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'''Mutual recursion''' is a form of [[recursion]] where two mathematical or computational functions are defined in terms of each other.<ref>Manuel Rubio-Sánchez, Jaime Urquiza-Fuentes,Cristóbal Pareja-Flores (2002), 'A Gentle Introduction to Mutual Recursion', Proceedings of the 13th annual conference on Innovation and technology in computer science education, June 30–July 2, 2008, Madrid, Spain.</ref>
For instance, consider two functions <code>even?</code> and <code>odd?</code> defined as follows:
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Some programming styles discourage mutual recursion, claiming that it can be confusing to distinguish the conditions which will return an answer from the conditions that would allow the code to run forever without producing an answer. It is usually possible to turn two mutually recursive functions into a single recursive function by inlining the code of one into the other, possibly at the expense of legibility. [[Peter Norvig]] points to a [[design pattern]] which discourages the use entirely, stating
{{quote|text=If you have two mutually-recursive functions that both alter the state of an object, try to move almost all the functionality into just one of the functions. Otherwise you will probably end up duplicating code.|sign=Peter Norvig
Any mutual recursion can be converted to direct recursion using procedural inlining.<ref>[http://delivery.acm.org/10.1145/180000/176510/p151-kaser.pdf?key1=176510&key2=1857140721&coll=GUIDE&dl=GUIDE&CFID=82873082&CFTOKEN=54657523 On the Conversion of Indirect to Direct Recursion]</ref>
In mathematics, the [[Hofstadter sequence#Hofstadter Female and Male sequences|Hofstadter Female and Male sequences]] are an example of a pair of integer sequences defined in a mutually recursive manner.
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== References ==
<references />
[[Category:Theory of computation]]
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