Recursion (computer science)
Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem (as opposed to iteration).^{[1]} The approach can be applied to many types of problems, and recursion is one of the central ideas of computer science.^{[2]}
"The power of recursion evidently lies in the possibility of defining an infinite set of objects by a finite statement. In the same manner, an infinite number of computations can be described by a finite recursive program, even if this program contains no explicit repetitions."^{[3]}
Most computer programming languages support recursion by allowing a function to call itself within the program text. Some functional programming languages do not define any looping constructs but rely solely on recursion to repeatedly call code. Computability theory proves that these recursiveonly languages are Turing complete;
they are as computationally powerful as Turing complete imperative
languages, meaning they can solve the same kinds of problems as
imperative languages even without iterative control structures such as
“while” and “for”.
Contents
[hide] 1 Recursive functions and algorithms
 2 Recursive data types
 3 Types of recursion
 4 Recursive programs
 5 Implementation issues
 6 Recursion versus iteration
 7 Tailrecursive functions
 8 Order of execution
 9 Timeefficiency of recursive algorithms
 10 See also
 11 Notes and references
 12 Further reading
 13 External links
Recursive functions and algorithms[edit]
A common computer programming tactic is to divide a problem into subproblems of the same type as the original, solve those subproblems, and combine the results. This is often referred to as the divideandconquer method; when combined with a lookup table that stores the results of solving subproblems (to avoid solving them repeatedly and incurring extra computation time), it can be referred to as dynamic programming or memoization.
A recursive function definition has one or more base cases, meaning input(s) for which the function produces a result trivially (without recurring), and one or more recursive cases, meaning input(s) for which the program recurs (calls itself). For example, the factorial function can be defined recursively by the equations 0! = 1 and, for all n > 0, n! = n(n − 1)!. Neither equation by itself constitutes a complete definition; the first is the base case, and the second is the recursive case. Because the base case breaks the chain of recursion, it is sometimes also called the "terminating case".
The job of the recursive cases can be seen as breaking down complex inputs into simpler ones. In a properly designed recursive function, with each recursive call, the input problem must be simplified in such a way that eventually the base case must be reached. (Functions that are not intended to terminate under normal circumstances—for example, some system and server processes—are an exception to this.) Neglecting to write a base case, or testing for it incorrectly, can cause an infinite loop.
For some functions (such as one that computes the series for e = 1/0! + 1/1! + 1/2! + 1/3! + ...) there is not an obvious base case implied by the input data; for these one may add a parameter (such as the number of terms to be added, in our series example) to provide a 'stopping criterion' that establishes the base case. Such an example is more naturally treated by corecursion, where successive terms in the output are the partial sums; this can be converted to a recursion by using the indexing parameter to say "compute the nth term (nth partial sum)".
Recursive data types[edit]
Many computer programs must process or generate an arbitrarily large quantity of data. Recursion is one technique for representing data whose exact size the programmer does not know: the programmer can specify this data with a selfreferential definition. There are two types of selfreferential definitions: inductive and coinductive definitions.
Inductively defined data[edit]
An inductively defined recursive data definition is one that specifies how to construct instances of the data. For example, linked lists can be defined inductively (here, using Haskell syntax):

