CODEWITHSUNDEEP
javarecursion
Zach Caudle
Zach Caudle

Reputation: 145

Clarification with recursion in java

So this entire time I thought that my issue with recursion was understanding cases. It turns out that my problem is understanding the values of recursive cases. For example, printing a portion of an array backwards.

Original try

    public static void printBackwards(int i, int j, char[] A){
        if(i == j){
            System.out.println(A[i]); 
        }
        else{
            printBackwards(i+1,j,A);
        }

    }

A working attempt

    public static boolean printBackwards(int i, int j, char[] A){
    if(i == j){
            System.out.println(A[i]);
            return true;
    }
    else{
            if(printBackwards(i+1,j,A) == true){
                System.out.println(A[i]);
                return true;
            }
            else{
                printBackwards(i+1,j,A);
            }
    }
    return false;
    }

However, is this even efficient recursion? Or is there a better way through it? This is the only way I could figure out when I looked it it from writing it out.

Upvotes: 2

Views: 137

Answers (3)

Crazenezz
Crazenezz

Reputation: 3456

For my opinion, to solve this problem doesn't need to use recursion. You can just do it with simple loop.

public class PrintBackwards {

    public static void main(String[] args) {
        char[] a = new char[] {'A', 'B', 'C', 'D'};

        for(int i = a.length - 1; i >= 0; i--) {
            System.out.println(a[i]);
        }
    }
}

Is there any reason behind this why you use recursion? If not, more faster to do it like example above.

If you want to use recursion, I hope this example can make you understand easier than yours.

public class PrintBackwards {

    private static char[] a = new char[]{'A', 'B', 'C', 'D'};

    public static void main(String[] args) {
        printBackwards(0);
    }

    public static void printBackwards(int i) {
        if (i < a.length) {
            printBackwards(++i);
            System.out.println(a[i - 1]);
        }
    }
}

Upvotes: 2

The Tran
The Tran

Reputation: 470

Here's the Java code to print array in reverse order:

public class TestProgram {

    private int[] a = {4, 2, 7, 1, 9, 5, 8};

    public static void main(String[] args) {
        TestProgram p = new TestProgram();
        p.print(a.length - 1);
    }

    public void print(int i) {
        // the anchor of the recursive method
        // it indicates that we are done printing array
        if (i < 0) 
            return;

        // print the current value
        System.out.printf("%d ", a[i]);

        // recursively call print() method
        print(i - 1);
    }
}

Upvotes: 1

Scott Hunter
Scott Hunter

Reputation: 49920

Start by asking "When can I solve the problem right away (w/o a recursive call)?". In this case, it would be when the area has only one element -- that's your base case.

When that ISN'T true, you need to break the problem into making a smaller version (which can be solved by calling printBackwards) and whatever remains to completing the problem. You already have a call that will print A[i+1..j] (for the original value of i). Thus, all that remains is to print A[i]. Figure out if it should be printed before or after the rest of the array, and you're all set.

Upvotes: 1

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