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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." |
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." 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. |
ALGOL and programming language research. |
As Peter Landin noted, ALGOL was the first language to combine seamlessly imperative effects with the (call-by-name) lambda calculus. Perhaps the most elegant formulation of the language is due to John C. Reynolds, and it best exhibits its syntactic and semantic purity. Reynolds's idealized ALGOL also made a convincing methodologic argument regarding the suitability of local effects in the context of call-by-name languages, in contrast with the global effects used by call-by-value languages such as ML. The conceptual integrity of the language made it one of the main objects of semantic research, along with Programming Computable Functions (PCF) and ML. |
IAL implementations timeline. |
To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60. |
The Burroughs dialects included special Bootstrapping dialects such as ESPOL and NEWP. The latter is still used for Unisys MCP system software. |
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" (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 has certain effects in contrast to call-by-reference. For example, without specifying the parameters as "value" or "reference", it is impossible 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. Think of passing a pointer to swap(i, A[i]) in to a function. Now that every time swap is referenced, it is reevaluated. Say i := 1 and A[i] := 2, so every time swap is referenced it will return the other combination of the values ([1,2], [2,1], [1,2] and so on). A similar situation occurs with a random function passed as actual argument. |
Call-by-name is known by many compiler designers 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. |
Examples and portability. |
Code sample comparisons. |
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 copied to y, and the subscripts of this element to i and k; |
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]); |
Here is an example of how to produce a table using Elliott 803 ALGOL. |
FLOATING POINT ALGOL TEST' |
BEGIN REAL A,B,C,D' |
FOR A:= 0.0 STEP D UNTIL 6.3 DO |
PRINT PUNCH(3),££L??' |
PRINT PUNCH(3),SAMELINE,ALIGNED(1,6),A,B,C' |
PUNCH(3) sends output to the teleprinter rather than the tape punch.<br> |
SAMELINE suppresses the carriage return + line feed normally printed between arguments.<br> |
ALIGNED(1,6) controls the format of the output with one digit before and six after the decimal point. |
The following code samples are ALGOL 68 versions of the above ALGOL 60 code samples. |
ALGOL 68 implementations used ALGOL 60's approaches to 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 |
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 |
Note: lower (⌊) and upper (⌈) bounds of an array, and array slicing, are directly available to the programmer. |
floating point algol68 test: |
# "printf" – sends output to the file "stand out". # |
# "printf($p$);" – selects a "new page" # |
printf(($pg$,"Enter d:")); |
for step from 0 while a:=step*d; a <= 2*pi do |
printf($l$); # "$l$" - 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 had no I/O facilities. |
Since ALGOL 60 had no I/O facilities, there is no portable hello world program in ALGOL. |
The next three examples are in Burroughs Extended Algol. The first two direct output at the interactive terminal they are run on. The first uses a character array, similar to C. The language allows the array identifier to be used as a pointer to the array, and hence in a REPLACE statement. |
A simpler program using an inline format: |
An even simpler program using the Display statement. Note that its output would end up at the system console ('SPO'): |
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 . |
Below is a version from Elliott 803 Algol (A104). The standard Elliott 803 used five-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). |
PRINT £HELLO WORLD£L??' |
The ICT 1900 series 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. The open and close quote characters were represented using '(' and ')' and spaces by %. |
WRITE TEXT('('HELLO%WORLD')'); |
ALGOL 68 code was published with reserved words typically in lowercase, but bolded or underlined. |
printf(($gl$,"Hello, world!")) |
In the language of the "Algol 68 Report" the input/output facilities were collectively called the "Transput". |
Timeline of ALGOL special characters. |
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 unsupported 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 it in order to support ALGOL's boolean operators /\ and \/. |
1962: ALCOR – This character set included the unusual "᛭" runic cross character for multiplication and the "⏨" Decimal Exponent Symbol for floating point notation. |
1964: GOST – The 1964 Soviet standard GOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL. |
2009 October: Unicode – The codice_3 (Decimal Exponent Symbol) for floating point notation was added to Unicode 5.2 for backward compatibility with historic Buran programme ALGOL software. |
AWK ("awk" ) is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Like sed and grep, it is a filter, and is a standard feature of most Unix-like operating systems. |
The AWK language is a data-driven scripting language consisting of a set of actions to be taken against streams of textual data – either run directly on files or used as part of a pipeline – for purposes of extracting or transforming text, such as producing formatted reports. The language extensively uses the string datatype, associative arrays (that is, arrays indexed by key strings), and regular expressions. While AWK has a limited intended application domain and was especially designed to support one-liner programs, the language is Turing-complete, and even the early Bell Labs users of AWK often wrote well-structured large AWK programs. |
