Recursion

Description

Recursion is the process of having a method call itself on a continually refining set of data. As the data set is being refined, the same process is repeated over it (the data set). This works in a similar fashion as the loop body. There are three parts to a recursive solution: the Base Case, the Recursive Case, and the Recusive Step.

1. The Base Case(s) are the points at which we stop making recursive calls and begin to return through the recursive calls that have been made.
2. The Recursive Case(s) are the points at which we determine our steps are not complete and we cannot yet identify what should be returned. Note the recursive case is not always the opposite of the base case.
3. Thus we will take the Recursive Step to perform operations and make additional recursive call(s).

If the recursive process will return information, we need to ensure a connection exists across all the recursive calls for actually performing the return. This can be completed in two ways. First, return statements are listed in multiple locations, placed at exactly the point where the return is desired. This implementation will involve returning a specific or literal value within the base case and a subsequent recursive call within the recursive step. Second, a single return of a variable whose value is continually upated can be made at the end of the method. In this solution, the return variable will be assigned a specific or literal value within the base case and the result of a subsequent recursive call within the recursive step.

When creating a recursive solution, there are two points when actions can be taken. First, actions can be taken before making the recursive call and as we move into the recursion. Second, steps can also be taken as we return from recursive calls and as we move out of the recursion.

The main method should never be accessed in a recursive fashion. The JVM (c:\>java classname) will handle starting up a program by calling the main method.

During COP 3502 & 3503 (when I am teaching these courses):

• If a method does not call itself (so it has no recursive calls), but also does not have any iteration (loops), then we will still refer to the method as recursive. Observe that such a method has only a base case and does not have a recursive case, therefore also does not have a recursive step.
• You are allowed to create helper methods to assist you in a recursive solution whenever you need additional methods. However, recall that none of the methods used may contain any iteration. A helper method could be a method which starts the recursive process (a starter helper method) or a method which assists in the actual recursive work being performed.

Recursive Example #1

Consider the recursive solution to finding the factorial.

  public static int factorial(int n) {
    if (n == 0 || n == 1) {
      return 1;
    }
    else if (n > 1) {
      return n * factorial(n - 1);
    }
    return 0;
  }

Recursive Example #2

Consider the recursive solution to finding Fibonacci numbers (recall the definition of a Fibonacci number is: F(0) = 0; F(1) = 1; and F(i) = F(n - 1) + F(n - 2), for all n >= 2 (Recursive Example #2).

  public static int Fibonacci(int n) {
    if (n == 0 || n == 1) {
      return n;
    }
    else {
      return Fibonacci(n - 1) + Fibonacci(n - 2);
    }
  }

Recursive Example #3

  public static int recursiveMethod1(int a[]) {
     return recursiveMethod1(a, 1, a[0]);
  }
  public static int recursiveMethod1(int a[], int index, int data) {
    if (index < a.length) {
      if (data < a[index]) {
        data = a[index];
      }
      data = recursiveMethod1(a, (index + 1), data);
    }
    return data;
  }

Recursive Example #4

  public static int recursiveMethod2(int n) {
    int result;

    if(n <= 1) {
      result = 0;
    }
    else if((n % 2) == 0) {
      result = n * n + recursiveMethod2(n - 1);
    }
    else {
      result = recursiveMethod2(n - 1);
    }
    return result;
  }

Here are other recursive solutions, some of which include an iterative implementation for your reference.

• Factorial - includes a slightly different implementation than the one given above.
• Minimum
• Multiplication Table
• Palindrome
• Printout
• Squares Incomplete - a partial solution to the example that follows. Use this as a guide to solving the problem of printing a square to the screen recursively.
• Squares
• Summation