Category:ALGOL 68
This programming language may be used to instruct a computer to perform a task.
Parameter passing methods:  By reference, By value 

Type safety:  Safe 
Type strength:  Strong 
Type compatibility:  Structural 
Type expression:  Explicit 
Type checking:  Dynamic, Static 
Lang tag(s):  algol68 
See Also: 
ALGOL 68 (short for ALGOrithmic Language 1968) is an imperative computer programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and more rigorously defined syntax and semantics.
The main aims and principles of design of ALGOL 68:
 Completeness and clarity of design,
 Orthogonal design,
 Security,
 Efficiency:
 Static mode checking,
 Modeindependent parsing,
 Independent compilation,
 Loop optimization,
 Representations  in minimal & larger character sets.
Execute an ALGOL 68 program online
 http://www.compileonline.com/execute_algol_online.php
 Algol 68G is available as one of the languages at https://tio.run
Tasks not implemented in ALGOL 68
Tasks not implemented in ALGOL 68
Grammar
The formal definition of Algol 68 is given by a "TwoLevel" or "Van Wijngaarden" grammar.
This specifies much more than "just" the syntax and includes such semantic details as the requirement of identifiers to be declared, the necessary type checking and coercions to be applied, etc. The degree of precision allowed by the grammar came at the cost of increased complexity relative to Backus Naur Form, which was used to define ALGOL 60. It is recomended that less formal material (such as the books mentioned under "Resources" below) be consulted before delving into the Revised Report.
A syntax chart is available here
Resources
 ALGOL BULLETIN  March 1959 to August 1988, in 52 issues
 Algol68 mailinglist  December 2008  algol68user AT lists.sourceforge.net
 Algol68 group at linkedin  includes various famous compiler composers.
Books available online:
 Algol 68G Manual  By Marcel van der Veer Includes the Revised Report
 Programming Algol 68 Made Easy  by Sian Mountbatten (on softwarepreservation.org)
 Informal Introduction to Algol 68  by C. H. Lindsey & S. V. Van der Meulen (on softwarepreservation.org)  if you prefer (and find) a hardcopy, be sure to get the 1977 edition. Highly recomended!
Editor modes:
 Emacs mode for Algol 68 supporting syntax highlighting and contextsensitive indentation.
 Vim script providing support for syntax highlighting.
 GeSHi syntax highlighting
 VSCode Algol 68 syntax highlighting
Status
 20th December 1968  ALGOL 68's Final Report was ratified by UNESCO's IFIP working group 2.1 in Munich.
 20th December 2008  Zig Zag  the 100th ALGOL 68 code contribution on rosettacode.org!
 Happy 40th Birthday ALGOL 68,
 AND 50th Birthday ALGOL 58.
 23rd August 2009  algol68g1.18.09h released
 20th December 2009  Happy 51st/41st Birthdays with Hamming numbers  the 200th ALGOL 68 code contribution on rosettacode.org!
 This time code was by Marcel van der Veer, author of Algol 68 Genie
 25th October 2011  Jejones3141 added Soundex  the 300th ALGOL 68 code specimen.
Revisions
 Mar. 1968: Draft Report on the Algorithmic Language ALGOL 68  Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster.
 Oct. 1968: Penultimate Draft Report on the Algorithmic Language ALGOL 68  Chapters 19  Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster.
 Dec. 1968: Report on the Algorithmic Language ALGOL 68  Offprint from Numerische Mathematik, 14, 79218 (1969); SpringerVerlag.  Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck and C.H.A. Koster.
