IAT 800 Recursion Oct 28, 2009 IAT 800.

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Presentation transcript:

IAT 800 Recursion Oct 28, 2009 IAT 800

Today’s Excitement Recursion Oct 28, 2009 IAT 800

Recursion Recursion basically means calling a method from inside itself. int factorial(int n) { if( n > 1 ) return( n* factorial( n-1 ) ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=2) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=2) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=1) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=2) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=1) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=2) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); 1 Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=2) { if(n > 1) return( n* 1 ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); int factorial(int n) (n=2) { if(n > 1) return( 2* 1 ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* factorial( n-1 ) ); } else return( 1 ); 2 Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( n* 2 ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); int factorial(int n) (n=3) { if(n > 1) return( 3* 2 ); } else return( 1 ); Oct 28, 2009 IAT 800

Calling Itself Let’s step through what happens. factorial(3); 6 Oct 28, 2009 IAT 800

Base Case Must have Base Case A case or condition that returns without further recursion Stops the recursive chain Eg factorial( int n ) Returned 1 when n = 1 In every other call, n decreases by 1 Oct 28, 2009 IAT 800

Web Crawling HTML reader called parsePage() Reads HTML Finds links Per Link it should Call parsePage() Oct 28, 2009 IAT 800

Web Crawling 1 2 5 4 3 Oct 28, 2009 IAT 800

Web Crawling 1 6 2 5 4 3 7 11 13 15 10 14 9 8 12 Oct 28, 2009 IAT 800

Web Crawling What base case? Count the number of recursive calls so far Place a limit on depth Explore no further after depth 4 Example stopped at depth 2 Oct 28, 2009 IAT 800

Remember—base cases prevent infinite cats. http://infinitecat.com/ Recursion Remember—base cases prevent infinite cats. http://infinitecat.com/ Oct 28, 2009 IAT 800