How to generate random numbers between 2 values, inclusive?

Question

I need help figuring out how to generate a set amount of random numbers between two user-inputted values, inclusively. Just so you know, I have searched for this but am not finding what I have just stated. Here is my code:

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

int main()
{
    int userBeg, userEnd, outPut;

    cout << "Enter a start value: ";
    cin >> userBeg;
    cout << "Enter an end value: ";
    cin >> userEnd;

    srand(time(NULL)); //generates random seed val

    for (int i = 0; i < 5; i++) {
      //prints number between user input, inclusive
    outPut = rand()%((userEnd - userBeg) + 1); 
    cout << outPut << "  ";
    }//end for

    return 0;
}//end main 

I'm confused with the output I get for the following ranges: 1-100 yield output numbers which fall in-between, but not including, the boundaries such as 50, 97, 24, 59, 22. But, 10-20 yield numbers such as 1, 14, 6, 12, 13. Here, the output is outside of the boundaries as well as in-between them. What am I doing wrong?

Thank you in advance for your help!

Answer 1

Using the c++11 random library.

You could use std::default_random_engine (or any other random engine) with std::uniform_int_distribution.

The values you pass to std::uniform_int_distribution will be the lower and upper bounds of your random range.

e.g.

#include <iostream>
#include <random>

int main()
{
    const int nrolls = 100;

    std::default_random_engine generator;
    std::uniform_int_distribution<int> distribution(0,9);

    int p[nrolls]={};

    for (int i=0; i<nrolls; ++i)
    {
        p[i] = distribution(generator);
    }

    std::cout << "uniform_int_distribution (0,9):" << '\n';
    for (int i=0; i<nrolls; ++i)
    {
        std::cout << i << ": " << p[i] << '\n';
    }
}

If you wish to seed the random engine, create it like.

unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();

std::default_random_engine generator(seed);

Answer 2

rand returns number between 0 and RAND_MAX. By taking modulo userEnd - userBeg + 1 the boundaries will be limited to 0 and userEnd - userBeg. If the random number should be within given boundaries, then userBeg should be added, so the calculus becomes

    outPut = rand()%((userEnd - userBeg) + 1) + userBeg; 

Answer 3

It's still experimental, but it is exactly what you want

http://en.cppreference.com/w/cpp/experimental/randint

#include <iostream>
#include <experimental/random>

int main()
{
    int random_number = std::experimental::randint(100, 999);
    std::cout << "random 3-digit number: " << random_number << '\n';
}

Edited: it's really random http://en.cppreference.com/w/cpp/numeric/random

Answer 4

For the rand to work you need to do:

#include <cstdlib>

int main() {

    int Min = 103;
    int Max = 113;

    int Number = std::rand() % (Max + 1 - Min) + Min;

    return 0;
}

I omitted the cin for clarity. (Max + 1 - Min) makes it inclusive. + Min sets the minimum value!

So stuffed into a function you may use:

int randint(int Min, int Max) {
    return std::rand() % (Max + 1 - Min) + Min;
}

After testing it works with negative numbers as well as positive.

Sample outputs (with a loop and cout):

11-23

The number is: 13
The number is: 18
The number is: 14
The number is: 17
The number is: 18
The number is: 18
The number is: 23
The number is: 15
The number is: 11
The number is: 22
The number is: 22
The number is: 11
The number is: 22
The number is: 16
The number is: 14
The number is: 21
The number is: 16
The number is: 19
The number is: 15
The number is: 13
The number is: 17

1239-1242

The number is: 1240
The number is: 1242
The number is: 1239
The number is: 1241
The number is: 1241
The number is: 1241
The number is: 1239
The number is: 1240
The number is: 1240
The number is: 1240
The number is: 1242
The number is: 1240
The number is: 1242