data ListOfStrings = EmptyList  Cons String ListOfStrings
The code above specifies a list of strings to be either empty, or a structure that contains a string and a list of strings. The selfreference in the definition permits the construction of lists of any (finite) number of strings.
Another example of inductive definition is the natural numbers (or positive integers):
A natural number is either 1 or n+1, where n is a natural number.
Similarly recursive definitions are often used to model the structure of expressions and statements in programming languages. Language designers often express grammars in a syntax such as BackusNaur form; here is such a grammar, for a simple language of arithmetic expressions with multiplication and addition:
<expr> ::= <number>
 (<expr> * <expr>)
 (<expr> + <expr>)
This says that an expression is either a number, a product of two
expressions, or a sum of two expressions. By recursively referring to
expressions in the second and third lines, the grammar permits
arbitrarily complex arithmetic expressions such as (5 * ((3 * 6) + 8))
, with more than one product or sum operation in a single expression.
Coinductively defined data and corecursion[edit]
A coinductive data definition is one that specifies the operations that may be performed on a piece of data; typically, selfreferential coinductive definitions are used for data structures of infinite size.
A coinductive definition of infinite streams of strings, given informally, might look like this:
A stream of strings is an object s such that: head(s) is a string, and tail(s) is a stream of strings.
This is very similar to an inductive definition of lists of strings;
the difference is that this definition specifies how to access the
contents of the data structure—namely, via the accessor functions head
and tail
—and
what those contents may be, whereas the inductive definition specifies
how to create the structure and what it may be created from.
Corecursion is related to coinduction, and can be used to compute particular instances of (possibly) infinite objects. As a programming technique, it is used most often in the context of lazy programming languages, and can be preferable to recursion when the desired size or precision of a program's output is unknown. In such cases the program requires both a definition for an infinitely large (or infinitely precise) result, and a mechanism for taking a finite portion of that result. The problem of computing the first n prime numbers is one that can be solved with a corecursive program (e.g. here).
Types of recursion[edit]
Single recursion and multiple recursion[edit]
Recursion that only contains a single selfreference is known as single recursion, while recursion that contains multiple selfreferences is known as multiple recursion. Standard examples of single recursion include list traversal, such as in a linear search, or computing the factorial function, while standard examples of multiple recursion include tree traversal, such as in a depthfirst search.
Single recursion is often much more efficient than multiple recursion, and can generally be replaced by an iterative computation, running in linear time and requiring constant space. Multiple recursion, by contrast, may require exponential time and space, and is more fundamentally recursive, not being able to be replaced by iteration without an explicit stack.
Multiple recursion can sometimes be converted to single recursion (and, if desired, thence to iteration). For example, while computing the Fibonacci sequence naively is multiple iteration, as each value requires two previous values, it can be computed by single recursion by passing two successive values as parameters. This is more naturally framed as corecursion, building up from the initial values, tracking at each step two successive values – see corecursion: examples. A more sophisticated example is using a threaded binary tree, which allows iterative tree traversal, rather than multiple recursion.
Indirect recursion[edit]
Most basic examples of recursion, and most of the examples presented here, demonstrate direct recursion, in which a function calls itself. Indirect recursion occurs when a function is called not by itself but by another function that it called (either directly or indirectly). For example, if f calls f, that is direct recursion, but if f calls g which calls f, then that is indirect recursion of f. Chains of three or more functions are possible; for example, function 1 calls function 2, function 2 calls function 3, and function 3 calls function 1 again.
Indirect recursion is also called mutual recursion, which is a more symmetric term, though this is simply a difference of emphasis, not a different notion. That is, if f calls g and then g calls f, which in turn calls g again, from the point of view of f alone, f is indirectly recursing, while from the point of view of g alone, it is indirectly recursing, while from the point of view of both, f and g are mutually recursing on each other. Similarly a set of three or more functions that call each other can be called a set of mutually recursive functions.
Anonymous recursion[edit]
Recursion is usually done by explicitly calling a function by name. However, recursion can also be done via implicitly calling a function based on the current context, which is particularly useful for anonymous functions, and is known as anonymous recursion.
Structural versus generative recursion[edit]
Some authors classify recursion as either "structural" or "generative". The distinction is related to where a recursive procedure gets the data that it works on, and how it processes that data:
[Functions that consume structured data] typically decompose their arguments into their immediate structural components and then process those components. If one of the immediate components belongs to the same class of data as the input, the function is recursive. For that reason, we refer to these functions as (STRUCTURALLY) RECURSIVE FUNCTIONS.^{[4]}
Thus, the defining characteristic of a structurally recursive function is that the argument to each recursive call is the content of a field of the original input. Structural recursion includes nearly all tree traversals, including XML processing, binary tree creation and search, etc. By considering the algebraic structure of the natural numbers (that is, a natural number is either zero or the successor of a natural number), functions such as factorial may also be regarded as structural recursion.
Generative recursion is the alternative:
Many wellknown recursive algorithms generate an entirely new piece of data from the given data and recur on it. HtDP (How To Design Programs) refers to this kind as generative recursion. Examples of generative recursion include: gcd, quicksort, binary search, mergesort, Newton's method, fractals, and adaptive integration.^{[5]}
This distinction is important in proving termination of a function.
 All structurally recursive functions on finite (inductively defined) data structures can easily be shown to terminate, via structural induction: intuitively, each recursive call receives a smaller piece of input data, until a base case is reached.
 Generatively recursive functions, in contrast, do not necessarily feed smaller input to their recursive calls, so proof of their termination is not necessarily as simple, and avoiding infinite loops requires greater care. These generatively recursive functions can often be interpreted as corecursive functions – each step generates the new data, such as successive approximation in Newton's method – and terminating this corecursion requires that the data eventually satisfy some condition, which is not necessarily guaranteed.
 In terms of loop variants, structural recursion is when there is an obvious loop variant, namely size or complexity, which starts off finite and decreases at each recursive step.
 By contrast, generative recursion is when there is not such an obvious loop variant, and termination depends on a function, such as "error of approximation" that does not necessarily decrease to zero, and thus termination is not guaranteed without further analysis.
Recursive programs[edit]
Recursive procedures[edit]
Factorial[edit]
A classic example of a recursive procedure is the function used to calculate the factorial of a natural number:
Pseudocode (recursive): 