 Sep 1973: Revised Report on the Algorithmic Language Algol 68  SpringerVerlag 1976  Edited by: A. van Wijngaarden, B.J. Mailloux, J.E.L. Peck, C.H.A. Koster, M. Sintzoff, C.H. Lindsey, L.G.L.T. Meertens and R.G. Fisker.
Coding style of samples, alphabets and stropping
Click "Expand" for more details.
Many of the code samples provided here have a leading main:(
and a matching )
at the end. These are not actually required in the language, but are included so as to highlight the main routine.
On some compilers, it may be necessary to include appropriate "job cards" or preludes in order for the programs to compile successfully. Hopefully not too much else is required. Examples:
Brief Algol68  Algol68 as in rosettacode  Actual ELLA Algol 68RS code 
print(("Hello, world!",new line)) 
main:( print(("Hello, world!",new line)) ) 
PROGRAM helloworld CONTEXT VOID USE standard BEGIN print(("Hello, world!", new line)) END FINISH 
Alphabets
Notionally, Algol 68 source is written in two alphabets. The reserved words, mode indicants (type names) and operators that are nonsymbolic (.e.g. and, or, ...) are generally referred to as "bold words" and usually shown in a bold font in literature. Words that are identifiers (used for "variable" names, procedure names, structure member names, ...) are in a separate, nonbold font.
The Manual for CMU ALGOL 68S (on softwarepreservation.org) refers to the nonbold words as being in timid face.
Examples of different program representations
At the time when ALGOL 68 was defined some predominant computers had
24 or 36 bit words, with 6 bit character sets. Hence it was desirable that
ALGOL 68 should be able to run on machines with only uppercase.
As multiple fonts were generally unavailable, a method of identifying the bold words was required.
The official spec provided for different representations of the same
program.
Quote stropping (enclosing the bold words in single quotes)
and Point stropping (preceeding the bold words with a dot)
were used.
A variant of Point stropping called RES stropping was also defined.
In RES stropping some languagedefined bold words are not preceded by a dot.
A pragmatic comment may have been required to indicate which
stropping convention was to be used, as in some of the examples below.
Upper stropping (representing the bold words by upper case and
nonbold words in lower case) was introduced by Algol 68R.
Upper stropping is used by Algol 68RS and is one of the options for Algol 68G.
Rutgers ALGOL 68 uses quote stropping.
Most of the samples on Rosetta Code use Upper stropping.
Examples (pragmatic comments to set the stropping regime not shown):
Algol68 as typically published
mode xint = int; xint sum sq:=0; for i while sum sq≠70×70 do sum sq+:=i↑2 od 
QUOTE stropping (similar to wiki)
'mode' 'xint' = 'int'; 'xint' sum sq:=0; 'for' i 'while' sum sq≠70×70 'do' sum sq+:=i↑2 'od' 
POINT stropping
.MODE .XINT = .INT; .XINT SUM SQ:=0; .FOR I .WHILE SUM SQ .NE 70*70 .DO SUM SQ .PLUSAB I .UP 2 .OD 
RES stropping
mode .xint = int; .xint sum sq:=0; for i while sum sq≠70×70 do sum sq+:=i↑2 od 
Upper stropping
MODE XINT = INT; XINT sum sq:=0; FOR i WHILE sum sq /= 70*70 DO sum sq PLUSAB i UP 2 OD 
Coercion (casting)
ALGOL 68 has a hierarchy of contexts which determine which kind of coercions are available at a particular point in the program.
Click "Expand" for more details.
These contexts are:
N a 
Context location  Coercions available in this context  Coercion examples  

