Reputation: 1584

Fill a vector with random numbers c++

I've got a vector that I'm trying to fill up with random numbers. I keep running into an issue however that the vector mostly outputs 0 each time that I'm running it (it shouldn't output a 0 ever). What am I doing wrong in my code written below to make it output 0 (it outputs 0 more so than any other number):

vector<int> myVector;
srand((unsigned)time(NULL));
int a = rand() % 20 + 1; //1 to 20    
for (int i =0; i < a; i++){
        int b = rand() % 20 + 1;
        myVector.push_back(b);
        cout << myVector[b] << endl;
    }

I am a beginner and have not done much C++ programming in a long time so I'm not sure what is making my code malfunction. If someone could explain what I've done wrong it would be greatly appreciated.

Upvotes: 45

Answers (8)

Marko Tunjic

Reputation: 1889

You can use the std::generate algorithm to fill a vector of n elements with random numbers.

In modern C++ it’s recommended not to use any time-based seeds and std::rand, but instead to use random_device to generate a seed. For software-based engine, you always need to specify the engine and distribution. Read More...

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

int main()
{
    // First create an instance of an engine.
    std::random_device rnd_device;
    // Specify the engine and distribution.
    std::mt19937 mersenne_engine {rnd_device()};  // Generates random integers
    std::uniform_int_distribution<int> dist {1, 52};
    
    auto gen = [&](){
                   return dist(mersenne_engine);
               };

    std::vector<int> vec(10);
    std::generate(vec.begin(), vec.end(), gen);
    
    // Optional
    for (const auto& i : vec) {
        std::cout << i << " ";
    }
}

If you want to rearrange the elements of a range in a random order:

std::shuffle(vec.begin(), vec.end(), mersenne_engine);

Upvotes: 98

Marek Kurdej

Reputation: 1499

Just adding my 2 cents... This response is similar to the one given by Marko Tunjic, but it doesn't use std::rand from C, but C++11 features instead. It allows you to use the distribution of your choice, uniform_int_distribution in the example below.

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

static std::vector<int> generate_data(size_t size)
{
    using value_type = int;
    // We use static in order to instantiate the random engine
    // and the distribution once only.
    // It may provoke some thread-safety issues.
    static std::uniform_int_distribution<value_type> distribution(
        std::numeric_limits<value_type>::min(),
        std::numeric_limits<value_type>::max());
    static std::default_random_engine generator;

    std::vector<value_type> data(size);
    std::generate(data.begin(), data.end(), []() { return distribution(generator); });
    return data;
}

int main()
{
    for (auto i = 0u; i < 5; ++i)
    {
        std::vector<int> myVector = generate_data(10);
        myVector = generate_data(10);

        std::cout << "myVector (iteration " << i << "): ";
        for (auto v: myVector)
        {
            std::cout << v << ",";
        }
        std::cout << "\n";
    }
}

Upvotes: 14

Martin.Martinsson

Reputation: 2154

What about simply:

#include <vector>
#include <algorithm>
#include <ctime>

std::srand(unsigned(std::time(nullptr)));
std::vector<int> v(1000);
std::generate(v.begin(), v.end(), std::rand);

Upvotes: 48

Giroud Vren

Reputation: 1

// here I generate a vector that contains random vectors
// for example, after this code, vec = { {1,4,8}, {1,3,6,7,9}, {2,5,6} }

#include <vector>
#include <time.h>

void generate_random_vectors(const int num_of_rand_vectors, vector<vector<int>> &vec) {
    for (int j = 0; j < num_of_rand_vectors; ++j) {
        // the vector size will be randomal: between 0 to 19
        int vec_size = (rand() % 20);
        vector<int> rand_vec(vec_size);
        for (int k = 0; k < vec_size; ++k) {
            // each vec element will be randomal: between 1 to 20
            rand_vec[k] = 1 + (rand() % 20);
        }
        // each vector will be sorted if you want to
        sort(rand_vec.begin(), rand_vec.end());
        // push each of the 'num_of_rand_vectors'-th vectors into vec
        vec.push_back(rand_vec);
    }
}

void main() {

    srand(static_cast<unsigned int>(time(NULL)));

    // input vector containing random sorted vectors
    vector<vector<int>> vec;
    generate_random_vectors(3, vec);
}

Upvotes: -2

Tony Lim

Reputation: 11

cout << myVector[b] ?!

it should be : cout << myVector[i];

Upvotes: -1

Sanjaya R

Reputation: 6426

Its not clear what you are trying to do with the loop, the code is creating a vector of random size, filled with random numbers.

You are outputting "myVector[b]", but 'b' is the random value, not the index of just added number. You could just :

cout << b << endl;

But really you should size the vector, and just access by index.

int vec_size = rand() % 20 + 1;
vec<int> myvec(vec_size);
for( int i = 0; i < vec_size; ++i ) {
    vec[i] = rand() % 20 + 1;
}

/* output the list after you made it */
std::copy(myvec.begin(), myvec.end(),
        std::ostream_iterator<int>(cout, "\n"));

Upvotes: 1