ALGOL
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- For other uses of "Algol", see Algol (disambiguation).
ALGOL (short for ALGOrithmic Language) is a family of imperative computer programming languages originally developed in the mid 1950s which became the de facto standard way to report algorithms in print for almost the next 30 years. It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages (including Pascal). ALGOL uses bracketed statement 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 are three official main branches of ALGOL family: ALGOL 58, ALGOL 60, and ALGOL 68. Of these, ALGOL 60 was the most widely known in the United States. 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 58 was originally known as the IAL (for International Algorithmic Language.)
Note: Throughout its effective life, the name of the programming language ALGOL was always presented in all-uppercase letters, and this is the practice adopted here.
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[edit] History
ALGOL was developed jointly by a committee of European and American computer scientists. 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.
John Backus developed the Backus normal form method of describing programming languages specifically for ALGOL 58. It was revised and expanded by Peter Naur to the Backus-Naur form for ALGOL 60. 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's original quote on this is recalled in the aphorism: "ALGOL 60 was a great improvement on its successors." (This is sometimes erroneously attributed to Edsger Dijkstra, also known for his pointed comments, who helped to implement an early ALGOL 60 compiler.)
[edit] True ALGOL 60s specification and implementation timeline
There were about 70 augmented, extensions, derivations and sublanguages of Algol 60[1]
| Name | Year | Author | State | Description | Target CPU | Licencing |
|---|---|---|---|---|---|---|
| Elliott ALGOL | 1960 | C. A. R. Hoare | UK | Subject of the famous Turing lecture | National-Elliott 803 & the Elliott 503 | |
| Case ALGOL | 1961 | US | Simula was originally contracted as a simulation extension of the Case ALGOL | UNIVAC 1107 | ||
| EMIDEC Algol | 1961 | US | EMIDEC | |||
| GOGOL | 1961 | Bill McKeeman | US | 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 | US | 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 Postrijen, Telegrafie en Telefonie | ZEBRA | |
| Kidsgrove Algol | 1963 | F. G. Duncan | UK | English Electric KDF9 | ||
| VALGOL | 1963 | Val Schorre | US | A test of the META II compiler compiler | ||
| Whetstone | 1964 | Brian Randell and L J Russell | UK | Atomic Power Division of English Electric. Precursor to Ferranti Pegasus (computer), National Physical Laboratories ACE (computer) and English Electric DEUCE implementations. | English Electric KDF9 | |
| NU ALGOL | 1965 | Norway | UNIVAC | |||
| MALGOL | 1966 | A. Viil Kotli & M. Rakhendi | Russian Federation | |||
| ALGOL/ZAM | 1967 | Poland | Polish ZAM computer | |||
| RegneCentralen ALGOL | 1967 | Peter Naur | Denmark | |||
| DG/L | 1973 | US | DG Eclipse family of computers | |||
| Chinese Algol | 1974 | China | Chinese characters, expressed via the Symbol system | ? |
The Burroughs large systems are stack machines designed to be programmed in an extended variant of ALGOL 60, known as Elliott ALGOL; indeed their operating system the MCP, was written in Elliott ALGOL as far back as 1961. The Unisys Corporation still markets machines in this family today, running the MCP and supporting a diverse set of Elliott ALGOL compilers.
[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 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 ensure that call-by-name was implemented correctly across all ALGOL 60 compilers.
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] Code sample (ALGOL 60)
(The way the bolded text has to be written depends on the implementation, e.g. 'INTEGER' (including the quotation marks) for integer.)
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.
[edit] Hello world
Since ALGOL 60 had no I/O facilities, there is no portable "Hello world" program in ALGOL. The following code could run on an ALGOL implementation for a Burroughs A-Series mainframe, and is taken from this site.
BEGIN FILE F (KIND=REMOTE); EBCDIC ARRAY E [0:11]; REPLACE E BY "HELLO WORLD!"; WHILE TRUE DO BEGIN WRITE (F, *, E); END; END.
An alternative example, using Elliott Algol I/O is as follows. In fact, Elliott Algol used different characters for 'open-string-quote' and 'close-string-quote', but ASCII does not allow these to be shown here.
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 sequencies were placed in double quotes (e.g. ££L?? produced a new line on the teleprinter).
HIFOLKS’
BEGIN
PRINT £HELLO WORLD££L??’
END’
[edit] 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] See also
[edit] 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
- "803 ALGOL" The manual for Elliott 803 ALGOL.
- AN IMPLEMENTATION OF ALGOL 60 FOR THE FP6000 Discussion of some implementation issues.
- "Hints on Programming Language Design", C.A.R. Hoare, December 1973 (source of the quote stating that Algol 60 is better than its successors).
- Some Algol 60 source code, both in Machine Syntax and Reference Syntax.
- "The European Side of the Last Phase of the Development of ALGOL 60" by Peter Naur
[edit] References
- 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 description on implementing a compiler.
- 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. [[2]]
| Image:Steps.svg | |
| Preceding: | ALGOL 58 |
| Subsequent: | Simula 67 ALGOL 68, ISWIM, Algol-W |
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