ALGOL

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For other uses of "Algol", see Algol (disambiguation).
ALGOL
Paradigm procedural, imperative, structured
Appeared in 1958
Designed by Bauer, Bottenbruch, Rutishauser, Samelson, Backus, Katz, Perlis, Wegstein, Naur, Vauquois, van Wijngaarden, Woodger, Green, McCarthy
Influenced Most subsequent imperative languages (so-called ALGOL-like languages)

ALGOL (short for ALGOrithmic Language)[1] is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years[2]. It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages (including BCPL, B, C, Simula and Pascal). ALGOL introduced code blocks and was the first language to use begin end pairs for delimiting them. Fragments of ALGOL-like syntax are sometimes still used as a notation for algorithms, so-called Pidgin Algol.

There were three major specifications:

  • ALGOL 58 - originally proposed to be called IAL (for International Algorithmic Language).
  • ALGOL 60, described in detail on this page - revised 1963[3]
  • ALGOL 68 - revised 1973[4]

Niklaus Wirth based his own Algol-W on ALGOL 60 before moving to develop Pascal. Algol-W was intended to be the next generation ALGOL but the ALGOL 68 committee decided on a design that was more complex and advanced rather than a cleaned simplified ALGOL 60. The official ALGOL versions are named after the year they were first published.

Algol68 is substantially different from Algol60 but was not well received so that in general "Algol" means dialects of Algol60.

Contents

[edit] Import and Implementations

The International Algorithmic Language (IAL) was extremely influential and is generally considered the ancestor of most of the modern programming languages (the so-called Algol-like languages). The Burroughs corporation built their line of computers to directly execute it. Additionally, in computer science, ALGOL object code was a simple and compact and stack-based instruction set architecture mainly used in teaching compiler construction and other high order language (of which Algol is generally considered the first) physical implementations such as Lisp machines and P-code machines. [5]

[edit] History

ALGOL was developed jointly by a committee of European and American computer scientists in a meeting in 1958 at ETH Zurich (cf. ALGOL 58). It specified three different syntaxes: a reference syntax, a publication syntax, and an implementation syntax. The different syntaxes permitted it to use different keyword names and conventions for decimal points (commas vs periods) for different languages.

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. 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.[6]

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 January 1 to 16):

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."

Both John Backus and Peter Naur served on the committee which created ALGOL 60 as did Wally Feurzeig, who later created Logo.

ALGOL 60 inspired many languages that followed it. Tony 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."[7]

[edit] IAL implementations timeline

To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.[8]

Name Year Author State Description Target CPU
ZMMD-implementation 1958 Friedrich L. Bauer, Heinz Rutishauser, Klaus Samelson, Hermann Bottenbruch Germany implementation of ALGOL 58 Z22
Elliott ALGOL 1960 C. A. R. Hoare UK Subject of the famous Turing lecture National-Elliott 803 & the Elliott 503
JOVIAL 1960 Jules Schwarz USA Was the DOD HOL prior to Ada (programming language) Various (see article)
Burroughs Algol
(Several variants)		 1961 Burroughs Corporation (with participation by Hoare, Dijkstra, and others) USA Basis of the Burroughs (and now Unisys MCP based) computers Burroughs large systems
and their midrange as well.
Case ALGOL 1961 USA Simula was originally contracted as a simulation extension of the Case ALGOL UNIVAC 1107
GOGOL 1961 Bill McKeeman USA For ODIN time-sharing system PDP-1
X1 Algol 60 1961 Edsger Dijkstra and J.A. Zonneveld Netherlands Mathematical Centre, Amsterdam X1
Dartmouth ALGOL 30 1962 Thomas Eugene Kurtz et al. USA LGP-30
USS 90 Algol 1962 L. Petrone Italy
Algol Translator 1962 G. van der May 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
VALGOL 1963 Val Schorre USA A test of the META II compiler compiler
Whetstone 1964 Brian Randell and L J Russell UK Atomic Power Division of English Electric Company. Precursor to Ferranti Pegasus (computer), National Physical Laboratories ACE (computer) and English Electric DEUCE implementations. English Electric Company KDF9
NU ALGOL 1965 Norway UNIVAC
ALGEK 1965 USSR Minsk-22 АЛГЭК, based on ALGOL-60 and COBOL support, for economical tasks
MALGOL 1966 publ. A. Viil, M Kotli & M. Rakhendi, Estonian SSR Minsk-22
ALGAMS 1967 GAMS group (ГАМС, группа автоматизации программирования для машин среднего класса), cooperation of Comecon Academies of Science Comecon Minsk-22, later ES EVM, BESM
ALGOL/ZAM 1967 Poland Polish ZAM computer
RegneCentralen ALGOL 1967 Peter Naur Denmark
Simula 67 1967 Ole-Johan Dahl and Kristen Nygaard Norway Algol 60 with classes UNIVAC 1107
Chinese Algol 1972 China Chinese characters, expressed via the Symbol system
DG/L 1972 USA DG Eclipse family of Computers

