Generate different random numbers - c++

Question

I want to generate different random numbers . I used srand and rand , but in my output some numbers are identical .
This is my output :
How to do with srand to generate different numbers ?
#include<iostream>
#include<time.h>
#include <windows.h>
int main(){
time_t t;
std::vector<int> myVector;
srand((unsigned)time(NULL));
for (int i = 0; i < 40; i++){
int b = rand() % 100;
myVector.push_back(b);
std::cout << myVector[i] << std::endl;
}
Sleep(50000);
}

Answer 1

One easy way is to add all numbers from 0-99 to a vector and shuffle it, then you can get as many (up to 100) non repeating random numbers as you require.
#include <algorithm>
#include <chrono>
#include <iostream>
#include <random>
#include <vector>
int main(void) {
std::vector<int> numbers;
for(int i=0; i<100; i++) // add 0-99 to the vector
numbers.push_back(i);
unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
std::shuffle(numbers.begin(), numbers.end(), std::default_random_engine(seed));
for(int i=0; i<40; i++) // print the first 40 randomly sorted numbers
std::cout << numbers[i] << std::endl;
}

Answer 2

You could use a set:
std::set<int> numbers;
while (numbers.size() < 40)
{
numbers.add(rand() % 100);
}
and then copy it into a vector if necessary.

Answer 3

srand number generator can give identical numbers.
You could implement a solution which deletes duplicates not adding them to the vector. For example:
#include <iostream>
#include <algorithm>
#include <vector>
int main()
{
std::vector<int> myVector;
srand((unsigned)time(NULL));
while(myVector.size() < 40)
{
int b = rand() % 100;
if ( !(std::find(myVector.begin(), myVector.end(), b) != myVector.end()))
{
myVector.push_back(b);
std::cout << myVector.at(myVector.size()-1) << std::endl;
}
}
Sleep(50000);
return 0;
}

Answer 4

An easy way of getting rid of duplicates is using std::unique in <algorithm>.
Here is an example of that in use:
#include <vector>
#include <iostream>
#include <algorithm>
#include <random>
int ran(int min, int max)
{
std::random_device r;
std::mt19937 gen(r());
std::uniform_int_distribution<> dis(min, max);
return dis(gen);
}
int main()
{
const int fill_size = 10;
const int min = 1; // min random number
const int max = 100; // max random number
std::vector<int> vec;
while (vec.size() != fill_size) {
vec.emplace_back(ran(min, max)); // create new random number
std::sort(begin(vec), end(vec)); // sort before call to unique
auto last = std::unique(begin(vec), end(vec));
vec.erase(last, end(vec)); // erase duplicates
}
std::random_shuffle(begin(vec), end(vec)); // mix up the sequence
for (const auto& i : vec) // and display elements
std::cout << i << " ";
}

Answer 5

As for me, the idea of using set is not so good, because the generating time of every new value increases. If you have enough memory it seems that usage of an array can be preferable.
In the next code, I don't use shuffle, instead, I use a random function just size times to choose one value. I add it to the destination vector, then in the source array, swap the value with the last element and decrease arr_size.
/*
* Return random unsigned int value using intrinsic
* */
unsigned getRandom() {
unsigned val;
_rdrand32_step(&val);
return val;
}
/*
* Return a vector<int> of uniq numbers in a range of [min ... max).
*
* #param min - min value.
* #param max - max value.
* #param size - amount of uniq numbers (size <= max-min).
* */
vector<int> getUniqNumbers(int min, int max, unsigned size) {
int arr_size = max - min;
int *a = new int[arr_size];
for (int i = 0; i < arr_size; i++) {
a[i] = min + i;
}
vector<int> numbers(size);
for (int i = 0; i < size; i++) {
unsigned u_rand = getRandom() % arr_size;
numbers[i] = a[u_rand];
a[u_rand] = a[--arr_size];
}
delete[] a;
return numbers;
}

Answer 6

You can easily achieve a unique set of random numbers writing:
#include<iostream>
#include<vector>
int main(){
std::vector<int> myVector;
srand((unsigned)time(NULL));
for (int i = 0; i < 40; i++) {
int b = rand() % 100;
if(!std::find(std::begin(myvector),std::end(myvector),b)) {
myVector.push_back(b);
std::cout << myVector[i] << std::endl;
}
}
}

Answer 7

This is a statistical (mathematical) issue. Random numbers may be identical to eachother. If you need unique numbers, you must check to see if they are used before. For example like this:
for (int i = 0; i < 40; i++){
int b = rand() % 100;
for (int j = 0; j < i; j++){
if(myVector[j]==b)i--;
else{
myVector.push_back(b);
std::cout << myVector[i] << std::endl;
}
}
}