AWK was created at Bell Labs in the 1970s, and its name is derived from the surnames of its authors: Alfred Aho (author of egrep), Peter Weinberger (who worked on tiny relational databases), and Brian Kernighan. The acronym is pronounced the same as the name of the bird species auk, which is illustrated on the cover of "The AWK Programming Language". When written in all lowercase letters, as codice_1, it refers to the Unix or Plan 9 program that runs scripts written in the AWK programming language. |
According to Brian Kernighan, one of the goals of AWK was to have a tool that would easily manipulate both numbers and strings. AWK was also inspired by Marc Rochkind's programming language that was used to search for patterns in input data, and was implemented using yacc. |
As one of the early tools to appear in Version 7 Unix, AWK added computational features to a Unix pipeline besides the Bourne shell, the only scripting language available in a standard Unix environment. It is one of the mandatory utilities of the Single UNIX Specification, and is required by the Linux Standard Base specification. |
In 1983, AWK was one of several UNIX tools available for Charles River Data Systems' UNOS operating system under Bell Laboratories license. |
AWK was significantly revised and expanded in 1985–88, resulting in the GNU AWK implementation written by Paul Rubin, Jay Fenlason, and Richard Stallman, released in 1988. GNU AWK may be the most widely deployed version because it is included with GNU-based Linux packages. GNU AWK has been maintained solely by Arnold Robbins since 1994. Brian Kernighan's nawk (New AWK) source was first released in 1993 unpublicized, and publicly since the late 1990s; many BSD systems use it to avoid the GPL license. |
AWK was preceded by sed (1974). Both were designed for text processing. They share the line-oriented, data-driven paradigm, and are particularly suited to writing one-liner programs, due to the implicit main loop and current line variables. The power and terseness of early AWK programs – notably the powerful regular expression handling and conciseness due to implicit variables, which facilitate one-liners – together with the limitations of AWK at the time, were important inspirations for the Perl language (1987). In the 1990s, Perl became very popular, competing with AWK in the niche of Unix text-processing languages. |
Structure of AWK programs. |
An AWK program is a series of pattern action pairs, written as: |
where "condition" is typically an expression and "action" is a series of commands. The input is split into records, where by default records are separated by newline characters so that the input is split into lines. The program tests each record against each of the conditions in turn, and executes the "action" for each expression that is true. Either the condition or the action may be omitted. The condition defaults to matching every record. The default action is to print the record. This is the same pattern-action structure as sed. |
In addition to a simple AWK expression, such as codice_2 or codice_3, the condition can be codice_4 or codice_5 causing the action to be executed before or after all records have been read, or "pattern1, pattern2" which matches the range of records starting with a record that matches "pattern1" up to and including the record that matches "pattern2" before again trying to match against "pattern1" on subsequent lines. |
In addition to normal arithmetic and logical operators, AWK expressions include the tilde operator, codice_6, which matches a regular expression against a string. As handy syntactic sugar, "/regexp/" without using the tilde operator matches against the current record; this syntax derives from sed, which in turn inherited it from the ed editor, where codice_7 is used for searching. This syntax of using slashes as delimiters for regular expressions was subsequently adopted by Perl and ECMAScript, and is now common. The tilde operator was also adopted by Perl. |
AWK commands are the statements that are substituted for "action" in the examples above. AWK commands can include function calls, variable assignments, calculations, or any combination thereof. AWK contains built-in support for many functions; many more are provided by the various flavors of AWK. Also, some flavors support the inclusion of dynamically linked libraries, which can also provide more functions. |
The "print" command. |
The "print" command is used to output text. The output text is always terminated with a predefined string called the output record separator (ORS) whose default value is a newline. The simplest form of this command is: |
Although these fields ("$X") may bear resemblance to variables (the $ symbol indicates variables in the usual Unix shells and in Perl), they actually refer to the fields of the current record. A special case, "$0", refers to the entire record. In fact, the commands "codice_8" and "codice_12" are identical in functionality. |
The "print" command can also display the results of calculations and/or function calls: |
# Actions to perform in the event the record (line) matches the above regex_pattern |
print foobar(variable) |
Output may be sent to a file: |
# Actions to perform in the event the record (line) matches the above regex_pattern |
print "expression" > "file name" |
# Actions to perform in the event the record (line) matches the above regex_pattern |
print "expression" | "command" |
Awk's built-in variables include the field variables: $1, $2, $3, and so on ($0 represents the entire record). They hold the text or values in the individual text-fields in a record. |
Other variables include: |
Variables and syntax. |
Variable names can use any of the characters [A-Za-z0-9_], with the exception of language keywords. The operators "+ - * /" represent addition, subtraction, multiplication, and division, respectively. For string concatenation, simply place two variables (or string constants) next to each other. It is optional to use a space in between if string constants are involved, but two variable names placed adjacent to each other require a space in between. Double quotes delimit string constants. Statements need not end with semicolons. Finally, comments can be added to programs by using "#" as the first character on a line, or behind a command or sequence of commands. |
User-defined functions. |