Editing Lisp (programming language) (section)

== Examples ==

Here are examples of Common Lisp code.

The basic "[[Hello, World!]]" program:
<syntaxhighlight lang="Lisp">
(print "Hello, World!")
</syntaxhighlight>

Lisp syntax lends itself naturally to recursion. Mathematical problems such as the enumeration of recursively defined sets are simple to express in this notation. For example, to evaluate a number's [[factorial]]:
<syntaxhighlight lang="Lisp">
(defun factorial (n)
    (if (zerop n) 1
        (* n (factorial (1- n)))))
</syntaxhighlight>

An alternative implementation takes less stack space than the previous version if the underlying Lisp system optimizes [[tail recursion]]:
<syntaxhighlight lang="Lisp">
(defun factorial (n &optional (acc 1))
    (if (zerop n) acc
        (factorial (1- n) (* acc n))))
</syntaxhighlight>

Contrast the examples above with an iterative version which uses [[Common Lisp]]'s {{Lisp2|loop}} macro:
<syntaxhighlight lang="Lisp">
(defun factorial (n)
    (loop for i from 1 to n
        for fac = 1 then (* fac i)
        finally (return fac)))
</syntaxhighlight>

The following function reverses a list. (Lisp's built-in ''reverse'' function does the same thing.)
<syntaxhighlight lang="Lisp">
(defun -reverse (list)
    (let ((return-value))
      (dolist (e list) (push e return-value))
      return-value))
</syntaxhighlight>