The Burroughs dialects included special Bootstrapping dialects such as ESPOL and NEWP.

[edit] Properties

ALGOL 60 as officially defined had no I/O facilities; implementations defined their own in ways that were rarely compatible with each other. In contrast, ALGOL 68 offered an extensive library of transput (ALGOL 68 parlance for 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 had certain limitations in contrast to call-by-reference, making it an undesirable feature in imperative language design. For example, it is impossible in ALGOL 60 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.[9] However, call-by-name is still beloved of ALGOL implementors 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.

[edit] ALGOL 60 Reserved words and restricted identifiers

There are 35 such reserved words in the standard Burroughs large systems sub-language: ALPHA, ARRAY, BEGIN, BOOLEAN, COMMENT, CONTINUE, DIRECT, DO, DOUBLE, ELSE, END, EVENT, FALSE, FILE, FOR, FORMAT, GO, IF, INTEGER, LABEL, LIST, LONG, OWN, POINTER, PROCEDURE, REAL, STEP, SWITCH, TASK, THEN, TRUE, UNTIL, VALUE, WHILE, ZIP.

There are 71 such restricted identifiers in the standard Burroughs large systems sub-language: ACCEPT, AND, ATTACH, BY, CALL, CASE, CAUSE, CLOSE, DEALLOCATE, DEFINE, DETACH, DISABLE, DISPLAY, DIV, DUMP, ENABLE, EQL, EQV, EXCHANGE, EXTERNAL, FILL, FORWARD, GEQ, GTR, IMP, IN, INTERRUPT, IS, LB, LEQ, LIBERATE, LINE, LOCK, LSS, MERGE, MOD, MONITOR, MUX, NEQ, NO, NOT, ON, OPEN, OR, OUT, PICTURE, PROCESS, PROCURE, PROGRAMDUMP, RB, READ, RELEASE, REPLACE, RESET, RESIZE, REWIND, RUN, SCAN, SEEK, SET, SKIP, SORT, SPACE, SWAP, THRU, TIMES, TO, WAIT, WHEN, WITH, WRITE and also the names of all the intrinsic functions.

[edit] Examples and portability issues

[edit] Code sample (ALGOL 60)

(The way the bold text has to be written depends on the implementation, e.g. 'INTEGER' (including the quotation marks) 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[p, q]) > y then
            begin y := abs(a[p, q]);
            i := p; k := q
            end
end Absmax

Here's an example of how to produce a table using Elliott 803 ALGOL.[10]

 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.

[edit] Timeline: Hello world

The variations and lack of portability of the programs from one implementation to another is easily demonstrated by the classic hello world program.

[edit] ALGOL 58 (IAL)

Main article: ALGOL 58

ALGOL 58 had no I/O facilities.

[edit] ALGOL 60 family

Since ALGOL 60 had no I/O facilities, there is no portable hello world program in ALGOL. The following program could (and still will) compile and run on an ALGOL implementation for a Unisys A-Series mainframe, and is a straightforward simplification of code taken from The Language Guide at the University of Michigan-Dearborn Computer and Information Science Department Hello world! ALGOL Example Program page.

BEGIN
  FILE F(KIND=REMOTE);
  EBCDIC ARRAY E[0:11];
  REPLACE E BY "HELLO WORLD!";
  WRITE(F, *, E);
END.

A simpler program using an inline format:

BEGIN
  FILE F(KIND=REMOTE);
  WRITE(F, <"HELLO WORLD!">);
END.

A even simpler program using the Display statement:

BEGIN DISPLAY("HELLO WORLD!") END.

