Random() efficiency in C++

Question

I am writing function where I need to find the random number between 1 - 10. One of the easiest way is to use random() libc call. I am going to use this function a lot. But I don't know how efficient it will be. If any one has idea about efficiency of random() that will be a help ?

Also I notice that random() give the same pattern in 2 runs.

int main()
{
   for(int i=0;i<10;i++)
   {
    cout << random() % 10 << endl;     
   }
}

Output 1st time :- 3 6 7 5 3 5 6 2 9 1

Second time also I got same output.

Then how come it's random ?

Answer 1

Others have explained why it's the same sequence every time, but this is how you generate a random number with C++ :

#include <random>

int main() {
    std::random_device rd{}; //(hopefully) truly random device
    std::mt19937 engine{rd()}; //seed a pseudo rng with random_device
    std::uniform_int_distribution<int> d(1,10); //1 to 10, inclusive
    int RandNum = d(engine); //generate
    return 0;
}

http://en.cppreference.com/w/cpp/numeric/random

Answer 2

The actual execution time depends on your platform of course, but it is pretty much straight forward, couple multiplication and divisions or shifts:

What common algorithms are used for C's rand()?

I don't think you should be worried. If you need a lot of random numbers, then another random source probably would be a better choice for you.

If you are looking for tweaks, how about splitting the result from rand() into individual digits to get several results per call.

Answer 3

This way is very simple and effective, you only need to set the seed:

#include <iostream>
#include <stdlib.h>
#include <time.h>

using namespace std;

int main(){
    srand(time(NULL));
    for(int i=0;i<10;i++)
        cout << rand() % 10 << endl;     
}

Answer 4

要解决在2次运行中获得相同模式的问题，只需添加函数randomize（）