Fill string with zeros on the left

Question

I’m doing a work of Operating Systems and need to turn decimal numbers into binaries. So far so good, because the method below takes an integer and converts to binary. My problem is the following:

When I put, for example (integer 1), it returns me correctly 1. But I have a stored value that would be the number of digits, for example (Qtd = 4). I would like the string to appear this way: 0001 with four digits.

It would be possible in this method to perform this procedure, regardless of the value of the variable number of digits?

public String converteDecimalParaBinario(int valor) {
   int resto = -1;
   StringBuilder sb = new StringBuilder();

   if (valor == 0) {
      return "0";
   }

   // enquanto o resultado da divisão por 2 for maior que 0 adiciona o resto ao início da String de retorno
   while (valor > 0) {
      resto = valor % 2;
      valor = valor / 2;
      sb.insert(0, resto);
   }

   return sb.toString();
} 

Answer 1

You can use the Decimalformat class to format its output value. So:

    //coloque isso no final do seu método converteDecimalParaBinario
    DecimalFormat df = new DecimalFormat("0000");
    return df.format(Integer.parseInt(sb.toString()));

You say in the statement that you have a variable qtd which indicates the number of digits that must possess the output, so the construction of your Decimalformat object must be variable according to that quantity. To make this variable amount of digits, you can make a for and build your Pattern. Example:

    int qtd = 5;
    StringBuilder pattern = new StringBuilder();
    for(int i=0; i<qtd; i++) {
        pattern.append("0");
    }
    DecimalFormat df = new DecimalFormat(pattern.toString());

If it is not part of your job to convert the number to binary you can also use the method toBinaryString() from the Integer class to do the conversion for you. Example:

import java.text.DecimalFormat;
public class Bin {
    public static void main(String[] args) {
        System.out.println(converteDecimalParaBinario(4));
    }
    public static String converteDecimalParaBinario(int valor) {
        int qtd = 5;
        StringBuilder pattern = new StringBuilder();
        for(int i=0; i<qtd; i++) {
            pattern.append("0");
        }
        DecimalFormat df = new DecimalFormat(pattern.toString());
        return df.format(Integer.parseInt(Integer.toBinaryString(valor)));
    } 
}

Exit:

00100

To change the number of digits the output will have, simply change the variable value qtd.

Answer 2

You can perform as follows if it is a string:

String str = "123";
String formatted = ("0000" + str).substring(str.length());

^{Note: Enter a zero for each desired digit, that is, 4 digits you mention in the question lack 4 zeros.}

Or if it’s a number:

Int number = 123;
String formatted = String.format("%04d", number);

^{Note: $04d is for picture digits, $05d is for 5 digits, etc...}

Anyway, the second option is always better: have the variable with the correct type and format the number as intended by making use of the method format.

---

For your particular case, you could do it as follows:

String formatado = String.format("%04d", Integer.parseInt(sb.toString()));
return formatado;

Example

/* package superBuBu; */

import java.util.*;
import java.lang.*;
import java.io.*;

class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {

        int valor = 1;
        int resto = -1;

        StringBuilder sb = new StringBuilder();

        if (valor == 0) {
            System.out.println("0");
        }

        // enquanto o resultado da divisão por 2 for maior que 0 adiciona o resto ao início da String de retorno
        while (valor > 0) {
            resto = valor % 2;
            valor = valor / 2;
            sb.insert(0, resto);
        }

        String formatado = String.format("%04d", Integer.parseInt(sb.toString()));

        System.out.println(formatado);
    }
}

Example working on Ideone.

Answer 3

Dude I didn’t test but this should work!

public String converteDecimalParaBinario(int valor) {
   int resto = -1;
   StringBuilder sb = new StringBuilder();
   int len=0;   

   if (valor == 0) {
      return "0000";
   }

   // enquanto o resultado da divisão por 2 for maior que 0 adiciona o resto ao início da String de retorno
   while (valor > 0) {
      resto = valor % 2;
      valor = valor / 2;
      sb.insert(0, resto);
   }
   len = sd.length();
   while(len<4){
       sd="0"+sd;
       len++;
   }
   return sb.toString();
} 

What I did:

I started a variable of type int calling for lenand assigns the size of the sd for len with the command len = sd.length();.
If len is less than 4 adds 0 left, repeats until Len is equal to 4. sd returns.