Emacs Lisp has two primary ways to cause an expression, or a series of
expressions, to be evaluated repeatedly: one uses a
loop, and the other uses recursion.
Repetition can be very valuable. For example, to move forward four sentences, you need only write a program that will move forward one sentence and then repeat the process four times. Since a computer does not get bored or tired, such repetitive action does not have the deleterious effects that excessive or the wrong kinds of repetition can have on humans.
People mostly write Emacs Lisp functions using
while loops and
their kin; but you can use recursion, which provides a very powerful
way to think about and then to solve problems1.
 You can write
recursive functions to be frugal or wasteful of mental or computer
resources; as it happens, methods that people find easy—that are
frugal of `mental resources'—sometimes use considerable computer
resources. Emacs was designed to run on machines that we now consider
limited and its default settings are conservative. You may want to
increase the values of
max-lisp-eval-depth. In my .emacs file, I set them to
15 and 30 times their default value.