Access violation writing location 0x00000000 when reading/writing to vector

I need to convert Java alghoritm into C++. I'm having a hard time doing this, because C++ is definitely not my language, it's much harder and illogical sometimes compared to Java. (At least for me, full-time Java developer).

Anyway, I failed. Code is almost identical to Java in terms of logic, and still doesn't work :( The worst part was converting traditional Java arrays to C++'s vector<int>. And I think I messed up here.

This is my C++ code:

#include "stdafx.h"
#include <string>
#include <vector>
#include <iostream>

using namespace std;

double calculate(int n, int k, vector<int>& values);

int main()
{
    printf("%f\n", calculate(7, 4, vector<int>{ 2, 2, 2, 1, 2, 2, 2 }));
    system("pause");
    return 0;
}

double calculate(int n, int k, vector<int>& values) {
    int amount = k;
    int offset = 0;
    int predictions = (n - k) + 1;

    vector<vector<int>> arrays;

    int index = 0;
    while (predictions > 0) {
        for (int i = 0; i < predictions; i++) {
            vector<int> arr;

            for (int y = 0; y < amount; y++) {
                arr[y] = values[offset + y]; // HERE IS THE ERROR!!!
            }

            arrays[index] = arr;
            offset++;
            index++;
        }
        amount++;
        offset = 0;
        predictions--;
    }
    vector<double> averages;
    for (size_t a = 0; a < arrays.size(); a++) {
        vector<int> arr = arrays[a];

        int sum = 0;
        for (int value : arr) 
            sum += value;

        averages[a] = (double)sum / (double)arr.size();
    }
    double biggestAverage = 0;
    for (double average : averages) {
        if (average > biggestAverage)
            biggestAverage = average;
    }
    return biggestAverage;
}

And Java code here: (works flawlessly)

import java.text.DecimalFormat;
import java.util.Arrays;

public class Main {

    private static final DecimalFormat df = new DecimalFormat("#.000");

    public static void main(String[] args) {
        System.out.println(calc(7, 4, 2, 2, 2, 1, 2, 2, 2));
    }

    public static double calc(int n, int k, int... values) {
        int amount = k;
        int offset = 0;
        int predictions = (n - k) + 1;

        int[][] arrays = new int[n * n][n];

        int index = 0;
        while (predictions > 0) {
            for (int x = 0; x < predictions; x++) {
                int[] arr = new int[amount];

                System.arraycopy(values, offset, arr, 0, amount);

                //CLEAR CODE!!! WORKS TOO!!!
                //for (int y = 0; y < amount; y++) {
                //    arr[y] = values[offset + y];
                //}


                arrays[index] = arr;
                offset++;
                index++;
            }
            amount++;
            offset = 0;
            predictions--;
        }
        double[] averages = new double[arrays.length];
        for (int a = 0; a < arrays.length; a++) {
            int[] arr = arrays[a];

            int sum = 0;
            for (int anArr : arr)
                sum += anArr;

            averages[a] = (double) sum / (double) arr.length;
        }

        double biggestAverage = 0;
        for (double average : averages) {
            if (average > biggestAverage)
                biggestAverage = average;
        }
        return Double.valueOf(
                df.format(biggestAverage)
                        .replace(",", "."));
    }

}

Full error stacktrace (C++):

Exception thrown at 0x010C10B5 in Application.exe: 0xC0000005: Access violation writing location 0x00000000.

You need to allocate the vector elements before assigning values to them, when you do

vector<int> arr; // defines an empty vector.
arr[y] = ...     // access element at position y.

arr here is empty, pointing to a zero array, you are trying to write at memory 0x0000.

Checkout vector documentation, to see the different ways to initialize it.

In C++ you need to handle your memory, you cannot just port code from different higher-level languages.

There are different ways to do it, for instance

vector<int> arr;
arr.reserve(amount);
for (int y = 0; y < amount; ++y) {
  arr.push_back(values[offset + y]);
}