不使用数组将十进制数转换为二进制数

Question

I want to make a java code that converts any decimal number inputted, into binary number. Although the array method is easy, it doesn't actually prints a number(it prints array), nor does it seem to correctly use the algorithm we use manually. So I'm trying to do it without array. The problem is it sometimes giving correct answers, and sometimes giving wrong answers. I think the problem might be in multiplying by zero in the last cycle of loop, but I'm not sure. The code I wrote as comment was a failed try to resolve the issue.

import java.util.Scanner;

public class DecToBin {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long dec = 0, num = 0, rem = 0;
        
        System.out.print("Enter A Decimal Number: ");
        dec = sc.nextLong();
        while(dec > 0){
            rem = dec % 2;
            num += rem;
            num *= 10;
            dec /= 2;

        }
        // if(num % 100 == 10){
        //     num = num/10;
        // }
        System.out.println("Binary Equivalent: " + num);


    }
}
 

Answer 1

This is my first time answering, so please let me know if anything is off:-)

You could try to use a String as intermediate. Short and simple.... But i dont know if that counts as an Array:-)
Your while-part would be something like this:

    String strBin = "";
    while(dec > 0){
        strBin = String.valueOf(dec%2) + strBin;
        dec /= 2;
    }
    num = Long.valueOf(strBin);        

The order of bits in the binairy number is correct by adding in front of the previous String, so building up the result from right to left.
Also, i would not use a Long (last line... num), because it limits your max result. But if you must..... meh

Answer 2

This problem lends its self to a recursive solution

  private static int DecToBin(final int input, final int result) {
    if (input == 0) {
      return result;
    }
    return DecToBin(input / 2, result * 10 + input % 2);
  }

Answer 3

I found following "direct" issues with your code:

1. You are not reversing the final number. In the manual way of converting DEC->BIN, we reverse the final representation. Hint: Dry run for input 11
2. You are doing num += rem; num *= 10 num += rem; num *= 10 . The order is wrong. You should be multiplying it before you add the remainder. eg. Your code will output 1010 for 5 , instead of 101 .
3. By your method, you are trying to represent decimal number into its binary representation as int , which limits your input to 2047 , as 2048 and above need 11+ digits for their binary representation and you can't have that in int . You should use String if you don't want to use array. Also, reversing would be easier.

something like:

import java.util.Scanner;

public class DecToBin {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int dec = 0, num = 0;
        
        System.out.print("Enter A Decimal Number: ");
        dec = sc.nextInt();
        String bi = "";
        while(dec > 0){
            int rem = dec % 2;
            // num *= 10;
            // num += rem;
            bi += Character.toString(rem + 48);
            dec /= 2;

        }
        // if(num % 100 == 10){
        //     num = num/10;
        // }
        System.out.println("Binary Equivalent: " + new StringBuilder(bi).reverse().toString());


    }
}