Java method using a for loop

Question

I am learning Java and discovering that a little bit of knowledge is confusing. The goal is to write a method that is the equivalent of the n! function. I am using a for loop to multiply a variable declared outside the method. All I get back is 0.

What am I doing wrong?

//
// Complete the method to return the product of
// all the numbers 1 to the parameter n (inclusive)
// @ param n
// @ return n!

public class MathUtil
{
   public int total;

   public int product(int n)
  {
   for (int i = 1; i == n; i ++)
   {
       total = total * i;

    }
    return total;

  }
}

Read this line of code adn tell me what seems wrong with it: for (int i = 1; i == n; i ++) Hint: initialization, condition, incrementation. — Kon
– Kon, Commented Mar 11, 2016 at 3:38
Please learn some basic debugging skills. Learn to use a debugger, or at least print out information at some meaningful positions. You should be able to solve the problems easily yourself — Adrian Shum
– Adrian Shum, Commented Mar 11, 2016 at 3:50

Adrian Shum · Accepted Answer · 2016-03-11 03:52:31Z

4

There is actually a lot of problems in your code:

It does not make sense to make it an instance method.
You have not initialize your total to a reasonable value.
The condition in your for loop is wrong
Method is not given a meaningful name
Messy indentation
(The list keeps growing...)

So here is a slightly improved version

public class MathUtil
{
  //
  // Complete the method to return the product of
  // all the numbers 1 to the parameter n (inclusive)
  // @ param n
  // @ return n!

  public static int factorial(int n)
  {
    int total = 1;
    for (int i = 1; i <= n; i ++)
    {
      total = total * i;
    }
    return total;
  }
}

so that you can call it as MathUtil.product(123) instead of some weird new MathUtil().product(123)

Personally I would rather do something like

result = n;
while (--n > 0) {
    result *= n;
}
return result;

edited Mar 11, 2016 at 3:52

answered Mar 11, 2016 at 3:44

Adrian Shum

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3 Comments

BryKKan Over a year ago

Also, product is is a bad name in the first place. It doesn't appropriately describe the functionality. factorial would be a better name, since that's what is actually does.

Adrian Shum Over a year ago

@Bryan Yes! I was planning to mention that too! Dunno why I missed that at last. Thanks for reminding

BryKKan Over a year ago

That last bit is clever! I may use that! I never considered using the commutative property to do the multiplication in reverse. I've always done factorials by counting up. It might be helpful to make a small note on the difference between pre vs post increment though, since your compact solution hinges on it whereas the for loop does not.

Jernej K · Accepted Answer · 2016-03-11 03:43:26Z

2

You are missing the initialization. Now I added the default value to 1. And you also have to change the condition. The for loop has to go on as long as the var i is smaller or equal to n.

public class MathUtil
{
  public int total = 1;

  public int product(int n)
  {
    for (int i = 1; i <= n; i ++)
    {
     total = total * i;
    }
   return total;

  }
 }

edited Mar 11, 2016 at 3:43

answered Mar 11, 2016 at 3:41

Jernej K

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1 Comment

Jernej K Over a year ago

I did correct it. Now it should be fine, because you go from one to N.

Thomas Kis · Accepted Answer · 2016-03-11 03:43:25Z

1

you didn't initialized total, therefore it's 0. Whenever you multiply 0 with anything, you will get 0 as result.

answered Mar 11, 2016 at 3:43

Thomas Kis

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Comments

Renuka Deshmukh · Accepted Answer · 2016-03-11 03:45:39Z

1

public int total = 1;

    public int product(int n) {
        for (int i = 1; i <= n; i++) {
            total = total * i;

        }
        return total;

    }

you havent initialized total. It defaults to 0. Then when you multiple anything with total, you get 0 as result

edited Mar 11, 2016 at 3:45

answered Mar 11, 2016 at 3:44

Renuka Deshmukh

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1 Comment

element11 Over a year ago

Please add some explanation rather than just solving it and posting code

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