Editing Quine (computing) (section)

==="Cheating" quines===

==== Self-evaluation ====
In many functional languages, including [[Scheme (programming language)|Scheme]] and other [[Lisp (programming language)|Lisps]], and interactive languages such as [[APL (programming language)|APL]], numbers are self-evaluating. In [[TI-BASIC]], if the last line of a program returns a value, the returned value is displayed on the screen. Therefore, in such languages a program consisting of only a single digit results in a 1-byte quine.  Since such code does not ''construct'' itself, this is often considered cheating.

<syntaxhighlight lang="basic">
1
</syntaxhighlight>

==== Empty quines ====
In some languages, particularly [[scripting language]]s but also [[C (programming language)|C]], an empty source file is a fixed point of the language, being a valid program that produces no output.{{efn|1=Examples include [[Bash (Unix shell)|Bash]], [[Perl]], and [[Python (programming language)|Python]]}} Such an empty program, submitted as "the world's smallest self reproducing program", once won the "worst abuse of the rules" prize in the [[International Obfuscated C Code Contest]].<ref>{{cite web |url = http://www0.us.ioccc.org/1994/smr.hint |title = IOCCC 1994 Worst Abuse of the Rules |archive-url=https://web.archive.org/web/20201112015540/http://www0.us.ioccc.org/1994/smr.hint |archive-date=12 November 2020 |url-status=dead}}</ref> The program was not actually compiled, but used <code>cp</code> to copy the file into another file, which could be executed to print nothing.<ref>{{cite web |title=Makefile |url=http://www0.us.ioccc.org/1994/Makefile |website=IOCCC.org |access-date=4 April 2019 |archive-date=23 April 2019 |archive-url=https://web.archive.org/web/20190423002150/http://www0.us.ioccc.org/1994/Makefile |url-status=dead }}</ref>

==== Source code inspection ====
Quines, per definition, cannot receive ''any'' form of input, including reading a file, which means a quine is considered to be "cheating" if it looks at its own source code. The following [[Unix shell|shell]] script is not a quine:
<syntaxhighlight lang="bash">
#!/bin/sh
# Invalid quine.
# Reading the executed file from disk is cheating.
cat $0
</syntaxhighlight>

A shorter variant, exploiting the behaviour of [[Shebang (Unix)|shebang]] directives:
<syntaxhighlight lang="bash">
#!/bin/cat
</syntaxhighlight>

Other questionable techniques include making use of compiler messages; for example, in the [[GW-BASIC]] environment, entering "Syntax Error" will cause the interpreter to respond with "Syntax Error".

Quine code can also be outputted visually, for example it's used to visualize the neutral zone in [[Yars' Revenge]], along with [[Syntactic_sugar#Syntactic_saccharin|syntactic saccharin]], to obfuscate the source code.