments. ALIGNED(1,6) controls the format of the output with 1 digit before and 6 after the decimal point.
The following code samples are ALGOL 68 versions of the above ALGOL 60 code samples.
PROC abs max = (REAL a, REF REAL y, REF INT i, k)REAL: COMMENT The absolute greatest element of the matrix a, of size ⌈a by 2⌈a is transferred to y, and the subscripts of this element to i and k; COMMENT BEGIN REAL y := 0; i := ⌊a; k := 2⌊a; FOR p FROM ⌊a TO ⌈a DO FOR q FROM 2⌊a TO 2⌈a DO IF ABS a > y THEN y := ABS a; i := p; k := q FI OD OD; y END # abs max #
Note: lower (⌊) and upper (⌈) bounds of an array, and array slicing, are directly available to the programmer.
floating point algol68 test: ( REAL a,b,c,d; # printf - sends output to the FILE stand out. # # printf($p$); – selects a new page # printf(($pg$,"Enter d:")); read(d); FOR step FROM 0 WHILE a:=step*d; a
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