function factorial is: 
The function can also be written as a recurrence relation:
This evaluation of the recurrence relation demonstrates the computation that would be performed in evaluating the pseudocode above:
Computing the recurrence relation for n = 4: 

b_{4} = 4 * b_{3} 
This factorial function can also be described without using recursion by making use of the typical looping constructs found in imperative programming languages:
Pseudocode (iterative): 

function factorial is: 
The imperative code above is equivalent to this mathematical definition using an accumulator variable t:
The definition above translates straightforwardly to functional programming languages such as Scheme; this is an example of iteration implemented recursively.
Greatest common divisor[edit]
The Euclidean algorithm, which computes the greatest common divisor of two integers, can be written recursively.
Function definition:
Pseudocode (recursive): 

function gcd is: input: integer x, integer y such that x > 0 and y >= 0 
Recurrence relation for greatest common divisor, where expresses the remainder of :
 if
Computing the recurrence relation for x = 27 and y = 9: 

gcd(27, 9) = gcd(9, 27% 9) = gcd(9, 0) = 9 
Computing the recurrence relation for x = 111 and y = 259: 
gcd(111, 259) = gcd(259, 111% 259) = gcd(259, 111) = gcd(111, 259% 111) = gcd(111, 37) = gcd(37, 111% 37) = gcd(37, 0) = 37 
The recursive program above is tailrecursive; it is equivalent to an iterative algorithm, and the computation shown above shows the steps of evaluation that would be performed by a language that eliminates tail calls. Below is a version of the same algorithm using explicit iteration, suitable for a language that does not eliminate tail calls. By maintaining its state entirely in the variables x and y and using a looping construct, the program avoids making recursive calls and growing the call stack.
Pseudocode (iterative): 

function gcd is: 
The iterative algorithm requires a temporary variable, and even given knowledge of the Euclidean algorithm it is more difficult to understand the process by simple inspection, although the two algorithms are very similar in their steps.
Towers of Hanoi[edit]
The Towers of Hanoi is a mathematical puzzle whose solution illustrates recursion.^{[6]}^{[7]} There are three pegs which can hold stacks of disks of different diameters. A larger disk may never be stacked on top of a smaller. Starting with n disks on one peg, they must be moved to another peg one at a time. What is the smallest number of steps to move the stack?
Function definition:
Recurrence relation for hanoi:
Computing the recurrence relation for n = 4: 

hanoi(4) = 2*hanoi(3) + 1 = 2*(2*hanoi(2) + 1) + 1 = 2*(2*(2*hanoi(1) + 1) + 1) + 1 = 2*(2*(2*1 + 1) + 1) + 1 = 2*(2*(3) + 1) + 1 = 2*(7) + 1 = 15 
Example implementations:
Pseudocode (recursive): 

function hanoi is: 
Although not all recursive functions have an explicit solution, the Tower of Hanoi sequence can be reduced to an explicit formula.^{[8]}
An explicit formula for Towers of Hanoi: 

