Pair or odd loop does not show the expected result

Question

public class ParImpar {

public int totali;
public int totalp;

public ParImpar(int totali, int totalp) {
    this.totali = totali;
    this.totalp = totalp;
}

public static void main(String[] args) {
    new ParImpar(0,1).parImpar();
    //System.out.println(new ParImpar(0, 1).totalp);
}

public void parImpar(){
    for (int i = 0; i < 101; i++) {
        if ((i % 2) != 0){
            totali = totali + i;
            System.out.println(totalp);
        }else{
            totalp = totalp * i;
        }
    }
    System.out.println("Total pares " + totalp);
    System.out.println("Total impar " + totali);
}}

My totalp is always printed at 0, I’ve tried other ways and always end up giving error.

Answer 1

The problem is that this multiplication that adopts a factorial pattern will generate a very large, very large number. It does not fit in a int, or even a long, needs a BigInteger to make it work, there’s this beautiful code. With a int the value quickly exceeds the limit, and does not give error for reasons of performance, then the number that should be more than 2 billion becomes negative (this is how it works even, it reverses the signal because of the overflow), and being negative the whole calculation happens to be wrong.

Beyond this could not begin the for with 0, after all the multiplication by 0 will be 0, and all the results will be contaminated, has to start with 1.

import java.math.*;

class ParImpar {
    public BigInteger totali;
    public BigInteger totalp;
    public ParImpar(BigInteger totali, BigInteger totalp) {
        this.totali = totali;
        this.totalp = totalp;
    }
    public static void main(String[] args) {
        new ParImpar(BigInteger.ZERO, BigInteger.ONE).parImpar();
    }
    public void parImpar() {
        for (BigInteger i = BigInteger.ONE; i.compareTo(BigInteger.valueOf(100)) != 0; i = i.add(BigInteger.ONE)) {
            if (i.mod(BigInteger.valueOf(2)).compareTo(BigInteger.ZERO) != 0) totali = totali.add(i);
            else totalp = totalp.multiply(i);
        }
        System.out.println("Total pares " + totalp);
        System.out.println("Total impar " + totali);
    }
}

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

Making a class just to do this I think a construction very bad.