Java program to insert lower and upper bound and return the sum of even numbers from the range

Question

I’m having trouble getting this code to work properly. The goal is to sum all the even numbers of a given range (upper and lower limit), but is not returning the correct result using the for.

For example, if I insert: Lower Bound (ltinf) = 0 and Upper Bound (ltsup) = 5, the program returns the value 12 and not the sum 2+4 = 6.

Could someone shed some light on this problem?

package ps1ex3matheusportugal;
import javax.swing.JOptionPane;

public class PS1EX3MatheusPortugal {

    public static void main(String[] args) {

        int ltsup = Integer.parseInt(JOptionPane.showInputDialog("Insira o limite superior: "));
        int ltinf = Integer.parseInt(JOptionPane.showInputDialog("Insira o limite inferior: "));

        if (ltsup < ltinf){
            JOptionPane.showMessageDialog(null, "O valor do limite inferior deve ser menor que o limite superior");
        }
        else if (ltsup > ltinf){
            int cont, somapar = ltinf;
            for (cont = ltinf; cont <= ltsup; cont++){
                if (cont % 2 == 1){
                    cont++;
                    somapar = somapar + 2;
                }
                else{
                    for (cont = ltinf; cont <= ltsup; cont++){
                        somapar = somapar + 2;
                    }
                }
            }
            JOptionPane.showMessageDialog(null, "A soma de todos os números dentro do intervalo inserido é: " + somapar);
        }
    }

}

Answer 1

for (cont = ltinf; cont <= ltsup; cont++){
    if (cont % 2 == 1){
        cont++;
        somapar = somapar + 2;
    }
    else{
        for (cont = ltinf; cont <= ltsup; cont++){
            somapar = somapar + 2;
        }
    }
}

This logic is confused, there’s even a go inside Else. I can’t even tell you how to fix it, what I can suggest is reworking the code.

int somapar = 0;
for (int cont = ltinf; cont <= ltsup; cont++) {
    if (cont % 2 == 0){
        somapar += cont;
    }
}

You can also increment the counter two by two, ensuring that the counter will always be even:

int somapar = 0;
if (ltinf % 2 != 0) {
    ltinf++;
}
for (int cont = ltinf; cont <= ltsup; cont += 2) {
    somapar += cont;
}