IAT 265
Recursion
June 18, 2015
IAT 265
1
Today’s Excitement
 Recursion
June 18, 2015
IAT 265
2
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 );
}
June 18, 2015
IAT 265
3
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 );
}
June 18, 2015
IAT 265
4
Calling Itself
 Let’s
step through what happens.
factorial(3);
int factorial(int n) (n=3)
int factorial(int n) (n=2)
{
{
if(n > 1)
if(n > 1)
{
{
return( n* factorial( n-1 ) );
return( n* factorial( n-1 ) );
}
}
else
else
return( 1 );
return( 1 );
}
}
June 18, 2015
IAT 265
5
Calling Itself
 Let’s
step through what happens.
factorial(3);
int factorial(int n) (n=3)
int factorial(int n) (n=2)
int factorial(int n) (n=1)
{
{
{
if(n > 1)
if(n > 1)
if(n > 1)
{
{
{
return( n* factorial( n-1 ) );
return( n* factorial( n-1 ) );
return( n* factorial( n-1
}
}
}
else
else
else
return( 1 );
return( 1 );
return( 1 );
}
}
}
June 18, 2015
IAT 265
6
Calling Itself
 Let’s
step through what happens.
factorial(3);
int factorial(int n) (n=3)
int factorial(int n) (n=2)
int factorial(int n) (n=1)
{
{
{
if(n > 1)
if(n > 1)
if(n > 1)
{
{
{
return( n* factorial( n-1 ) );
return( n* factorial( n-1 ) );
return( n* factorial( n-1
}
}
}
else
else
else
return( 1 );
return( 1 );
return( 1 );
}
}
}
June 18, 2015
IAT 265
7
Calling Itself
 Let’s
step through what happens.
factorial(3);
int factorial(int n) (n=3)
int factorial(int n) (n=2)
{
{
if(n > 1)
if(n > 1)
{
{
return( n* factorial( n-1 ) );
return( n* factorial( n-1 ) );
}
}
else
else
return( 1 );
return( 1 );
}
}
June 18, 2015
IAT 265
1
8
Calling Itself
 Let’s
step through what happens.
factorial(3);
int factorial(int n) (n=3)
int factorial(int n) (n=2)
{
{
if(n > 1)
if(n > 1)
{
{
return( n* factorial( n-1 ) );
return( n* 1 );
}
}
else
else
return( 1 );
return( 1 );
}
}
June 18, 2015
IAT 265
9
Calling Itself
 Let’s
step through what happens.
factorial(3);
int factorial(int n) (n=3)
int factorial(int n) (n=2)
{
{
if(n > 1)
if(n > 1)
{
{
return( n* factorial( n-1 ) );
return( 2* 1 );
}
}
else
else
return( 1 );
return( 1 );
}
}
June 18, 2015
IAT 265
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 );
}
June 18, 2015
2
IAT 265
11
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 );
}
June 18, 2015
IAT 265
12
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 );
}
June 18, 2015
IAT 265
13
Calling Itself
 Let’s
step through what happens.
factorial(3);
6
June 18, 2015
IAT 265
14
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
June 18, 2015
IAT 265
15
Web Crawling
 HTML
reader called parsePage()
– Reads HTML
– Finds links
– Per Link it should
• Call parsePage()
June 18, 2015
IAT 265
16
Web Crawling
1
2
4
3
June 18, 2015
IAT 265
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17
Web Crawling
1
6
2
5
4
3
7
11
13
10
8
June 18, 2015
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9
15
12
IAT 265
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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
June 18, 2015
IAT 265
19
Recursion
Remember—base cases prevent infinite cats.
http://infinitecat.com/
June 18, 2015
IAT 265
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