# lecture20 - Administrative Issues Introduction to Recursion...

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Introduction to Recursion Lecture 20 Administrative Issues • Due Today – Homework 4 • Ongoing – Program 4 • Due Friday, June 20 th at 11pm Method Calls • Many examples of calling a method to perform a service println() shuffle() addPhoneBookEntry() unplug() • Called one method from inside another Review: The Stack Trace main() method1() helpMethod() Class 1 Class 2 obj.method1() helpMethod() Recursive Method Calls • Methods can also call themselves • This is called recursion – Very powerful tool – Easy-to-implement and elegant solutions to some problems Recursion • Defining something in terms of itself • Bad practice for a dictionary – Writer: A person that writes – Write: The action performed by a writer • However, can be useful to define certain concepts list : number or number, list • 5, 6, 7, 4, 3, 6, 22 •6 • 76, 22, 32, 3

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The Base Case • Recursive definitions must have terminating condition, or base case – If not, they never end! •Ex am p l e list : number or number, list • Without the base case, lists would all be infinitely long – Similar to an infinite loop Two Parts • Two parts of a recursive definition – The base case – The recursive step • Both parts are required Recursion in Math • Recursion can define many mathematical concepts • Factorial 1! = 1 n! = n·(n–1)! • Powers of two 2 0 = 1 2 n = 2·2 n–1 Thinking Recursively • Clichés often think recursively – “Take it one problem at a time.” – “We play the season one game at a time.”
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## This note was uploaded on 04/15/2009 for the course ASTRONOMY 1000 taught by Professor Kim during the Spring '01 term at Yonsei University.

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lecture20 - Administrative Issues Introduction to Recursion...

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