ALGOL

This article is about the programming language family. For other uses, 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) e.g. Simula, CPL, Pascal, Ada

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 became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years. It was designed to avoid some of the perceived problems with FORTRAN^{[citation needed]} and eventually gave rise to many other programming languages (including BCPL, B, Pascal, Simula and C).^{[citation needed]} ALGOL introduced code blocks and was the first language to use begin and end pairs for delimiting them. Fragments of ALGOL-like syntax are sometimes still used as a notation for algorithms, so-called Pidgin code.

There were three major specifications:

• ALGOL 58 - originally proposed to be called IAL (for International Algorithmic Language).
• ALGOL 60, described in detail in this article - revised 1963
• ALGOL 68 - revised 1973 - introduced things like flexible arrays, slices, parallelism, the extensibility features (especially operator identification), and so forth.

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.

Algol 68 is substantially different from Algol 60 but was not well received so that in general "Algol" means dialects of Algol 60.

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). 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).

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.

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):

• 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."

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. 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.

IAL implementations timeline

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

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
X1 ALGOL 60	Summer 1960	Edsger W. Dijkstra and Jaap A. Zonneveld	Netherlands	First implementation of ALGOL 60	Electrologica X1
Elliott ALGOL	1960s	C. A. R. Hoare	UK	Subject of the famous Turing lecture	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 W. Dijkstra and J.A. Zonneveld	Netherlands	Mathematical Centre, Amsterdam	X1
RegneCentralen ALGOL	1961	Peter Naur, Jørn Jensen	Denmark	Inplementation of full Algol 60	DASK at Regnecentralen
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 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
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, National Physical Laboratories ACE 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
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.

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. 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.

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.

Examples and portability issues

Code sample comparisons

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.

 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.

ALGOL 68

The following code samples are ALGOL 68 versions of the above ALGOL 60 code samples.

ALGOL 68 reuses ALGOL 60's stropping. In ALGOL 68's case tokens with the bold typeface are reserved words, types (modes) or operators.

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[p, q] > y then
           y := abs a[p, q];
           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.

ALGOL 68 uses C programming language style declarations real a,b,c,d; and stdio procedures like printf with formats.

floating point algol68 test:
(
  real a,b,c,d;

  printf(($pg$,"Enter d:")); 
  read(d);

  for step from 0 while a:=step*d; a <= 2*pi do
    printf($l$);
    b := sin(a);
    c := cos(a);
    printf(($z-d.6d$,a,b,c))
  od
)

printf - sends output to the file stand out.
printf($p$); - selects a new page.
printf($l$); - selects a new line.
printf(($z-d.6d$,a,b,c)) - formats output with 1 digit before and 6 after the decimal point.

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.

ALGOL 58 (IAL)

Main article: ALGOL 58

ALGOL 58 had no I/O facilities.

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.

An 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 ICT 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'

ALGOL 68

Main article: ALGOL 68

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

begin
  printf(($gl$,"Hello, world!"))
end

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

Timeline of ALGOL special characters

This article contains Unicode 5.2 "Miscellaneous Technical" characters. Without proper rendering support, you may see question marks, boxes, or other symbols instead of something like "10" (Decimal Exponent Symbol U+23E8).

The ALGOLs were conceived at a time when character sets were diverse and evolving rapidly, also the ALGOLs were defined so that only uppercase letters were required.

1960: IFIP - The Algol 60 language and report included several mathematical symbols which are available on modern computers and operating systems, but, unfortunately, were not supported on most computing systems at the time. For instance: ×, ÷, ≤, ≥, ≠, ¬, ∨, ∧, ⊂, ≡, ␣ and ⏨.

1961 September: ASCII - The ASCII character set, then in an early stage of development, had the \ (Back slash) character added to if in order to support ALGOL's the boolean operators /\ and \/.

1962: ALCOR - This character set included the unusual "᛭" (iron/runic cross) character and the "⏨" (Decimal Exponent Symbol) for floating point notation.

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

1968: The "Algol 68 Report" - used existing ALGOL characters, and further adopted →, ↓, ↑, □, ⌊, ⌈, ⎩, ⎧, ○ and ⊥ characters which can be found on the IBM 2741 keyboard with "golf-ball" print heads inserted (such as the APL golfball), these became available in the mid 1960s while ALGOL 68 was being drafted. The report was translated into Russian, German, French and Bulgarian, and allowed programming in languages with larger character sets, e.g. Cyrillic alphabet of the Russian BESM-4.

2009 October: Unicode - The "⏨" (Decimal Exponent Symbol) for floating point notation was added to Unicode 5.2 for backward compatibility with historic Buran (spacecraft) ALGOL software.

See also

• ALGOL 58
• ALGOL N
• ALGOL 68
• ALGOL W

• Atlas Autocode
• Coral 66
• Edinburgh IMP
• Jensen's Device
• ISWIM

• JOVIAL
• NELIAC
• Simula
• S-algol
• Scheme (programming language)

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]

External links

• Revised Report on the Algorithmic Language Algol 60 by Peter Naur, et al. ALGOL definition
• A BNF syntax summary of ALGOL 60
• "The Emperor's Old Clothes" - Hoare's 1980 ACM Turing Award speech, which discusses ALGOL history and his involvement
• MARST, a free Algol-to-C translator
• AN IMPLEMENTATION OF ALGOL 60 FOR THE FP6000 Discussion of some implementation issues.
• "The European Side of the Last Phase of the Development of ALGOL 60" by Peter Naur
• Edinburgh University wrote compilers for Algol60 (later updated for Algol60M) based on their Atlas Autocode compilers initially bootstrapped from the Atlas to the KDF-9. The Edinburgh compilers generated code for the ICL1900, the ICL4/75 (an IBM360 clone), and the ICL2900. Here is the BNF for Algol60 and the ICL2900 compiler source, library documentation, and a considerable test suite including Brian Wichmann's tests. Also there is a rather superficial Algol60 to Atlas Autocode source-level translator.
• Revised Report on the Algorithmic Language Algol 60 by Peter Naur, et al. ALGOL definition
• A Windows Algol Algol running on Vista win64 system by Marcel van der Veer, Algol 68 Genie works on normal Windows up through the latest.
• Eric S. Raymond's Retrocomputing Museum, among others a link to the NASE Algol-60 interpreter written in C.
• Stories about Burrough's B5000 [2]