An alternative example, using Elliott Algol I/O is as follows. Elliott Algol used different characters for "open-string-quote" and "close-string-quote", represented here by ‘ and ’.

 program HiFolks;
 begin
    print ‘Hello world’;
 end;

Here's a version for the Elliott 803 Algol (A104) The standard Elliott 803 used 5 hole paper tape and thus only had upper case. The code lacked any quote characters so £ (UK Pound Sign) was used for open quote and ? (Question Mark) for close quote. Special sequences were placed in double quotes (e.g. ££L?? produced a new line on the teleprinter).

  HIFOLKS'
  BEGIN
     PRINT £HELLO WORLD££L???'
  END'

The ICL 1900 Algol I/O version allowed input from paper tape or punched card. Paper tape 'full' mode allowed lower case. Output was to a line printer.

  'BEGIN'
     'WRITE TEXT'("HELLO WORLD");
  'END'

[edit] ALGOL 68

Main article: ALGOL 68

In the language of the "Algol 68 Report", Input/output facilities were collectively called the "Transput".

ALGOL 68 code was published with reserved words typically in lowercase, but bolded or underlined.

begin
  print(("Hello, world!",newline))
end

OR using a specific transput channel:

begin
  putf((stand out,$gl$,"Hello, world!"))
end

For ease of programming computers with 7-bit characters of the time there were "official" methods to "BOLD" reserved words, for example, by using uppercase:

BEGIN
  print(("Hello, world!",newline))
END

Programmers were sometimes required to totally "THINK IN UPPERCASE" on computers that only had 6-bit characters, eg the CDC 6600 "super computers". In this case the above code would be written:

'BEGIN'
  PRINT(("HELLO, WORLD!",NEWLINE))
'END'

The "Algol 68 Report" was translated into Russian, German, French and Bulgarian, and allowed programming in languages with larger character sets, eg Cyrillic alphabet. eg the Russian BESM-4.

BEGIN
  print(("Здравствуй, мир!",newline))
END

Note: The 1964 Russian standard GOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL.[11]

[edit] See also

[edit] References

  1. ^ The name of the family is sometimes given in mixed case (Algol 60), and sometimes in all uppercase (ALGOL 68). For simplicity this article uses ALGOL.
  2. ^ cf. Pseudocode
  3. ^ "Revised Report on the Algorithmic Language Algol 60". 1963. http://www.masswerk.at/algol60/report.htm. Retrieved on June 8 2007. 
  4. ^ "Revised Report on the Algorithmic Language ALGOL 68". 1973. http://burks.brighton.ac.uk/burks/language/other/a68rr/rrtoc.htm. Retrieved on June 8 2007. 
  5. ^ "Compiler Construction Theory and Practice, Ch.9". Science Research Associates, Inc. 1979. 
  6. ^ Knuth, Donald E. (1964) Backus Normal Form vs Backus Naur Form. Communications of the ACM 7(12):735-736
  7. ^ "Hints on Programming Language Design", C.A.R. Hoare, December 1973. Page 27. (This statement is sometimes erroneously attributed to Edsger Dijkstra, also involved in implementing the first ALGOL 60 compiler.)
  8. ^ The Encyclopedia of Computer Languages
  9. ^ Aho, Alfred V.; Ravi Sethi, Jeffrey D. Ullman (1986). Compilers: Principles, Techniques, and Tools (1st edition ed.). Addison-Wesley. ISBN 0-201-10194-7. , Section 7.5, and references therein
  10. ^ "803 ALGOL", the manual for Elliott 803 ALGOL
  11. ^ "GOST 10859 standard". http://homepages.cwi.nl/~dik/english/codes/stand.html#gost10859. Retrieved on June 5 2007. 

[edit] Further reading

  • B. Randell and L.J. Russell, ALGOL 60 Implementation: The Translation and Use of ALGOL 60 Programs on a Computer. Academic Press, 1964. The design of the Whetstone Compiler. One of the early published descriptions of implementing a compiler. See the related papers: Whetstone Algol Revisited, and The Whetstone KDF9 Algol Translator by B. Randell
  • E. W, Dijkstra, Algol 60 translation: an algol 60 translator for the x1 and making a translator for algol 60, report MR 35/61. Mathematisch Centrum, Amsterdam, 1961. [1]

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/ALGOL"
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