Recursive function that count with local var

Question

I'm having a question about recursive functions.

I've made this little example program that counts the individual numbers in an integers: example: 123 = 6 because 1 + 2 + 3 = 6.

Now I've made it with a static int and this recursive functions:

  static int totalNumbers(int a)
  { 
    if(a <= 0)
      return sum;
    else
    {
      sum += a % 10;
      return totalNumbers(a/10);
    }
  }

The function works like a charm but my question is, can I make it without a static int called sum? Is there a way that I can define a integer sum in the function and let them count up with a local var or is it not possible?

Kind regards,

Tomasz Nurkiewicz · Accepted Answer · 2011-12-18 22:55:19Z

6

Of course:

static int totalNumbers(int a)
  { 
    if(a <= 0)
      return 0;
    else
    {
      return (a % 10) + totalNumbers(a/10);
    }
  }

edited Dec 18, 2011 at 22:55

Tomasz Nurkiewicz

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answered Dec 18, 2011 at 22:13

yurib

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2 Comments

fge Over a year ago

Why else if you return? ;)

user1007522 Over a year ago

Thanks :-) so simple but didn't saw it. Learned something new !

fge · Accepted Answer · 2011-12-18 22:13:46Z

4

static int totalNumbers(int a)
{
    return a < 10 ? a : (a % 10) + totalNumbers(a / 10);
}

answered Dec 18, 2011 at 22:13

fge

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Comments

Tomasz Nurkiewicz · Accepted Answer · 2011-12-18 22:14:25Z

2

Of course:

static int totalNumbers(int a, int sum)
{ 
  if(a <= 0)
    return sum;
  else
  {
    return totalNumbers(a/10, sum + a % 10);
  }
}

static int totalNumbers(int a) {
  return totalNumbers(a, 0);
}

This version is tail-recursive, but it doesn't give you anything in Java.

answered Dec 18, 2011 at 22:14

Tomasz Nurkiewicz

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Comments

Mvoicem · Accepted Answer · 2011-12-18 22:16:12Z

1

Sure, it's even better to not have static "sum" variable. I would write it in such way:

 static int totalNumbers(int a) {
    if (a <= 0) return 0;

    return a%10 + totalNumbers(a/10);
 }

answered Dec 18, 2011 at 22:16

Mvoicem

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Comments

Óscar López · Accepted Answer · 2011-12-18 22:54:13Z

Of course you can!

public static int totalNumbers(int a) {
    if (a == 0)
        return 0;
    return (a % 10) + totalNumbers(a / 10);
}

By using a static int attribute you're missing the point of recursive functions, a function in the purest mathematical sense, doesn't depend on external values, and you can be certain that every time you call it with the same input, it will return the same output - something you can not say about your method, which will give different results every time you call it with the same input, unless you manually reset to zero the external static int attribute between calls.

For instance, try calling your implementation two or more times in a row, like this:

System.out.println(totalNumbers(123));
System.out.println(totalNumbers(123));
System.out.println(totalNumbers(123));

See what I mean?

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