1. Cs212: DataStructures
Computer Science Department
Lab 3 : Recursion

2. Overview Recursion is a function that calls itself
Repeats function (method) block of code until a specified condition is met
int factorial() {
... factorial() ... }
When a recursive call is made, the function (method) clones itself, making new copies of
Code
Local variables
Parameters
21-Feb-14
Computer Science Department

3. Rules Every recursive function must have a base case.
Every recursive function must make progress towards its base case to prevent infinite recursion.
Anything that can be programmed using recursive functions can be programmed without recursion.
8-Dec-18
Computer Science Department

4. Example1: Calculating Factorial
Factorial can be defined as follows
Factorial of 0 is 1
Factorial of N (for N>0) is N * N-1 * ... * 3 * 2 * 1
Iterative version of factorial
(JAVA)
(C++)
int factorial (int N) {
int result = 1;
for (int i=1; i<= N; i++)
{ result = result * i;}
return result; }
public static int factorial (int n) {
int result = 1;
for (int i=1; i<= n; i++)
{ result = result * i;}
return result; }
8-Dec-18
Computer Science Department

5. Calculating Factorial ( cont.)
Recursive version of factorial
Base case
(JAVA)
(C++)
int factorial (int N) {
if (N == 0)
return 1;
else
return N * factorial(N-1); }
Public static int factorial (int n) {
if (N == 0)
return 1;
else
return N * factorial(N-1); }
Recursive calls
8-Dec-18
Computer Science Department

6. Example 2: Printing integers
Print integers from 1 to N.
Iterative version:
In C++
In Java
void printInt( int N ) {
for ( int i = 1 ; i<= N ; i++)
cout << i << endl ; }
void printInt( int N ) {
for ( int i = 1 ; i<= N ; i++)
System.out.println(i) ; }
8-Dec-18
Computer Science Department

7. Printing integers (cont.)
Recursive version:
In C++
In Java
Public static void printInt( int N ) {
if (N == 0) return;
printInt( N - 1 );
System.out.println( N ); }
void printInt( int N ) {
if (N == 0) return;
printInt( N - 1 );
cout << N << endl; }
Base case
Recursive calls
8-Dec-18
Computer Science Department