Return value from recursive method (java)

Question

I have to write a recursive method to sum the figures of an integer.

The method works fine, but I don't understand why it returns the last integer instead of the total.

import java.util.Scanner;

public class SommaCifreRicorsivo{
    public static void main (String[] args){
        System.out.printf("Inserire un numero: ");
        Scanner tastiera = new Scanner(System.in);
        int numero = tastiera.nextInt();
        int somma = 0;

        int risultato = eseguiSomma(numero,somma);
        System.out.println("La somma delle sue cifre è " + risultato);
    }

    public static int eseguiSomma(int numero, int somma){
        //Caso base
        if (numero < 10) {
            somma = somma + numero;
            System.out.println("Aggiungo la cifra " + numero + " alla somma, ottenendo " + somma);
            return (somma += numero);
        }

        //Chiamate ricorsive
        somma = somma + numero%10;
        System.out.println("Aggiungo la cifra " + (numero%10) + " alla somma, ottenendo " + somma);
        eseguiSomma((numero/10), somma);

        return somma;

    }
}

meowgoesthedog · Accepted Answer · 2018-08-23 17:01:04Z

1

somma is a fundamental type (int), so Java passes it by value. This means the value you are modifying is just a copy of somma stored on the stack frame of each recursive call, not the final return value.

Because of this, the argument somma is entirely unnecessary. You should instead return the return value of the recursive call plus the new digit. This gives a much simpler solution.

public static int eseguiSomma(int numero){
    //Caso base
    if (numero < 10) {
        return numero;
    }

    //Chiamate ricorsive
    return numero % 10 + eseguiSomma(numero / 10);
}

// now simply call without somma
int risultato = eseguiSomma(numero);

answered Aug 23, 2018 at 17:01

meowgoesthedog

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Comments

Louis-wht · Accepted Answer · 2018-08-23 17:00:32Z

0

Simply correct it to :

import java.util.Scanner;

public class SommaCifreRicorsivo{
    public static void main (String[] args){
        System.out.printf("Inserire un numero: ");
        Scanner tastiera = new Scanner(System.in);
        int numero = tastiera.nextInt();
        int somma = 0;

        int risultato = eseguiSomma(numero,somma);
        System.out.println("La somma delle sue cifre è " + risultato);
    }

    public static int eseguiSomma(int numero, int somma){
        //Caso base
        if (numero < 10) {
            somma = somma + numero;
            System.out.println("Aggiungo la cifra " + numero + " alla somma, ottenendo " + somma);
            return (somma += numero);
        }

        //Chiamate ricorsive
        somma = somma + numero%10;
        System.out.println("Aggiungo la cifra " + (numero%10) + " alla somma, ottenendo " + somma);

        // the error was on this line!
        somma += eseguiSomma((numero/10), somma);

        return somma;

    }
}

You are trying to modify somma, but somma is a raw type so you only modify its local copy and somma remains unchanged.

edited Aug 23, 2018 at 17:00

answered Aug 23, 2018 at 16:56

Louis-wht

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1 Comment

Louis-wht Over a year ago

Yes sorry, I did write = instead of +=, correcting my answer right away, thanks!

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