Soft  Weak  Meek  Firm  Strong  
S t 
Right hand side of:
Also:

deproc eduring  all soft then weak deref erencing  all weak then deref erencing  all meek then uniting  all firm then widening, rowing and voiding 
Widening occurs if there is no loss of precision. For example: An INT will be coerced to a REAL, and a REAL will be coerced to a LONG REAL. But not viceversa. Examples: INT to LONG INT
INT to REAL
REAL to COMPL
BITS to []BOOL
BYTES to STRING A variable can also be coerced (rowed) to an array of length 1. For example: INT to [1]INT
REAL to [1]REAL 
F i 

Example:
UNION(INT,REAL) var := 1  
M e 
IF ~ THEN ... FI FROM ~ BY ~ TO ~ WHILE ~ DO ... OD etc

Examples:
REF REF BOOL to BOOL
REF REF REF INT to INT  
W e 

Examples:
REF BOOL to REF BOOL
REF REF INT to REF INT
REF REF REF REAL to REF REAL
REF REF REF REF STRUCT to REF STRUCT  
S o 
The LHS of assignments, as "~" in: ~ := ... 
Example:

For more details about Primaries and Secondaries refer to Operator precedence.
See also
Library code used in Rosetta Code samples
Various (including the standard prelude)
File related
LSystem related
Prime related
Row (array) related
Sorting related
Tools
Format an upperstropped Algol 68 source with Mediawiki markup
Implement read and include pragmaticcomments for compilers that don't support file inclusion
Subcategories
This category has the following 3 subcategories, out of 3 total.
@
 ALGOL 68 Implementations (9 P)
 ALGOL 68 User (8 P)
Pages in category "ALGOL 68"
The following 200 pages are in this category, out of 1,035 total.
(previous page) (next page)A
 A+B
 Abbreviations, easy
 Abbreviations, simple
 ABC correlation
 ABC incremental counts
 ABC problem
 ABC words
 Abelian sandpile model
 Abelian sandpile model/Identity
 Abundant odd numbers
 Abundant, deficient and perfect number classifications
 Accumulator factory
 Achilles numbers
 Ackermann function
 Additive primes
 Address of a variable
 AKS test for primes
 Align columns
 Aliquot sequence classifications
 AlmkvistGiullera formula for pi
 Almost prime
 Alternade words
 Amb
 Amicable pairs
 Anadromes
 Anagrams
 Anagrams/Deranged anagrams
 Anaprimes
 Angle difference between two bearings
 Angles (geometric), normalization and conversion
 Anonymous recursion
 Antiprimes
 Append numbers at same position in strings
 Apply a callback to an array
 Apply a digital filter (direct form II transposed)
 Approximate equality
 Apéry's constant
 Arbitraryprecision integers (included)
 Archimedean spiral
 Arithmetic derivative
 Arithmetic evaluation
 Arithmetic numbers
 Arithmeticgeometric mean
 Arithmetic/Complex
 Arithmetic/Integer
 Arithmetic/Rational
 Array concatenation
 Array length
 Arrays
 Ascending primes
 ASCII control characters
 Assertions
 Associative array/Creation
 Associative array/Iteration
 Associative array/Merging
 Attractive numbers
 Averages/Arithmetic mean
 Averages/Mean angle
 Averages/Mean time of day
 Averages/Median
 Averages/Mode
 Averages/Pythagorean means
 Averages/Root mean square
 Averages/Simple moving average
B
 Bspline
 Babbage problem
 Bacon cipher
 Balanced brackets
 Balanced ternary
 Barnsley fern
 Base 16 numbers needing a to f
 Base64 encode data
 Bell numbers
 Benford's law
 Bernoulli numbers
 Bernstein basis polynomials
 Best shuffle
 Bin given limits
 Binary coded decimal
 Binary digits
 Binary search
 Binary strings
 Binomial transform
 Bioinformatics/base count
 Birthday problem
 Bitmap
 Bitmap/Bresenham's line algorithm
 Bitmap/Bézier curves/Cubic
 Bitmap/Midpoint circle algorithm
 Bitwise IO
 Bitwise operations
 Blum integer
 Boolean values
 Boustrophedon transform
 Box the compass
 Brilliant numbers
 Bulls and cows
 Bulls and cows/Player
C
 Caesar cipher
 Calculating the value of e
 Calendar
 Calendar  for "REAL" programmers
 CalkinWilf sequence
 Call a foreignlanguage function
 Call a function
 Call an object method
 Calmo numbers
 CalmoSoft primes
 Camel case and snake case
 Canonicalize CIDR
 Cantor set
 Carmichael 3 strong pseudoprimes
 Cartesian product of two or more lists
 Casesensitivity of identifiers
 Casting out nines
 Catalan numbers
 Catalan numbers/Pascal's triangle
 Catamorphism
 Centre and radius of a circle passing through 3 points in a plane
 Centroid of a set of Ndimensional points
 Change e letters to i in words
 Changeable words
 Chaocipher
 Character codes
 Chebyshev coefficients
 Check if a polygon overlaps with a rectangle
 Check if two polygons overlap
 Check that file exists
 Chemical calculator
 Cheryl's birthday
 Chinese remainder theorem
 Chinese zodiac
 Cholesky decomposition
 Chowla numbers
 Circles of given radius through two points
 Circular primes
 Cistercian numerals
 Classes
 Closures/Value capture
 Code Golf: Code Golf
 Collect and sort square numbers in ascending order from three lists
 Collections
 Colorful numbers
 Combinations
 Combinations and permutations
 Comma quibbling
 Commandline arguments
 Commatizing numbers
 Comments
 Common list elements
 Compare a list of strings
 Compare length of two strings
 Compiler/code generator
 Compiler/lexical analyzer
 Compiler/Simple file inclusion pre processor
 Composite numbers k with no single digit factors whose factors are all substrings of k
 Compound data type
 Concatenate two primes is also prime
 Concurrent computing
 Conditional structures
 Conjugate a Latin verb
 Conjugate transpose
 Consecutive primes with ascending or descending differences
 Constrained random points on a circle
 Continued fraction
 Continued fraction/Arithmetic/Construct from rational number
 Convert CSV records to TSV
 Convert seconds to compound duration
 Conway's Game of Life
 Coprime triplets
 Coprimes
 Copy a string
 Copy stdin to stdout
 CORDIC
 Count how many vowels and consonants occur in a string
 Count in factors
 Count in octal
 Count occurrences of a substring
 Count the coins
 Count the coins/01
 Cousin primes
 Cramer's rule
 CRC32
 Create a file
 Create a twodimensional array at runtime
 Create an executable for a program in an interpreted language
 Create an HTML table
 CSV data manipulation
 CSV to HTML translation
 Cuban primes
 Cubic special primes
 Cullen and Woodall numbers
 Cumulative standard deviation
 Currency