CODEWITHSUNDEEP
c#c++data-structures
Mithrein Goul
Mithrein Goul

Reputation: 23

C++ vector vs C# List. Why are they producing different results?

I'm translating some code from C++ to C#. The code is simple enough and most of the syntax involved is almost the same, however the C++ version uses a vector<vector<int>> structure that doesn't exists in C#, so I replaced it with a List<List<int>> which I initialized with empty "rows" since I need it to represent a table with a fixed number of rows (See the code below). The logic is exactly the same but somehow i'm not getting the same outputs. I suspect it has to do with the behaviour of List<List<int>> but I'm not sure. Any hints would be appreciated.

This is the original C++ code:

void LIS(int arr[], int n)
{ 
    vector<vector<int>> L(n);
    L[0].push_back(arr[0]);

    for (int i = 1; i < n; i++)
    {
        for (int j = 0; j < i; j++)
        {
            if ((arr[i] > arr[j]) && (L[i].size() < L[j].size() + 1)){
                L[i] = L[j];
            }
        }
        L[i].push_back(arr[i]);
    }

    vector<int> max = L[0];
    for (vector<int> x : L)
        if (x.size() > max.size())
            max = x;

    for(int e : max){
       cout<<e<<endl;
    }
}

This is my C# version:

    public static void LIS(int[] arr, int n)
    {
        // Here I replace the vector<vector<int>> with a List<List<int>> 
        // and I fill it with n empty lists.

        List<List<int>> L = new List<List<int>>();
        for(int x = 0; x < n; x++)
        {
            L.Add(new List<int>());
        }

        L[0].Add(arr[0]);

        for (int i = 1; i < n; i++)
        {
            for (int j = 0; j < i; j++)
            {
                if ((arr[i] > arr[j]) && (L[i].Count < L[j].Count + 1))
                {
                    L[i] = L[j];
                }
            }

            L[i].Add(arr[i]);
        }

        List<int> max = L[0];

        foreach (List<int> x in L)
        {
            if (x.Count > max.Count)
            {
                max = new List<int>(x);
            }
        }

        foreach(int e in max){
           Console.WriteLine(e);
        }

I'm testing with this array:

int[] arr = { 0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15 };
int n = arr.Length;

In the C++ version I get: 0 2 6 9 11 15

and in the C# version I get: 0 8 4 12 2 10 6 14 1 9 5 13 3 11 7 15

Upvotes: 2

Views: 344

Answers (1)

Paramecium13
Paramecium13

Reputation: 345

In the C++ code doesn't L[i] = L[j]; make a copy of the vector at L[j] and store it in L[i] (similar thing for the variable max)? To achieve a similar result in C#, you could do L[i] = L[j].ToList() to copy the list.

Upvotes: 1

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