Algol (β Per / Beta Persei), known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first (non-nova) variable stars to be discovered. Algol is actually a three-star system (Beta Persei A, B and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2 days, 20 hours and 49 minutes during the roughly 10-hour long partial eclipses. There is also a secondary eclipse when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.
In 1881, the Harvard astronomer Edward Pickering presented evidence that Algol was actually an eclipsing binary. This was confirmed a few years later, in 1889, when the Potsdam astronomer Hermann Carl Vogel found periodic doppler shifts in the spectrum of Algol, inferring variations in the radial velocity of this binary system. Thus Algol became one of the first known spectroscopic binaries.
| Components | Semimajor axis | Ellipticity | Period | Inclination |
|---|---|---|---|---|
| A—B | 0.00218″ | 0.00 | 2.87 days | 97.69° |
| (AB)—C | 0.09461″ | 0.225 | 680.05 days | 83.98° |
Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can be seen.
This system also exhibits variable activity in the form of x-ray and radio flares. The former is thought to be caused by the magnetic fields of the AB components interacting with the mass transfer. The radio emissions may be created by magnetic cycles similar to sunspots, but, as the magnetic fields around these stars are up to ten times stronger than that of the Sun, these radio flares are more powerful and longer lasting.
Algol is 92.8 light years from Earth; however, about 7.3 million years ago it passed within 9.8 light years and its apparent magnitude was approximately −2.5, considerably brighter than Sirius is today. Because the total mass of the system is 5.8 solar masses, and despite the fairly large distance at closest approach, this may have been enough to perturb the solar system's Oort cloud slightly and to increase the number of comets entering the inner solar system. However, the actual increase in net cratering rate is believed to have been quite small.
It is known as 大陵五 (the Fifth Star of the Mausoleum) in Chinese astronomy, and also bore the grim name Tseih She (叠尸 - die2 shi1 in Modern Pinyin), meaning 'Piled up corpses'.
There were three major specifications:
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.
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.
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):
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.
| 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 | US | 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) | US | Basis of the Burroughs (and now Unisys MCP based) computers | Burroughs large systems and their midrange as well. |
| Case ALGOL | 1961 | US | Simula was originally contracted as a simulation extension of the Case ALGOL | UNIVAC 1107 | |
| 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 Posterijen, Telegrafie en Telefonie | ZEBRA |
| Kidsgrove Algol | 1963 | F. G. Duncan | UK | English Electric Company 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 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 | US | DG Eclipse family of computers |
The Burroughs dialects included special bootstraping dialects such as ESPOL and NEWP.
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.
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.
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 AbsmaxHere'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.
BEGIN
FILE F(KIND=REMOTE);
EBCDIC ARRAY E[0:11];
REPLACE E BY "HELLO WORLD!";
WRITE(F, *, E);
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'
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))
endOR 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))
ENDNote: The 1964 Russian standard GOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL.
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