# Array notation: Difference between revisions

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'''Array notation''' is a way to write most [[array]]s literally, with no or minimal use of [[primitive function]]s, possibly over multiple code lines. It differs from the [[strand notation]] existing since [[APL\360]] in that it can be used to write arrays of rank greater than one. Array notation is supported in [[dzaima/APL]] and [[BQN]], and by some tools for [[Dyalog APL]], where it is planned as an eventual language feature. | '''Array notation''' is a way to write most [[array]]s literally, with no or minimal use of [[primitive function]]s, possibly over multiple code lines. It differs from the [[strand notation]] existing since [[APL\360]] in that it can be used to write arrays of rank greater than one. Array notation is supported in [[dzaima/APL]] and [[BQN]], and by some tools for [[Dyalog APL]], where it is planned as an eventual language feature. | ||

Array notation generally consists of a | Array notation generally consists of a vector notation written with parentheses <source lang=apl inline>()</source>, roughly equivalent to stranding, and a high-rank notation using square brackets <source lang=apl inline>[]</source>, indicating the [[Mix]] of a vector. It may also support [[namespace]]s, with dzaima/APL and Dyalog using <source lang=apl inline>name:value</source> syntax in parentheses for this. In each case, elements are separated by [[statement separator]]s such as <source lang=apl inline>⋄</source> or line breaks. | ||

== Examples == | == Examples == | ||

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:''See also [[Array notation in Dyalog APL]]'' | :''See also [[Array notation in Dyalog APL]]'' | ||

A predecessor of array notation is the list notation used by both [[A+]] and [[K]], with parenthesis-and-semicolon syntax like <source lang=apl inline>(a;b;c)</source>. In A+ this is related to [[bracket indexing]] and an "expression group" notation written with curly braces and semicolons. It allows line breaks, but in addition to rather than in place of semicolons. The later K version corresponds more closely to APL: the semicolon is a statement separator and is interchangeable with a line break, and because K represents arrays with nested lists, it corresponds to both | A predecessor of array notation is the list notation used by both [[A+]] and [[K]], with parenthesis-and-semicolon syntax like <source lang=apl inline>(a;b;c)</source>. In A+ this is related to [[bracket indexing]] and an "expression group" notation written with curly braces and semicolons. It allows line breaks, but in addition to rather than in place of semicolons. The later K version corresponds more closely to APL: the semicolon is a statement separator and is interchangeable with a line break, and because K represents arrays with nested lists, it corresponds to both vector and high-rank array notation. | ||

== References == | == References == | ||

<references/> | <references/> | ||

{{APL syntax}}[[Category:APL syntax]][[Category:Nested array model]] | {{APL syntax}}[[Category:APL syntax]][[Category:Nested array model]] |

## Revision as of 02:27, 19 August 2022

**Array notation** is a way to write most arrays literally, with no or minimal use of primitive functions, possibly over multiple code lines. It differs from the strand notation existing since APL\360 in that it can be used to write arrays of rank greater than one. Array notation is supported in dzaima/APL and BQN, and by some tools for Dyalog APL, where it is planned as an eventual language feature.

Array notation generally consists of a vector notation written with parentheses `()`

, roughly equivalent to stranding, and a high-rank notation using square brackets `[]`

, indicating the Mix of a vector. It may also support namespaces, with dzaima/APL and Dyalog using `name:value`

syntax in parentheses for this. In each case, elements are separated by statement separators such as `⋄`

or line breaks.

## Examples

Medium-sized array constants are often needed in code. Due to the lack of a native multi-line notation, programmers have resorted to various ad-hoc methods of approximating such, usually at the cost of reduced readability. A very common technique is repeated concatenation:

```
poss←1 2⍴'fns' ((0 1)(0.7 0)(0.7 0)×size)
poss⍪← 'fnd' ((0 1)(0 0)(0 0)×size)
poss⍪← 'lines'((0 0)(0.7 0)(0.7 0)×size)
poss⍪← 'lnd' ((0 0)(0 0)(0 0)×size)
```

Array notation allows this multi-line construction to be made with a single assignment, and also provides an alternate syntax for the inner vectors of vectors:

```
poss←['fns' ((0 1 ⋄ 0.7 0 ⋄ 0.7 0)×size)
'fnd' ((0 1 ⋄ 0 0 ⋄ 0 0)×size)
'lines'((0 0 ⋄ 0.7 0 ⋄ 0.7 0)×size)
'lnd' ((0 0 ⋄ 0 0 ⋄ 0 0)×size)]
```

## Description

The notation is added to the language by giving meaning to previously invalid statements. The added syntax consists of three constructs that are currently SYNTAX ERRORs:

*broken*round parentheses*broken*square brackets- empty round parentheses:
`()`

where *broken* means interrupted by one or more diamonds (`⋄`

) or line breaks (outside of dfns).

- A
*broken*round parenthesis creates a namespace if every diamond/line break-separated statement is a*name-value pair*. - A
*broken*round parenthesis creates a vector if every diamond/line break-separated statement is a value expression. In that case, every such statement forms an element in the resulting vector. - A
*broken*square bracket creates a an array where every diamond/line break-separated statement forms a major cell in the resulting array. `()`

is equivalent to`(⎕NS 0⍴⊂'')`

- A
*name-value pair*consist of a valid APL identifier, followed by a`:`

and a value expression.

## History

*See also Array notation in Dyalog APL*

A predecessor of array notation is the list notation used by both A+ and K, with parenthesis-and-semicolon syntax like `(a;b;c)`

. In A+ this is related to bracket indexing and an "expression group" notation written with curly braces and semicolons. It allows line breaks, but in addition to rather than in place of semicolons. The later K version corresponds more closely to APL: the semicolon is a statement separator and is interchangeable with a line break, and because K represents arrays with nested lists, it corresponds to both vector and high-rank array notation.

## References