1. Lecture 13
Recursion part 2
CSE 1322
4/26/2018

2. Advanced Recursion
Sometimes recursion involves processing a collection. 
While it could be done using 'foreach' or some other iterative approach, recall that recursion works best for non-linear structures. 
Recursion is also useful for making use of the activation stack as a "reminder" of where we've been and what remains to be done.
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3. multiple recursion Consider this example: what is DoWork(4) ?
void DoWork(int i) {   Console.WriteLine(i);   if (i > 2)   {     DoWork(i-1);     DoWork(i-2);   }   Console.WriteLine(i); }
void DoWork(int i) {   System.out.println(i);   if (i > 2)   {     DoWork(i-1);     DoWork(i-2);   }   System.out.println(i); }
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4. Java - multiple recursion Consider this example: what is DoWork(4) ?
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5. C# - multiple recursion Consider this example: what is DoWork(4) ?
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6. multiple recursion (2)
Fibonacci series: an= a(n-1) +a(n-2) i.e. the next number in the series is the sum of the 2 preceding numbers.
the Fibonacci series: 1,1,2,3,5,8,13,…
Write a multiply recursive method:
int Fibo ( int n) { }
(Remember stopping state first!)
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7. The solution
int Fib(int i) {   if (i <= 2)     return 1;   else     return Fib(i-1) + Fib(i-2); }
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8. Java - multiple recursion Consider this example: what is Fibonacci(?) ?
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9. C# - multiple recursion Consider this example: what is Fibonacci(?) ?
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10. How can we reverse a string recursively?
String Processing
How can we reverse a string recursively?
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11. Pseudo-code solution (base case is when string is ? ):
reverse a string by repeatedly removing the first character
reversing this substring (recursively),
appending the first character to the end
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12. string reversal code
public string Reverse (string s) {   if (s.Length==1)     return s;   else     return Reverse(s.Substring(1)) + s[0]; }
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13. string reversal code
public String Reverse (String s) {   if (s.length()==1)     return s;   else     return Reverse(s.substring(1)) + s.charAt(0); }
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14. string reversal Complete Program
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15. string reversal Complete Program
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16. Non-linear Recursion
Folders and files within a computer system are an excellent example of a non-linear structure that is easily processed via recursion. 
This is also possible using iteration, but we would have more coding work to do in handling where we've been in the file system. 
We can leverage the activation stack to remember for us.
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17. Non-linear Recursion
Consider the steps to count the total number of files:
If we're in a folder with no files (or subfolders), then we return 0
Otherwise, we return the count of the files in the folder + all the files in the subfolders
It's the second part that's tricky... but notice that's where the recursive call comes in.  We just need to recursively call for each subfolder (so this is recursion wrapped inside of iteration):
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18. solution
private int CountFiles(DirectoryInfo di) { int file_count = 0; file_count += di.GetFiles().Length; foreach (DirectoryInfo sub_folder in di.GetDirectories())     file_count += CountFiles(sub_folder); return file_count; }
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