1. CIT 383: Administrative Scripting
Recursion
CIT 383: Administrative Scripting

2. CIT 383: Administrative Scripting
Topics
What is recursion?
Recursive definitions.
Examples.
Writing a recursive function.
Recursion in system administration.
CIT 383: Administrative Scripting

3. CIT 383: Administrative Scripting
Recursion
Recursion is a programming technique in which a function calls itself.
A recursive function must have both
A base case in which the function computes a result without calling itself.
One or more recursive cases where the function calls itself to compute the result.
CIT 383: Administrative Scripting

4. Recursive Definitions
How would you define a list of integers?
17, 42, 8, 97
We could define it by
A LIST is an: integer
or an: integer COMMA LIST
We defined a LIST using itself.
CIT 383: Administrative Scripting

5. Recursive Definitions
integer COMMA LIST
, ,8,97
integer COMMA LIST
, ,97
integer COMMA LIST ,
integer 97
CIT 383: Administrative Scripting

6. Recursive Definitions
The sum of the numbers 1 .. n is
sum(1..n) is: n + sum(1..n-1)
or: 1 if n is1
Ruby code:
def sum(n)
if n==1 1
else
n + sum(n-1)
end
CIT 383: Administrative Scripting

7. CIT 383: Administrative Scripting
sum(5) 5
sum
sum(4) 4
sum
sum(3) 3
sum
sum(2) 2
sum
sum(1) 1
sum
CIT 383: Administrative Scripting

8. Counting list elements
def count_list(list)
if list.empty?
else
1 + count_list(list[1..-1])
end
CIT 383: Administrative Scripting

9. CIT 383: Administrative Scripting
count_list([17, 42, 8, 97]) 4
count_list
count_list([42, 8, 97]) 3
count_list
count_list([8, 97]) 2
count_list
count_list([8]) 1
count_list
count_list()
count_list
CIT 383: Administrative Scripting

10. CIT 383: Administrative Scripting
Adding list elements
def sum_list(list)
if list.empty?
else
list[0] + sum_list(list[1..-1])
end
CIT 383: Administrative Scripting

11. Writing a Recursive Function
Write the base case first.
Recurse to a simpler case.
Avoid using globals.
CIT 383: Administrative Scripting

12. Write the base case first
Every recursive function must have something it can do without recursion.
There may be one or more base cases.
Integers: base case usually occurs for 0 or 1
Arrays: base case usually occurs for []
Writing a base case is writing ordinary, non- recursive code.
CIT 383: Administrative Scripting

13. Recurse to a simpler case
Break recursive case into two parts
A simpler problem (using a shorter array or smaller integer) of the same kind.
Extra work done by the recursive case.
Combine the results of the recursive work on the simpler problem and the extra work into a single solution.
CIT 383: Administrative Scripting

14. Recursion in System Administration
Searching the filesystem
Examples: find file that matches string or filename
Base cases: files
Recursive cases: directories
Spidering a web site
Examples: check for broken links, archiving
Base cases: documents
Recursive cases: hyperlinks
CIT 383: Administrative Scripting

15. CIT 383: Administrative Scripting
References
Michael Fitzgerald, Learning Ruby, O’Reilly, 2008.
David Flanagan and Yukihiro Matsumoto, The Ruby Programming Language, O’Reilly, 2008.
Hal Fulton, The Ruby Way, 2nd edition, Addison-Wesley, 2007.
Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.
CIT 383: Administrative Scripting