ⓘ J (programming language)

J (programming language)

ⓘ J (programming language)

The J programming language, developed in the early 1990s by Kenneth E. Iverson and Roger Hui, is an array programming language based primarily on APL.

To avoid repeating the APL special-character problem, J uses only the basic ASCII character set, resorting to the use of the dot and colon as inflections to form short words similar to digraphs. Most such primary or primitive J words serve as mathematical symbols, with the dot or colon extending the meaning of the basic characters available. Also, many characters which in other languages often must be paired such as {} " `` or are treated by J as stand-alone words or, when inflected, as single-character roots of multi-character words.

J is a very terse array programming language, and is most suited to mathematical and statistical programming, especially when performing operations on matrices. It has also been used in extreme programming and network performance analysis.

Like John Backuss languages FP and FL, J supports function-level programming via its tacit programming features.

Unlike most languages that support object-oriented programming, Js flexible hierarchical namespace scheme where every name exists in a specific locale can be effectively used as a framework for both class-based and prototype-based object-oriented programming.

Since March 2011, J is free and open-source software under the GNU General Public License version 3 GPLv3. One may also purchase source under a negotiated license.


1. Examples

J permits point-free style and function composition. Thus, its programs can be very terse and are considered difficult to read by some programmers.

The "Hello, World!" program in J is

Hello, world!

This implementation of hello world reflects the traditional use of J – programs are entered into a J interpreter session, and the results of expressions are displayed. Its also possible to arrange for J scripts to be executed as standalone programs. Heres how this might look on a Unix system:

Historically, APL used / to indicate the fold, so +/1 2 3 was equivalent to 1+2+3. Meanwhile, division was represented with the classic mathematical division symbol the obelus, ÷, which was implemented by overstriking a minus sign and a colon on both EBCDIC and ASCII paper text terminals. Because ASCII in general does not support overstrikes in a device-independent way, and does not include a division symbol per se, J uses % to represent division, as a visual approximation or reminder. This illustrates something of the mnemonic character of Js tokens, and some of the quandaries imposed by the use of ASCII.

Defining a J function named avg to calculate the average of a list of numbers yields:

avg =: +/ % #

This is a test execution of the function:

avg 1 2 3 4 2.5

# counts the number of items in the array. +/ sums the items of the array. % divides the sum by the number of items. Above, avg is defined using a train of three verbs +, %, and # termed a fork. Specifically V0 V1 V2 Ny is the same as V0Ny) V1 V2Ny) which shows some of the power of J. Here V0, V1, and V2 denote verbs and Ny denotes a noun.

Some examples of using avg:

v =:?. 20 $ 100 NB. a random vector v 46 55 79 52 54 39 60 57 60 94 46 78 13 18 51 92 78 60 90 62 avg v 59.2 4 avg \ v NB. moving average on periods of size 4 58 60 56 51.25 52.5 54 67.75 64.25 69.5 57.75 38.75 40 43.5 59.75 70.25 80 72.5 m =:?. 4 5 $ 50 NB. a random matrix m 46 5 29 2 4 39 10 7 10 44 46 28 13 18 1 42 28 10 40 12 avg 1 m NB. apply avg to each rank 1 subarray each row of m 17.22 21.2 26.4

Rank is a crucial concept in J. Its significance in J is similar to the significance of select in SQL and of while in C.

Implementing quicksort, from the J Dictionary yields:

The following is an implementation of quicksort demonstrating tacit programming. The later involves composing functions together and not referring explicitly to any variables. Js support for forks and hooks dictates rules on how arguments applied to this function will be applied to its component functions.

Sorting in J is usually accomplished using the built-in primitive verbs /: sort up and \: sort down. User-defined sorts such as quicksort, above, typically are for illustration only.

The following example demonstrates the usage of the self-reference verb $: to recursively calculate fibonacci numbers:

This recursion can also be accomplished by referring to the verb by name, although this is of course only possible if the verb is named:

The following expression exhibits pi with n digits and demonstrates the extended precision abilities of J:

n =: 50 NB. set n as the number of digits required
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