random array generation in c++ [duplicate] - c++

Question

This question already has answers here:
rand() returns same values when called within a single function
(5 answers)
Closed 8 years ago.
I'm trying to write a simple function that generates an array with random integers.
I encountered with an interesting thing, when I run this program outputs -each cout
statements- seem the same.
However, when I am debugging and watching it step by step array values are changing.
Am I doing something wrong?
Thanks.
void generateRandomArray( int *&inputArray, int size){
if(inputArray != NULL){
delete []inputArray;
}
//create a new array
inputArray = new int [size];
//fill with random numbers
srand(unsigned (time (NULL)));
for (int i = 0; i < size ; i++)
inputArray[i] = (rand() % 100) ;
}
int main(){
//Variables
int *inputArray = NULL;
int size;
//Test
size = 10;
//first call
generateRandomArray( inputArray, size);
for(int i = 0 ; i < size; i++){
cout << inputArray[i] << endl;
}
cout << "------------------" << endl;
//second call
generateRandomArray( inputArray, size);
//output is the same with previous
for(int i = 0 ; i < size; i++){
cout << inputArray[i] << endl;
}
return 0;
}

Answer 1

The program runs too fast, srand(unsigned (time (NULL))); will get a seed having the same seconds (since epoch). Hence, the rand sequences are the same.

Answer 2

The problem is that if the calls to srand all happen withing the same second, the generator will be seeded with the same values and so generate the same sequence, always.
When you're debugging it will be slower, and allow time to pass and have different seed for the generator. When not debugging the time between calls will be too fast.
Don't call srand more than once, unless you know what you're doing. Put it at the start of the main function, and don't call it again.

Answer 3

Call srand(time(NULL)) and it will seed the current time and make a random number from it. Since time is changing all the time, it will give you a different number every time.

Answer 4

With C++11, you may use something like: (https://ideone.com/EIb0nY)
#include <iostream>
#include <random>
#include <vector>
std::vector<int> generateRandomArray(std::default_random_engine& generator, int size)
{
std::uniform_int_distribution<int> distribution(1, 100);
std::vector<int> res(size);
for (auto& e : res) {
e = distribution(generator);
}
return res;
}
int main()
{
std::default_random_engine generator;
const int size = 10;
//first call
for (auto e : generateRandomArray(generator, size)) {
std::cout << e << std::endl;
}
std::cout << "------------------" << std::endl;
//second call
for (auto e : generateRandomArray(generator2, size)) {
std::cout << e << std::endl;
}
return 0;
}
That use the new random facilities.
It avoids manual memory management (with std::vector).