How can I create an array of random numbers in C++

Question

Instead of The ELEMENTS being 25 is there a way to randomly generate a large array of elements....10000, 100000, or even 1000000 elements and then use my insertion sort algorithms. I am trying to have a large array of elements and use insertion sort to put them in order and then also in reverse order. Next I used clock() in the time.h file to figure out the run time of each algorithm. I am trying to test with a large amount of numbers.

#define ELEMENTS 25

void insertion_sort(int x[],int length);
void insertion_sort_reverse(int x[],int length);

int main()
{
  clock_t tStart = clock();
  int B[ELEMENTS]={4,2,5,6,1,3,17,14,67,45,32,66,88,
               78,69,92,93,21,25,23,71,61,59,60,30};
  int x;

  cout<<"Not Sorted: "<<endl;
  for(x=0;x<ELEMENTS;x++)
       cout<<B[x]<<endl;

   insertion_sort(B,ELEMENTS);
   cout <<"Sorted Normal: "<<endl;
  for(x=0;x<ELEMENTS;x++)
       cout<< B[x] <<endl;

  insertion_sort_reverse(B,ELEMENTS);
  cout <<"Sorted Reverse: "<<endl;
  for(x=0;x<ELEMENTS;x++)
       cout<< B[x] <<endl;

  double seconds = clock() / double(CLK_TCK);
  cout << "This program has been running for " << seconds << " seconds." << endl;
  system("pause");
  return 0;
}

bames53 · Accepted Answer · 2016-01-21 18:23:41Z

19

#include <random>     // mt19937 and uniform_int_distribution
#include <algorithm>  // generate
#include <vector>     // vector
#include <iterator>   // begin, end, and ostream_iterator
#include <functional> // bind
#include <iostream>   // cout

std::vector<int> create_random_data(int n) {
  std::random_device r;
  std::seed_seq      seed{r(), r(), r(), r(), r(), r(), r(), r()};
  std::mt19937       eng(seed); // a source of random data

  std::uniform_int_distribution<int> dist;
  std::vector<int> v(n);

  generate(begin(v), end(v), bind(dist, eng));
  return v;
}

int main() {
  auto random_data = create_random_data(100000);

  std::cout << "unsorted: ";
  copy(begin(random_data), end(random_data),
       std::ostream_iterator<int>(std::cout, "\n"));
}

generate is a generic algorithm that fills a range with values generated by a functor. In this case we provide it with a functor that uses our source of random data to produce random numbers, and we provide a range corresponding to a container, which we can use after generate fills it with data.

We're using std::mt19937 and std::uniform_int_distribution, standard C++ facilities as of C++11 (and available in VS2010), to create random numbers instead of the older std::rand() and std::srand() because the newer method is easier to use correctly, higher quality and more flexible.

If you're using VS2012 or higher then the C++11 time library is available.

#include <chrono>

int main() {
  using std::chrono::high_resolution_clock;
  using std::chrono::duration_cast;
  using std::chrono::nanoseconds;

  auto start = high_resolution_clock::now();

  // ...

  auto end = high_resolution_clock::now();

  std::cout << duration_cast<nanoseconds>(end - start).count() << "ns\n";
}

<chrono>

edited Jan 21, 2016 at 18:23

answered Aug 27, 2012 at 21:09

bames53

88.7k15 gold badges191 silver badges255 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Mahesh Over a year ago

std::mt19937, std::uniform_int_distribution<> never heard of them. Wonder what is the significance of 19937 ? +1 Anyways :)

Qaz Over a year ago

@Mahesh, It has a very long period of 2^19937 − 1. - From Wikipedia's article on Mersenne Twister ;)

bames53 Over a year ago

@Mehesh 19937 is a prime number that is a common parameter for a certain random number algorithm, Mersenne Twister (mt)

Mooing Duck · Accepted Answer · 2012-08-27 21:18:27Z

Instead of

void insertion_sort(int x[],int length);
void insertion_sort_reverse(int x[],int length);

 int B[ELEMENTS]={4,2,5,6,1,3,17,14,67,45,32,66,88,
           78,69,92,93,21,25,23,71,61,59,60,30};

try

void insertion_sort(std::vector<int>& x);
void insertion_sort_reverse(std::vector<int>& x);

 srand(NULL);
 std::vector<int> B(num_elements); //num_elements can be a variable
 std::generate(B.begin(), B.end(), rand);

As relates to the task and not the question:
You'll want to run each sort twice in a row, the first without timing, the second with timing.
Your tests aren't fair since one starts from a randomized position, and the other from a sorted position.
You're including IO in your timings, and IO is always slow (cout)
std::endl forces the program to give all the output to the OS immediately, use '\n'.
You're displaying a completely unrelated number of seconds.

int main()
{
  srand(NULL);
  std::vector<int> B(num_elements); //num_elements can be a variable
  std::generate(B.begin(), B.end(), rand);
  std::vector<int> C(B); //make a copy of the data

  std::cout << "Not Sorted:" << '\n';
  for(int i=0;i<B.size();i++)
       cout<<B[i]<<'\n';

  clock_t tStart0 = clock();        
  insertion_sort(B,ELEMENTS);
  clock_t tStop0 = clock();     

  cout <<"Sorted Normal: "<<'\n';
  for(int i=0;i<B.size();i++)
       cout<< B[i] <<'\n';

  clock_t tStart1 = clock();        
  insertion_sort_reverse(C);
  clock_t tStop1 = clock();  

  cout <<"Sorted Reverse: "<<'\n';
  for(int i=0;i<C.size();i++)
       cout<< C[i] <<'\n';

  double seconds = (tStop1-tStart1 + tStop0-tStart0) / double(CLK_TCK);
  cout << "This program has been running for " << seconds << " seconds." << endl;
  system("pause");
  return 0;
}

alijandro · Accepted Answer · 2019-10-09 01:58:12Z

2

For C++11, I used the time value as initial random seed to generate any number of random data.

See the following sample code

#include <iostream>
#include <random>
#include <chrono>
#include <functional>
#include <iterator>

constexpr auto MAX = 20;

int main(int argc, char* argv[]) {

    std::default_random_engine re(std::chrono::system_clock::now().time_since_epoch().count());
    std::uniform_int_distribution<int> uid(0, MAX);
    int nums[MAX] {};
    std::generate(nums, nums+MAX, std::bind(uid, re));

    // apply sort algorithm

    std::copy(nums, nums+MAX, std::ostream_iterator<int>(std::cout, ","));
    std::cout << std::endl;
    return 0;
}

answered Oct 9, 2019 at 1:58

alijandro

12.3k2 gold badges61 silver badges78 bronze badges

Collectives™ on Stack Overflow

How can I create an array of random numbers in C++

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related