# New Mexico Computer Science For All

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New Mexico Computer Science For All
Recursion Maureen Psaila-Dombrowski

Recursion What is Recursion?
It is a concept/method used in computer science and mathematics Recursive problem: The problem can be described as a reduced or smaller form of the same problem Sometimes the problem gets so small that the solution of the small problem is trivial  can solved recursively or has a recursive implementation Recursion occurs in computer science when you use a recursive implementation ….WHAT?

Recursion - Example 5! = 5 * 4 * 3 * 2 * 1
Classic example is a factorial: n!  5! 5! = 5 * 4 * 3 * 2 * 1

Recursion - Example 5! = 5 * 4 * 3 * 2 * 1 NON – recursive definition
But ! = 4 * 3 * 2 * 1 5! = 5 * 4! Recursive definition SO… The problem can be broken down into a smaller or reduced version of the same problem    Maybe it can be solved recursively !

Recursion - Example TRIVIAL CASE
Look for the smallest MOST TRIVIAL form of the problem: 5! = 5 * 4 * 3 * 2 * 1  5! = 5 * 4! but 4! = 4 * 3 * 2 * 1 and 4! = 4 * 3!  since 3! = 3 * 2 * 1 but 3! = 3 * 2 * 1 and 3! = 3 * 2!  since 2! = 2 * 1 but 2! = 2 * 1!  since 1! = 1 TRIVIAL CASE

5! = 5 * 4! Can be solved Recursively
Recursion - Example So 5! = 5 * 4 * 3 * 2 * 1 Or 5! = 5 * 4! Can be solved Recursively Or in general n! = n * n-1 * n-2 * …* 3 * 2 * 1 n! = n * (n-1)!

Recursion in Computer Languages
Implemented by calling a function or procedure in the body of that same function or procedure. must be prevented from consuming excessive computing resources Factorial Psuedocode factorial N if N <=1 (then factorial = 1) (else factorial = N * factorial[N-1]) this is done by testing for trivial cases (or other stopping conditions), and avoiding recursive calls in those cases.

Recursive Programming
Pros Recursive description --- simple, elegant, and easy to explain and understand. Recursive implementation --- straightforward to build and verify. Cons Can be difficult to understand at first. In some programming languages not as efficient as iteratively Can be difficult to program circularity  infinite loop  MUST PUT IN A STOP Once you have a recursive description a recursive iimplementatio…

Why use Recursion? So if can be less efficient and difficult to understand and program, why use recursion? The problem/program may be easier to understand, solve and program using recursion When you have a recursive description  recursive solution is the most direct solution path Usually creating an easy to verify code is more important than creating the most efficient code

Recursion in NetLogo Draw a spiral from the outside in:

Recursion in NetLogo Iteratively Specify initial line length
Interative Draw While (condition?), repeat Turtle draws a line Turtle turns right Reduce line length

GO TO MODEL

But we forgot something!
Recursion in NetLogo Recursively Specify initial line length Recursive Draw Turtle draws a line Turtle turns right Draw a new line with a reduced length But we forgot something! We Forgot The STOP!

Recursion in NetLogo Recursively Specify initial line length
Recursive Draw If (condition?) STOP Turtle draws a line Turtle turns right draw a new live with a reduced length

GO to Code

Important Note For every recursive algorithm…… ….. There is an equivalent iterative (looping) algorithm!!

Summary Recursion: CS method
The solution depends on the solution to smaller instances of the same problem In most programming languages  a function or procedure calling itself in the code Pros: shorter, easier to understand, more elegant Cons: often not as efficient as iterative and can be difficult to program Why use recursion? The problem/program is easier to understand using recursion The problem is easier to solve using recursion

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