Fill a vector with random numbers c++

Question

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.

Answer 1

You can use 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>  using namespace std;  int main() {     // First create an instance of an engine.     random_device rnd_device;     // Specify the engine and distribution.     mt19937 mersenne_engine {rnd_device()};  // Generates random integers     uniform_int_distribution<int> dist {1, 52};          auto gen = [&dist, &mersenne_engine](){                    return dist(mersenne_engine);                };      vector<int> vec(10);     generate(begin(vec), end(vec), gen);          // Optional     for (auto i : vec) {         cout << i << " ";     }       } 

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

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