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Lisp - Recursion
Recursion, is one of the core concept of a function programming language. LISP, being a functional programming language, prefers recursions over iterative solutions. In this chapter, let's delve into the concept of recursion and its usage in LISP.
What is Recursion
Recursion is a programming technique where a function calls itself within its own definition.
Purpose of recursion is to solve problem by breaking it into sub-problems.
As a function calls itself, there must be a base case to break the infinite calling. Base case is to break the recursive calls.
Why Recursion
In LISP, functions are first class citizens, thus recursion is a natural fit here.
Problems can be expressed easily in recursive form.
As LISP is primarily using list processing capabilties, recursion is well suited for traversing and manipulating lists.
Key Components of Recursion
Base Case − A condition to stop the recursive call. It is generally the simplest case returning a result.
Recursive Case − A condition to call the function itself. Next function should reduce the scope and should bring the problem closer to the base case.
Example - Factorial using Recursion
Following is an example of a factorial function using recursive techniques.
main.lisp
; define a function to compute factorial (defun factorial (n) (if (= n 0) ; base case 1 (* n (factorial (- n 1))))) ; recursive call ; call factorial of 5; evaluates to 120 (print (factorial 5))
Output
When you execute the code, it returns the following result −
120
Explanation
(= n 0)− This is a base case. If n is 0, 1 is returned as factorial of 0 is 1.
(* n (factorial (- n 1)))− This is a recursive case. Here we're multiplying factorial of n-1 with n. This call continues until n reaches 0.
While using recursion, special care is required as stack overflow conditions is easy to occur and hard to debug. And when used appropriately, a recursive code is quite elegant and concise.