This document is part of the HTML publication "**An Introduction to the Imperative Part of C++**"

The original version was produced by **Rob Miller** at **Imperial College London**, September 1996.

Version 1.1 (modified by **David Clark** at **Imperial College London**, September 1997)

Version 1.2 (modified by **Bob White** at **Imperial College London**, September 1998)

Version 1.3, 1.4, 2.0, ..., 2.19 (modified by **William Knottenbelt** at **Imperial College London**, September 1999-September 2020)

We have already seen how, in a well designed C++ program, many
function definitions include calls to
other functions (for example, in the
last lecture
the definition of "`assign_list(...)`" included a call to
"`assign_new_node(...)`"). A function is *recursive*
(or has a *recursive definition*) if the definition includes
a call to itself.

Recursion is a familiar idea in mathematics and logic. For example, the natural numbers themselves are usually defined recursively. Very roughly speaking, the definition is:

- 0 is a natural number.
- if n is a natural number then s(n) (i.e. n+1) is a natural number, where s is the "successor function".

In this context, the notion of recursion is clearly
related to the notion of *mathematical induction*. Notice also
that the above definition includes a non-recursive
part or *base case* (the statement that 0 is a natural
number).

Another familiar mathematical example of a recursive function is the factorial function "!". Its definition is:

- 0! = 1
- for all n > 0, n! = nx(n-1)!

Thus, by repeatedly using the definition, we can work out that

6! = 6x5! = 6x5x4! = 6x5x4x3! = 6x5x4x3x2! = 6x5x4x3x2x1! = 6x5x4x3x2x1x1 = 720

Again, notice that the definition of "!" includes both a base case (the definition of 0!) and a recursive part.

The following program includes a call to the recursively
defined function "`print_backwards()`",
which inputs a series of characters from the keyboard,
terminated with a full-stop character, and then prints
them backwards on the screen.

#include<iostream> using namespace std; void print_backwards(); int main() { print_backwards(); cout << "\n"; return 0; } void print_backwards() { char character; cout << "Enter a character ('.' to end program): "; cin >> character; if (character != '.') { print_backwards(); cout << character; } }

A typical input/output session is:

Enter a character ('.' to end program):HEnter a character ('.' to end program):iEnter a character ('.' to end program):.iH

We will examine how this function works in more detail
in the next section. But notice that the recursive
call to "`print_backwards()`" (within its own definition)
is embedded in an "if" statement. In general,
recursive definitions must always use some sort of branch
statement with at least one non-recursive branch,
which acts as the base case of the definition. Otherwise
they will "loop forever". In Program 8.2.1 the base
case is in the implicit "else" part of the "if" statement.
We could have written the function as follows:

void print_backwards() { char character; cout << "Enter a character ('.' to end program): "; cin >> character; if (character != '.') { print_backwards(); cout << character; } else { ; } }

It is easy to see why Program 8.2.1 works with the aid of a few diagrams. When the main program executes, it begins with a call to "print_backwards()". At this point space is set aside in the computer's memory to execute this call (and in other cases in which to make copies of the value parameters). This space is represented as a box in Figure 8.3.1a:

**Figure 8.3.1a**

The internal execution of this call begins with a character
input, and then a second call to
"`print_backwards()`" (at this point, nothing has been
output to the screen). Again, space is set aside
for this second call:

**Figure 8.3.1b**

The process repeats, but inside the third call to
"`print_backwards()`" a full-stop character is input,
thus allowing the third call to terminate with no
further function calls:

**Figure 8.3.1c**

This allows the second call to "`print_backwards()`"
to terminate by outputting an "`i`" character,
which in turn allows the first call to terminate by
outputting an "H" character:

**Figure 8.3.1d**

Technically speaking, C++ arranges the memory spaces
needed for each function call in a *stack*. The
memory area for each new call is placed on the top
of the stack, and then taken off again when the execution
of the call is completed. In the example above, the
stack goes through the following sequence:

**Figure 8.3.2**

C++ uses this stacking principle for all nested function
calls - not just for recursively defined functions. A
stack is an example of a "last in/first out" structure
(as opposed to, for example, a *queue*, which is a "first
in/first out" structure).

Below are three more examples of recursive functions. We have already seen a function to calculate the factorial of a positive integer (Lecture 3, Program 3.3.1). Here's an alternative, recursive definition:

int factorial(int number) { if (number < 0) { cout << "\nError - negative argument to factorial\n"; exit(1); } else if (number == 0) return 1; else return (number * factorial(number - 1)); }

**Fragment of
Program 8.4.1**

As a second example, here's a function which raises
its first argument (of type "`float`") to the power of
its second (non-negative integer) argument:

float raised_to_power(float number, int power) { if (power < 0) { cout << "\nError - can't raise to a negative power\n"; exit(1); } else if (power == 0) return (1.0); else return (number * raised_to_power(number, power - 1)); }

**Fragment of
Program 8.4.2**

In both cases, care has been taken that a call to the function will not cause an "infinite loop" - i.e. that the arguments to the functions will either cause the program to exit with an error message, or are such that the series of recursive calls will eventually terminate with a base case.

