Convert Decimal to Octal in Java [Loop, Recursion]

Problem: Write a Java program to convert a decimal number into its corresponding octal representation.

Example:

Input:    7
Output:   7
 
Input:    8
Output:   10

Method 1: Using While Loop

To convert a decimal into octal we divide the decimal number by 8 until it reduces to 0 and stacks the sequence of remainders in a bottom-up manner.

In the Java program, we divide and concatenate remainders inside a while loop as follows:

import java.util.Scanner;
 
public class Main
{
	public static void main(String[] args) {
	    Scanner in = new Scanner(System.in);
	    int octal=0, i=1, remainder;
	    
	    //Input the decimal number
		System.out.println("Enter a Decimal Number");
		int decimal = in.nextInt();
		
		//loop until the decimal is not 0
		while(decimal>0){
		    remainder = decimal%8;     //compute remainder
		    octal +=  i*remainder;     //concatenate remainders to the octal
		    decimal = decimal/8;       //update decimal with quotient
		    i=i*10;                    //multiply 10 to i
		}
		
		//output the octal number
		System.out.println("Octal: "+octal);
	}
}

Output:

Enter a Decimal Number
15
Octal: 17

To concatenate remainders in a bottom-up manner, we first multiply the remainder with 10^{number_of_digits_in_octal} (i*10) then add it to the octal number.

Method 2: Using Recursion

We can also convert a decimal number into an octal number using the recursive approach.

To do so, we recursively call the function with the quotient value (decimal/8) and print the remainder after the recursive call.

import java.util.Scanner;
 
public class Main
{
	public static void main(String[] args) {
	    Scanner in = new Scanner(System.in);
	    
	    //Input the decimal number
		System.out.println("Enter a Decimal Number");
		int decimal = in.nextInt();
		
		System.out.print("Octal: ");
		decToOctal(decimal);              //call the recursive function
		
	}
	
	private static void decToOctal(int n){
	    if(n>0){
	        //recall by passing quotient
	        decToOctal(n/8);
	        
	        //output the remainder
	        System.out.print(n%8);
	    }
	}
}

Output:

Enter a Decimal Number
8
Octal: 10

By writing the print statement after the recursive call statement, the remainders gets printed in the bottom-up sequence.

It is so because the print action starts when the recursive function starts tracing back to its first recursive call.