J (programming Language) - Examples

Examples

J permits point-free style and function composition. Thus, its programs can be very terse and 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. It's also possible to arrange for J scripts to be executed as standalone programs, but the mechanisms for associating a script with the interpreter are system dependent. Here's how this might look on a UNIX system:

#!/bin/jc echo 'Hello, world!' exit ''

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 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 J's tokens, and some of the quandaries imposed by the use of ASCII.)

The following is a J program to calculate the average of a list of numbers:

avg=: +/ % #

and this is a test execution of the program

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. Note: avg is defined above using a train of three verbs ("+/", "%", and "#") known as a fork. Specifically (V0 V1 V2) Ny is the same as (V0(Ny)) V1 (V2(Ny)) 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 17.2 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.

Here is an implementation of quicksort, from the J Dictionary:

sel=: adverb def 'u # [' quicksort=: verb define if. 1 >: #y do. y else. (quicksort y sel e=.y{~?#y end. )

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

quicksort=: (($:@(<#[), (=#[), $:@(>#[)) ({~ ?@#)) ^: (1<#)

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 expression exhibits pi with n digits and demonstrates the extended precision capabilities of J:

n=: 50 NB. set n as the number of digits required <.@o. 10x^n NB. extended precision 10 to the nth * pi 314159265358979323846264338327950288419716939937510