By our function definition, the factorial(0) should be 0!
= 0 * factorial(-1). Wrong. This is a good time to talk
about how one should write a recursive function, and what two
cases must be considered when using recursive techniques.

There are four important criteria to think about when writing a
recursive function.

What is the base case, and
can it be solved?

What is the general case?

Does the recursive call make the problem smaller and
approach the base case?

Base Case

The base case, or halting case, of a function is the
problem that we know the answer to, that can be solved without
any more recursive calls. The base case is what stops the
recursion from continuing on forever. Every recursive function
must have at least one base case (many functions have
more than one). If it doesn't, your function will not work
correctly most of the time, and will most likely cause your
program to crash in many situations, definitely not a desired
effect.

Let's return to our factorial example from above. Remember the
problem was that we never stopped the recursion process; we
didn't have a base case. Luckily, the factorial function in
math defines a base case for us. n! = n*(n - 1)! as long as
n > 1. If n = = 1 or n = = 0, then n! = 1. The factorial
function is undefined for values less than 0, so in our
implementation, we'll return some error value. Using this
updated definition, let's rewrite our factorial function.

int factorial(int n)
{
if (n<0) return 0; /* error value for inappropriate input */
else if (n<=1) return 1; /* if n==1 or n==0, n! = 1 */
else return n * factorial(n-1); /* n! = n * (n-1)! */
}

That's it! See how simple that was? Lets visualize what would
happen if we were to invoke this function, for example
factorial(3):

The General Case

The general case is what happens most of the time, and is where the
recursive call takes place. In the case of factorial, the general
case occurs when n > 1, meaning we use the equation and recursive
definition n! = n*(n - 1)!.

Diminishing Size of Problem

Our third requirement for a recursive function is that the on
each recursive call the problem must be approaching the base
case. If the problem isn't approaching the base case, we'll
never reach it and the recursion will never end. Imagine the
following incorrect implementation of factorial:

/* this is incorrect */
int factorial(int n)
{
if (n<0) return 0;
else if (n<=1) return 1;
else return n * factorial(n+1);
}

Note that on each recursive call, the size of n gets bigger, not
smaller. Since we initially start out larger than our base cases
(n==1 & n==0), we will be going away from the base cases, not
towards them. Thus we will never reach them. Besides being an
incorrect implementation of the factorial algorithm, this is bad
recursive design. The recursively called problems should always be
heading towards the base case.

Avoiding Circularity

Another problem to avoid when writing recursive functions is
circularity. Circularity occurs when you reach a point in
your recursion where the arguments to the function are the same
as with a previous function call in the stack. If this happens
you will never reach your base case, and the recursion will
continue forever, or until your computer crashes, whichever
comes first.

If this function is called with the value 1, then it calls
itself with the value 2, which in turn calls itself with
the value 1. See the circularity?

Sometimes it is hard to determine if a function is circular.
Take the Syracuse problem for example, which dates back to the
1930s.

int syracuse(int n)
{
if (n==1) return 0;
else if (n % 2 != 0) return syracuse(n/2);
else return 1 + syracuse(3*n + 1);
}

For small values of n, we know that this function is not
circular, but we don't know if there is some special value of
n out there that causes this function to become circular.

Recursion might not be the most efficient way to implement an
algorithm. Each time a function is called, there is a certain
amount of "overhead" that takes up memory and system
resources. When a function is called from another function,
all the information about the first function must be stored so
that the computer can return to it after executing the new
function.

The Call Stack

When a function is called, a certain amount of memory is set
aside for that function to use for purposes such as storing
local variables. This memory, called a frame, is also used by
the computer to store information about the function such as
the function's address in memory; this allows the program to
return to the proper place after a function call (for example,
if you write a function that calls printf(), you would like
control to return to your function after printf()
completes; this is made possible by the frame).

Every function has its own frame that is created when the
function is called. Since functions can call other functions,
often more than one function is in existence at any given time,
and therefore there are multiple frames to keep track of.
These frames are stored on the call stack, an area of memory
devoted to holding information about currently running
functions.

A stack is a LIFO data-type, meaning that the last item to
enter the stack is the first item to leave, hence LIFO, Last In
First Out. Compare this to a queue, or the line for the teller
window at a bank, which is a FIFO data structure. The first
people to enter the queue are the first people to leave it,
hence FIFO, First In First Out. A useful example in
understanding how a stack works is the pile of trays in your
school's dining hall. The trays are stacked one on top of the
other, and the last tray to be put on the stack is the first
one to be taken off.

