Function modifies the passed argument, when it should not [duplicate]

Question

I am trying to write a function for creating a prime factors list. For this - I am using a recursive function. Here are the calls:

    private int Problem003()
    {
        //The prime factors of 13195 are 5, 7, 13 and 29.
        //What is the largest prime factor of the number 600851475143 ?

        return GeneratePrimeFactorsList(15).Last();
    }

    private List<int> GeneratePrimeFactorsList(int n)
    {
        List<int> _primefactors = new List<int>();
        _primefactors.Add(2);

        int i = 3;

        while(i <= n)
        {   
            if (CheckIfIntIsPrime(_primefactors, i))
            {   
                _primefactors.Add(i);
            }

            i=i+2;
        }

        return _primefactors;
    }

    private bool CheckIfIntIsPrime(List<int> _primefactors, int i)
    {   
        if (_primefactors.Count() == 0)
        {   
            return true;
        }
        else
        { 
            if(i % _primefactors.First() != 0)
            {
                _primefactors.Remove(_primefactors.First());
                return CheckIfIntIsPrime(_primefactors, i);
            }
            else
            {   
                return false;
            }
        }
    }

The problem is that, when I am calling for CheckIfIntIsPrime(List, i), which have bool return type - it modifies the List. That means, that after the check - the passed argument into the GeneratePrimeFactorsList(int) is getting empty after each while loop iteration.

The CheckIfPrime function works correctly, but modifies the passed argument, when it should not - I dont relate them.

Very strange case, but I feel, that I missing knowledge about some of the List properties.

Non-primitive types are not copied. If you want to work on copy, preferably create local list from passed reference by calling List<int> local = new List<int>(_primefactors); — Tomasz Plaskota
– Tomasz Plaskota, Commented Oct 25, 2016 at 18:56
@OndrejTucny dont agree - code does not fall into infinte loop. — Egor Osaulenko
– Egor Osaulenko, Commented Oct 25, 2016 at 18:58
if (_primefactors.Count() == 0) should always be false as you never remove anything from the list. — 001
– 001, Commented Oct 25, 2016 at 18:58

Community · Accepted Answer · 2017-05-23 12:06:43Z

1

An object is a reference type. All variables that are typed as a reference type don't contain the value itself; they each contain a reference or pointer to the value (or object). This is true even when the variable is passed as a parameter.

The function that receives the object parameter is not capable of modifying the variable itself (which, again, is just a pointer). This means it cannot cause the variable to point somewhere else. However, it can use the pointer to get a reference to the object and modify the object itself.

If you want to pass an object but want to make absolutely sure it doesn't get modified, you can pass a clone of it.

edited May 23, 2017 at 12:06

CommunityBot

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answered Oct 25, 2016 at 19:08

John Wu

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