The third example recursive function sums the
first n elements of an integer array "`a[]`".

int sum_of(int a[], int n) { if (n < 1 || n > MAXIMUM_NO_OF_ELEMENTS) { cout << "\nError - can only sum 1 to "; cout << MAXIMUM_NO_OF_ELEMENTS << " elements\n"; exit(1); } else if (n == 1) return a[0]; else return (a[n-1] + sum_of(a,n-1)); }

**Fragment of
Program 8.4.3**

From a purely mechanical point of view, recursion is not absolutely necessary, since any function that can be defined recursively can also be defined iteratively, i.e. defined using "for", "while" and "do...while" loops. So whether a function is defined recursively or iteratively is to some extent a matter of taste. Here are two of the recursive functions above, re-defined iteratively:

void print_backwards() { char chars[MAX_ARRAY_LENGTH]; int no_of_chars_input = 0; do { cout << "Enter a character ('.' to end program): "; cin >> chars[no_of_chars_input]; no_of_chars_input++; } while (chars[no_of_chars_input - 1] != '.' && no_of_chars_input < MAX_ARRAY_LENGTH); for (int count = no_of_chars_input - 2 ; count >=0 ; count--) cout << chars[count]; }

**Fragment of
Program 8.2.1b**

int factorial(int number) { int product = 1; if (number < 0) { cout << "\nError - negative argument to factorial\n"; exit(1); } else if (number == 0) return 1; else { for ( ; number > 0 ; number--) product *= number; return product; } }

**Fragment of
Program 8.4.1b**

It is a matter of debate whether, for a particular function,
a recursive definition is clearer than an iterative
one. Usually, an iterative definition will include more
local variable declarations - for example, the array
"`chars[MAX_ARRAY_LENGTH]`" in the first example above, and
the integer variable "`product`" in the
second example. In other words, temporary memory allocation
is made explicit in the iterative versions of
the functions by declaring variables, whereas it is implicit
in the recursive definitions (C++ is implicitly
asked to manipulate the stack by use of recursive calls).

Because of extra stack manipulation, recursive versions of functions often run slower and use more memory than their iterative counterparts. But this is not always the case, and recursion can sometimes make code easier to understand.

Recursive function definitions are often particularly
useful when the program is manipulating *recursive data
structures*. We have already seen one definition of
a recursive data structure - the definition of a node in a
linked list is given in terms of itself:

struct Node { char word[MAX_WORD_LENGTH]; Node *ptr_to_next_node; };

Later in the course you will study other recursive data structures in more detail, and see how associated recursive function definitions behave in these contexts.

We will end this lecture by briefly looking at a standard
recursive procedure for sorting. *Quick sort* is a
recursively defined procedure for rearranging the values
stored in an array in ascending or descending
order.

Suppose we start with the following array of 11 integers:

**Figure 8.7.1**

The idea is to use a process which separates the
list into two parts, using a distinguished value in the list
called a *pivot*. At the end of the process, one part
will contain only values less than or equal to the pivot,
and the other will contain only values greater than
or equal to the pivot. So, if we pick 8 as the pivot, at the
end of the process we will end up with something like:

**Figure 8.7.2**

We can then reapply exactly the same process to the left-hand and right-hand parts separately. This re- application of the same procedure leads to a recursive definition.

The detail of the rearranging procedure is as follows. The index of the pivot value is chosen simply by evaluating

(first + last) / 2

where "`first`" and "`last`" are the indices of the initial
and final elements in the array representing the
list. We then identify a "`left_arrow`" and a "`right_arrow`"
on the far left and the far right
respectively. This can be envisioned as:

**Figure 8.7.3**

so that "`left_arrow`" and "`right_arrow`" initially represent
the lowest and highest indices of the array
components. Starting on the right, the "`right_arrow`" is
moved left until a value less than or equal to the
pivot is encountered. This produces:

**Figure 8.7.4**

In a similar manner, "`left_arrow`" is moved right until
a value greater than or equal to the pivot is
encountered. This is already the situation in our example.
Now the contents of the two array components
are swapped to produce:

**Figure 8.7.5**

We continue by moving "`right_arrow`" left to produce:

**Figure 8.7.6**

and then "`left_arrow`" right to produce:

**Figure 8.7.7**

These values are exchanged to produce:

**Figure 8.7.8**

This part of the process only stops when the condition
"`left_arrow > right_arrow`" becomes
`True`. Since in Figure 8.7.8 this condition is still
`False`, we move "`right_arrow`" left again to
produce:

**Figure 8.7.9**

and "`left_arrow`" right again to produce:

**Figure 8.7.10**

Because we are looking for a value greater than or
equal to "`pivot`" when moving right, "`left_arrow`"
stops moving and an exchange is made (this time
involving the pivot) to produce:

**Figure 8.7.11**

It is acceptable to exchange the pivot because "`pivot`"
is the value itself, not the index. As before,
"`right_arrow`" is moved left and "`left_arrow`" is
moved right to produce:

**Figure 8.7.12**

The procedure's terminating condition "`left_arrow > right_arrow`"
is now `True`, and the first
sub-division of the list (i.e. array) is now complete.

Here is the procedure Quick Sort coded up as a C++ function:

void quick_sort(int list[], int left, int right) { int pivot, left_arrow, right_arrow; left_arrow = left; right_arrow = right; pivot = list[(left + right)/2]; do { while (list[right_arrow] > pivot) right_arrow--; while (list[left_arrow] < pivot) left_arrow++; if (left_arrow <= right_arrow) { swap(list[left_arrow], list[right_arrow]); left_arrow++; right_arrow--; } } while (right_arrow >= left_arrow); if (left < right_arrow) quick_sort(list, left, right_arrow); if (left_arrow < right) quick_sort(list, left_arrow, right); }

**Fragment of
Program 8.7.1**

We have seen that C++ functions may be defined recursively, and have briefly discussed how C++ executes recursively defined functions in terms of the stack. We have discussed the correspondence and relative advantages and disadvantages of recursive function definitions compared with iterative definitions. We have also described the standard sorting algorithm Quick Sort, and shown how it may be implemented in C++ using a recursive function definition. Most of the material here is also covered in more detail in Savitch, Chapter 14.