如何在一个范围内生成随机数？

Question

I'm creating a program for school that works as follows: user thinks of a number and computer tries to guess it. computer generates a random number, asks user if number is higher or lower, then if the number is higher it takes that number and sets that as the minimum value for the random number range(ex. generates a 47, then will only generate numbers between 47-100). if the number is lower it takes the random number that was generated and makes that the minimum value.

My problem is with the random number generator and the range. I can generate the random number by seeding it at the start { srand(time(NULL));} then generate a number { rand_num = rand()%100; } generates a number between 1-100. but I can't figure out how to generate a number in a range (example 47-89). thought it was {rand_num = rand()%89 + 47}, but this doesn't seem to work.

Github: https://gist.github.com/EthanA2020/e121b27ab80fa6e2d32df1396e62b632

{

    srand(time(NULL));
    do {
    rand_num=rand()%47+89;
    cout << rand_num << endl;
    cout << "enter: "; cin >> userquitval;
    }while (userquitval!=7);
}

Answer 1

To generate a random number in C++11 or later, use the C++ random library. This should be much preferred over the C utilities for any application.

#include <random>
#include <iostream>

int main() {
  std::random_device seed;
  std::mt19937 gen{seed()}; // seed the generator
  std::uniform_int_distribution dist{47, 89}; // set min and max
  int guess = dist(gen); // generate number
  std::cout << "Computer guess: " << guess << '\n';
}

Read the relevant cppreference page for more information.

Answer 2

In the modulo operator you have to use the length of the interval you want the values in.

rand_num=rand()%(90-47)+47;

As mentioned by Jesper, this introduces a bias. To understand it, think what happens if you want a number equal to 0 or 1, and the range of rand is [0, 1, 2]. With the above formula, the probability of getting a 0 would be 0.66 and the probability of getting a 1 would be 0.33).

To overcome this problem, you can use the following formula that scales the random number to a float in the [0,1] interval, and then use that to generate the number you need.

(double) uniform = (double) rand() / (double) RAND_MAX;
rand_num = (int) ((90.0 - 47.0)*uniform) + 47);