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simple task, to generate arrays with length I want to.

I don't also know how to get array I've created, except my own weird method. Does the first part of my code work alright and I should reconsider the way I want to get them (optional)?

although, I do understand why do I get the same values each time, but I don't think, it's related to my problem somehow.

I'm writing down this:

  cin >> x;
  int array1[x];
  int array2[x]; 
  for (int i = 0; i <= x; i++) {
    array1[i] = rand() % 10 + 1;
    array2[i] = rand() % 10 + 1;
  }
  

  cout << "[" << array1[a];
  for (int a = 0; a <= x; a++) {
    a += 1; 
    cout << ", " <<array1[a];
  }


  cout << "] [" << array2[b];
  for (int b = 0; b <= x; b++) {
    b += 1; 
    cout << ", " << array2[b];
  }
  cout << "]";

why do i get some abnormal answer for x = 6, 5, 15 cases like this:

[2, 5, 9, 6, 0] [8, 1, 9, 6, 32759]
[2, 5, 9, 6, 2, 8, 3, 2, 8] [8, 1, 9, 6, 2, 7, 4, 7, 7]
Claude Monet
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    VLA is not C++ (it is C and supported as extension by some compiler). use `std::vector` instead. – Jarod42 Nov 16 '22 at 16:25
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    C-style arrays, VLAs, no use of modern C++ random number functions. Old-style enumeration. Where do I stop? This is terrible. – Jesper Juhl Nov 16 '22 at 16:25
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    @JesperJuhl You left out breaching those non-standard arrays on the top-end, so you stopped a little early :-P – WhozCraig Nov 16 '22 at 16:26
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    `b <= x` would lead to out of bound access (so UB). Note also that you increase twice your index (`++` and `+= 1`). – Jarod42 Nov 16 '22 at 16:27
  • ***I do understand why do I get the same values each time, but I don't think, it's related to my problem somehow.*** You probably forgot to seed the random number generator 1 time with a seed that changes each run of the program. If that is not the case my guess is you are using an old version of MinGW with a broken rand() – drescherjm Nov 16 '22 at 16:29
  • Seems you have hidden `a`, `b`, as you use them before the declaration in the loop. – Jarod42 Nov 16 '22 at 16:29
  • @WhozCraig Hej, you are right. I just wanted to say that there's a lot of fail in that code. – Jesper Juhl Nov 16 '22 at 16:33
  • http://www.sscce.org/ – Jesper Juhl Nov 16 '22 at 17:17

1 Answers1

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Or using header and std::vector, std::generate (no raw loop). Also when you write code, write small readable functions. And to get unique random numbers random generators need to be seeded.

#include <algorithm>
#include <iostream>
#include <random>
#include <vector>

int generate_random_number()
{
    // only initialize the generator and distribution once (static)
    // and initialize the generator from a random source (device)
    static std::mt19937 generator(std::random_device{}());
    static std::uniform_int_distribution<int> distribution{ 1,10 };
    return distribution(generator);
}

// overload stream output for vector, so we can use vectors in std::cout
std::ostream& operator<<(std::ostream& os, const std::vector<int>& values)
{
    bool comma{ false };
    
    os << "[";
    for (const int value : values)
    {
        if (comma) os << ", ";
        os << value;
        comma = true;
    }
    os << "]";

    return os;
}

// helper function to create an array of given size
std::vector<int> create_random_array(const std::size_t size)
{
    std::vector<int> values(size); // create and allocate memory for an array of size ints
    std::generate(values.begin(), values.end(), generate_random_number); // fill each value in the array with a value from the function call to generate_random_number
    return values;
}

int main()
{
    std::size_t count;
    std::cout << "how many random numbers ? : ";
    std::cin >> count;

    auto array1 = create_random_array(count);
    std::cout << array1 << "\n";

    auto array2 = create_random_array(count);
    std::cout << array2 << "\n";

    return 0;
}
Pepijn Kramer
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