Get first digits of a number in Java

Question

How do I take part of an integer number in Java? For example, I have a variable int a = 12345678, and the user wants to receive only 4 digits.

Answer 1

There are several ways to do this. One of them is to turn into string with valueOf() and take a piece of it with the method substr() and then convert back to int, thus:

import java.lang.*;

class Program {
    public static void main (String[] args) {
        System.out.println(PegaPrimirosDigitos(12345678, 4));
        System.out.println(PegaPrimirosDigitos(-12345678, 4));
        System.out.println(PegaPrimirosDigitos(123, 4));
        System.out.println(PegaPrimirosDigitos(-12, 4));
        System.out.println(PegaPrimirosDigitos(0, 0));
    }
    public static int PegaPrimirosDigitos(int valor, int digitos) {
        digitos = Math.max(1, digitos);
        int positivo = Math.abs(valor);
        String texto = String.valueOf(positivo);
        if(digitos > texto.length()) {
            return valor;
        }
        return Integer.parseInt(texto.substring(0, digitos)) * Integer.signum(valor);
    }
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Can also be done mathematically without involving conversions but really you don’t know how to do it?

import java.lang.*;

class Ideone {
    public static void main (String[] args) throws java.lang.Exception {
        System.out.println(PegaPrimirosDigitos(12345678, 4));
        System.out.println(PegaPrimirosDigitos(-12345678, 4));
        System.out.println(PegaPrimirosDigitos(123, 4));
        System.out.println(PegaPrimirosDigitos(-12, 4));
        System.out.println(PegaPrimirosDigitos(0, 0));
    }
    public static int PegaPrimirosDigitos(int valor, int digitos) {
        digitos = Math.max(1, digitos);
        int positivo = Math.abs(valor);
        int tamanho = (int)(Math.log10(positivo) + 1);
        if(digitos > tamanho) {
            return valor;
        }
        return valor / (int)Math.pow(10, tamanho - digitos);
    }
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

I do not guarantee that it will work in all situations, but the central point of the solution is there. For lack of a better criterion definition I chose that at least should pick 1 digit. I also understood that the negative sign should be considered but it is not a digit.

This problem appears to be artificial and serves no real purpose but the solution is there. If you have more details, I may be able to give a better solution. The solution here is already better than defined in the question.

Answer 2

At first, you can convert the number to a String and then use the method substring to "cut" from index 0 to the digit numbers that must be returned. For example:

public int foo(int number, int digits){
   String str = Integer.toString(number);
   if(digits> str.length())
       return number;

    boolean isNegative = str.startsWith("-"); 
    return Integer.valueOf(str.substring(0, isNegative ? digits + 1 : digits));
}

Example working on Ideone