h_{1} = 1 = 2^{1}  1 h_{2} = 3 = 2^{2}  1 h_{3} = 7 = 2^{3}  1 h_{4} = 15 = 2^{4}  1 h_{5} = 31 = 2^{5}  1 h_{6} = 63 = 2^{6}  1 h_{7} = 127 = 2^{7}  1 In general: h_{n} = 2^{n}  1, for all n >= 1 
Binary search[edit]
The binary search algorithm is a method of searching a sorted array for a single element by cutting the array in half with each recursive pass. The trick is to pick a midpoint near the center of the array, compare the data at that point with the data being searched and then responding to one of three possible conditions: the data is found at the midpoint, the data at the midpoint is greater than the data being searched for, or the data at the midpoint is less than the data being searched for.
Recursion is used in this algorithm because with each pass a new array is created by cutting the old one in half. The binary search procedure is then called recursively, this time on the new (and smaller) array. Typically the array's size is adjusted by manipulating a beginning and ending index. The algorithm exhibits a logarithmic order of growth because it essentially divides the problem domain in half with each pass.
Example implementation of binary search in C:
/*
Call binary_search with proper initial conditions.
INPUT:
data is an array of integers SORTED in ASCENDING order,
toFind is the integer to search for,
count is the total number of elements in the array
OUTPUT:
result of binary_search
*/
int search(int *data, int toFind, int count)
{
// Start = 0 (beginning index)
// End = count  1 (top index)
return binary_search(data, toFind, 0, count1);
}
/*
Binary Search Algorithm.
INPUT:
data is a array of integers SORTED in ASCENDING order,
toFind is the integer to search for,
start is the minimum array index,
end is the maximum array index
OUTPUT:
position of the integer toFind within array data,
1 if not found
*/
int binary_search(int *data, int toFind, int start, int end)
{
//Get the midpoint.
int mid = start + (end  start)/2; //Integer division
//Stop condition.
if (start > end)
return 1;
else if (data[mid] == toFind) //Found?
return mid;
else if (data[mid] > toFind) //Data is greater than toFind, search lower half
return binary_search(data, toFind, start, mid1);
else //Data is less than toFind, search upper half
return binary_search(data, toFind, mid+1, end);
}
Recursive data structures (structural recursion)[edit]
An important application of recursion in computer science is in defining dynamic data structures such as lists and trees. Recursive data structures can dynamically grow to a theoretically infinite size in response to runtime requirements; in contrast, the size of a static array must be set at compile time.
"Recursive algorithms are particularly appropriate when the underlying problem or the data to be treated are defined in recursive terms."^{[9]}
The examples in this section illustrate what is known as "structural recursion". This term refers to the fact that the recursive procedures are acting on data that is defined recursively.
As long as a programmer derives the template from a data definition, functions employ structural recursion. That is, the recursions in a function's body consume some immediate piece of a given compound value.^{[5]}
Linked lists[edit]
Below is a C definition of a linked list node structure. Notice especially how the node is defined in terms of itself. The "next" element of struct node is a pointer to another struct node, effectively creating a list type.
struct node
{
int data; // some integer data
struct node *next; // pointer to another struct node
};
Because the struct node data structure is defined recursively, procedures that operate on it can be implemented naturally as recursive procedures. The list_print procedure defined below walks down the list until the list is empty (i.e., the list pointer has a value of NULL). For each node it prints the data element (an integer). In the C implementation, the list remains unchanged by the list_print procedure.
void list_print(struct node *list)
{
if (list != NULL) // base case
{
printf ("%d ", list>data); // print integer data followed by a space
list_print (list>next); // recursive call on the next node
}
}
Binary trees[edit]
Below is a simple definition for a binary tree node. Like the node for linked lists, it is defined in terms of itself, recursively. There are two selfreferential pointers: left (pointing to the left subtree) and right (pointing to the right subtree).
struct node
{
int data; // some integer data
struct node *left; // pointer to the left subtree
struct node *right; // point to the right subtree
};
Operations on the tree can be implemented using recursion. Note that because there are two selfreferencing pointers (left and right), tree operations may require two recursive calls:
// Test if tree_node contains i; return 1 if so, 0 if not.
int tree_contains(struct node *tree_node, int i) {
if (tree_node == NULL)
return 0; // base case
else if (tree_node>data == i)
return 1;
else
return tree_contains(tree_node>left, i)  tree_contains(tree_node>right, i);
}
At most two recursive calls will be made for any given call to tree_contains as defined above.
// Inorder traversal:
void tree_print(struct node *tree_node) {
if (tree_node != NULL) { // base case
tree_print(tree_node>left); // go left
printf("%d ", tree_node>data); // print the integer followed by a space
tree_print(tree_node>right); // go right
}
}
The above example illustrates an inorder traversal of the binary tree. A Binary search tree is a special case of the binary tree where the data elements of each node are in order.
Filesystem traversal[edit]
Since the number of files in a filesystem may vary, recursion is the only practical way to traverse and thus enumerate its contents. Traversing a filesystem is very similar to that of tree traversal, therefore the concepts behind tree traversal are applicable to traversing a filesystem. More specifically, the code below would be an example of a preorder traversal of a filesystem.
import java.io.*;
public class FileSystem {
public static void main (String [] args) {
traverse ();
}
/**
* Obtains the filesystem roots
* Proceeds with the recursive filesystem traversal
*/
private static void traverse () {
File [] fs = File.listRoots ();
for (int i = 0; i < fs.length; i++) {
if (fs[i].isDirectory () && fs[i].canRead ()) {
rtraverse (fs[i]);
}
}
}
/**
* Recursively traverse a given directory
*
* @param fd indicates the starting point of traversal
*/
private static void rtraverse (File fd) {
File [] fss = fd.listFiles ();
for (int i = 0; i < fss.length; i++) {
System.out.println (fss[i]);
if (fss[i].isDirectory () && fss[i].canRead ()) {
rtraverse (fss[i]);
}
}
}
}
This code blends the lines, at least somewhat, between recursion and iteration. It is, essentially, a recursive implementation, which is the best way to traverse a filesystem. It is also an example of direct and indirect recursion. The method "rtraverse" is purely a direct example; the method "tr