In the call stack, the frames are put on top of each other in
the stack. Adhering to the LIFO principle, the last function
to be called (the most recent one) is at the top of the stack
while the first function to be called (which should be the
main() function) resides at the bottom of the stack. When
a new function is called (meaning that the function at the top
of the stack calls another function), that new function's frame
is pushed onto the stack and becomes the active frame. When a
function finishes, its frame is destroyed and removed from the
stack, returning control to the frame just below it on the
stack (the new top frame).

Let's take an example. Suppose we have the following functions:

We can trace the flow of functions in the program by looking at
the call stack. The program begins by calling main() and
so the main() frame is placed on the stack.

The main() function then calls the function stephen().
The stephen() function then calls the function theSpark().
When the function theSpark() is finished executing, its
frame is deleted from the stack and control returns to the
stephen() frame.
After regaining control, stephen() then calls SparkNotes().
When the function SparkNotes() is finished executing, its
frame is deleted from the stack and control returns to
stephen().
When stephen() is finished, its frame is deleted and
control returns to main().
When the main() function is done, it is removed from the
call stack. As there are no more functions on the call stack,
and thus no where to return to after main() finishes, the
program is finished.

Recursion and the Call Stack

When using recursive techniques, functions "call themselves".
If the function stephen() were recursive, stephen()
might make a call to stephen() during the course of its
execution. However, as mentioned before, it is important to
realize that every function called gets its own frame, with its
own local variables, its own address, etc. As far as the
computer is concerned, a recursive call is just like any other
call.

Changing the example from above, let's say the stephen
function calls itself. When the program begins, a frame for
main() is placed on the call stack.
main() then calls stephen() which is placed on the stack.

stephen() then makes a recursive call to itself, creating a
new frame which is placed on the stack.

Overhead of Recursion

Imagine what happens when you call the factorial function on
some large input, say 1000. The first function will be called
with input 1000. It will call the factorial function on an
input of 999, which will call the factorial function on an
input of 998. Etc. Keeping track of the information about all
active functions can use many system resources if the recursion
goes many levels deep. In addition, functions take a small
amount of time to be instantiated, to be set up. If you have a
lot of function calls in comparison to the amount of work each
one is actually doing, your program will run significantly
slower.

So what can be done about this? You'll need to decide up front
whether recursion is necessary. Often, you'll decide that an
iterative implementation would be more efficient and almost as
easy to code (sometimes they'll be easier, but rarely). It has
been proven mathematically that any problem that can be solved
with recursion can also be solved with iteration, and vice
versa. However, there are certainly cases where recursion is a
blessing, and in these instances you should not shy away from
using it. As we'll see later, recursion is often a useful tool
when working with data structures such as trees (if you have no
experience with trees, please see the SparkNote on the
subject).

As an example of how a function can be written both recursively and
iteratively, let's look again to the factorial function.

We originally said that the 5! = 5*4*3*2*1 and 9! = 9*8*7*6*5*4*3*2*1. Let's use this definition
instead of the recursive one to write our function iteratively.
The factorial of an integer is that number multiplied by all
integers smaller than it and greater than 0.

int factorial(int n)
{
int fact=1;
/* error check */
if (n<0) return 0;
/* multiply n by all numbers smaller than n and greater than 0 */
for( ; n>0; n--) fact *= n;
/* return the result */
return(fact);
}

This program is more efficient and should execute faster
than the recursive solution above.

For mathematical problems like factorial, there is sometimes an
alternative to both an iterative and a recursive
implementation: a closed-form solution. A closed-form solution
is a formula that involves no looping of any kind, only
standard mathematical operations in a formula to compute the
answer. The Fibonacci function, for example, does have a
closed-form solution:

This solution and implementation uses four calls to sqrt(),
two calls to pow(), two additions, two subtractions, two
multiplications, and four divisions. One might argue that this
is more efficient than both the recursive and iterative
solutions for large values of n. Those solutions involve a
lot of looping/repetition, while this solution does not.
However, without the source code for pow(), it is
impossible to say that this is more efficient. Most likely,
the bulk of the cost of this function is in the calls to
pow(). If the programmer for pow() wasn't smart about
the algorithm, it could have as many as n - 1 multiplications,
which would make this solution slower than the iterative, and
possibly even the recursive, implementation.

Given that recursion is in general less efficient, why would we
use it? There are two situations where recursion is the best
solution:

The problem is much more clearly solved using
recursion: there are many problems where the recursive solution
is clearer, cleaner, and much more understandable. As long as
the efficiency is not the primary concern, or if the
efficiencies of the various solutions are comparable, then you
should use the recursive solution.

Some problems are much
easier to solve through recursion: there are some problems
which do not have an easy iterative solution. Here you should
use recursion. The Towers of Hanoi problem is an example of
a problem where an iterative solution would be very difficult.
We'll look at Towers of Hanoi in a later section of this guide.