View source for Eqn (software)

{{Short description|Preprocessor that formats equations for drawing}}
{{lowercase title}}
{{Infobox software
| name = eqn
| logo = 
| screenshot = 
| screenshot size = 
| caption = 
| author = [[Brian Kernighan]],<br />[[Lorinda Cherry]]<br />([[AT&T Bell Laboratories]])
| developer = Various [[open-source software|open-source]] and [[commercial software|commercial]] developers
| released = {{Start date and age|1974|6}}
| latest release version = 
| latest release date = 
| programming language = [[C (programming language)|C]], [[Yacc]]<ref name="Kernighan_Brailsford" />
| operating system = [[Unix]], [[Unix-like]], [[Plan 9 from Bell Labs|Plan 9]]
| platform = [[Cross-platform]]
| genre = [[Command (computing)|Command]]
| license = Plan 9: [[MIT License]]
| website = 
}}
Part of the [[troff]] suite of [[Unix]] document layout tools, '''eqn''' is a [[preprocessor]] that formats equations for printing.  A similar program, '''neqn''', accepted the same input as eqn, but produced output tuned to look better in [[nroff]]. The eqn program was created in 1974 by [[Brian Kernighan]] and [[Lorinda Cherry]].
It was implemented using [[yacc]] compiler-compiler.<ref name="Kernighan_Brailsford" />

The input language used by eqn allows the user to write mathematical expressions in much the same way as they would be spoken aloud.  The language is defined by a [[context-free grammar]], together with [[operator precedence]] and [[operator associativity]] rules.<ref name="Kernighan_Cherry_1975" />  The eqn language is similar to the mathematical component of [[TeX]], which appeared several years later, but is simpler and less complete.

An independent compatible implementation of the eqn preprocessor has been developed by [[GNU]] as part of [[groff (software)|groff]], the GNU version of troff.  The GNU implementation extends the original language by adding a number of new [[keyword (computer programming)|keywords]] such as ''smallover'' and ''accent''. [[mandoc]], a specialised compiler for UNIX [[man pages]], also contains a standalone eqn parser/formatter.

== History ==
Eqn was written using the [[yacc]] parser generator.<ref name="Kernighan_Brailsford" />

== Syntax examples ==
Here is how some examples would be written in eqn<ref name="Kernighan_Cherry_1978" /> (with equivalents in TeX for comparison):

{| class="wikitable"
|- 
!TeX
!eqn
!formula
|- 
|{{code|lang=latex|a^2}}
|<code>a sup 2</code>
|<math>a^2</math>
|-
|{{code|lang=latex|1=\sum_{k = 1}^N k^2}}
|<code>sum from { k = 1 } to N { k sup 2 }</code>
|<math>\sum_{k=1}^N k^2</math>
|-
|<syntaxhighlight lang="latex" inline>x = {-b \pm \sqrt{b^2 - 4ac} \over 2a}</syntaxhighlight>
|<code>x = {-b +- sqrt{b sup 2 - 4ac}} over 2a</code>
|<math>x= {-b \pm \sqrt{b^2 - 4ac} \over 2a}</math>
|}

Spaces are important in eqn; [[lexical analysis#Token|tokens]] are delimited only by [[whitespace character]]s, tildes ~, braces {} and double-quotes "". Thus <code>f(pi&nbsp;r&nbsp;sup&nbsp;2)</code> results in <math display="inline">f(pi r^{2)}</math>, whereas <code>f(&nbsp;pi&nbsp;r&nbsp;sup&nbsp;2&nbsp;)</code> is needed to give the intended <math display="inline">f(\pi r^2)</math>.

==References==
{{Reflist|refs=
<ref name="Kernighan_Cherry_1975">{{cite journal|year=1975|title=A System for Typesetting Mathematics|journal=[[Communications of the ACM]]|volume=18|issue=3|pages=151–157 |first1=Brian W. |last1=Kernighan |first2=Lorinda L. |last2=Cherry| author-link = Brian Kernighan| author2-link = Lorinda Cherry |doi=10.1145/360680.360684|s2cid=155801|doi-access=free}}</ref>

<ref name="Kernighan_Cherry_1978">{{Citation
| last    = Kernighan
| first   = Brian W.
| last2   = Cherry
| first2  = Lorinda
| author-link  = Brian Kernighan
| author2-link = Lorinda Cherry
| year     = 1978
| title    = Typesetting Mathematics — User’s Guide
| edition  = 2nd
| url = https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=699ba8cbd336af184c02165bb766832f385880ab
| format   = PDF
}}</ref>

<ref name="Kernighan_Brailsford">{{cite web|url=https://www.youtube.com/watch?v=vT_J6xc-Az0  |archive-url=https://ghostarchive.org/varchive/youtube/20211222/vT_J6xc-Az0 |archive-date=2021-12-22 |url-status=live|title=UNIX Special: Profs Kernighan & Brailsford |work=Computerphile |date=September 30, 2015 }}{{cbignore}}</ref>

}}

== External links ==
*[http://www.kohala.com/start/troff/v7man/eqn/eqn2e.ps Typesetting Mathematics, User's Guide (Second Edition)]
*{{webarchive |url=https://web.archive.org/web/20070808042840/http://www.uwm.edu/cgi-bin/IMT/wwwman?topic=neqn(1)&msection= |date=August 8, 2007 |title=UNIX man page for neqn(1) }}
* {{man|1|eqn|Plan 9}}

{{Plan 9 commands}}

[[Category:Troff|eqn]]
[[Category:Plan 9 commands]]


{{Unix-stub}}