Generate Random Double Numbers in C++

Double is a data type just like a float but double has 2x more precision than float. One bit for the sign, 11 bits for the exponent and 52* bits for the value constitute the 64-bit IEEE 754 double precision Floating Point Number known as “double.” Double has 15 decimal digits of precision.

In this article, we generate random double numbers in C++.

Methods to generate random double 

The random double number can be generated using a few methods which are mentioned below:

1. Using random() function
2. Using uniform_real_distribution and default_random_engine. 

1. Using random() function

Using the random function is the method where we get random integers and by using them we create a random double number by performing certain operations in it. You can set lower and upper bounds in it.

random(): It generate a random integer.

Example:

C++

// C++ program to generate random double
// Using random function
#include <iostream>
#include <time.h>
using namespace std;

// Driver Code
int main()
{
    const long max_rand = 1000000L;

    double lower_bound = 0;
    double upper_bound = 100;

    srandom(time(NULL));

      // Using random function to
      // get random double value
    double random_double = lower_bound
                           + (upper_bound - lower_bound)
                                 * (random() % max_rand)
                                 / max_rand;

    cout << random_double << endl;

    return 0;
}

Output

35.9952

2. Using uniform_real_distribution and default_random_engine

When we want to find random numbers after defining the upper and lower bound then we can use this method.

uniform_real_distribution: The random library now includes the uniform real distribution class, whose member functions generate random real numbers or continuous values with uniform probability from a specified input range.

default_random_engine: This class of pseudo-random number generators produces random numbers.

• min(): It gives back the lowest value specified by the operator ().
• max(): It gives back the highest value specified by the operator ().
• operator(): A fresh random number is given back.

Example:

C++

// C++ program to generate random double
// uniform_real_distribution and
// default_random_engine
#include <iostream>
#include <random>
using namespace std;

// Driver Code
int main()
{
    // Declaring the upper and lower
    // bounds
    double lower_bound = 0;
    double upper_bound = 100;

    uniform_real_distribution<double> unif(lower_bound,
                                           upper_bound);

    default_random_engine re;

    // Getting a random double value
    double random_double = unif(re);

    cout << random_double << endl;

    return 0;
}

Output

13.1538

Time complexity: O(1).

Auxiliary Space: O(1).