1. IAT 800
Recursion
Oct 28, 2009
IAT 800

2. Today’s Excitement
Recursion
Oct 28, 2009
IAT 800

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

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

15. 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

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

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

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

19. 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

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