Using the rand() Function to Generate Random Numbers¶

Function Name: rand() Necessary libraries: #include \, #include \ Description: rand()is a function that returns a random integer number in the range between 0 and RAND_MAX (RAND_MAX = ~32767). Before calling rand(), it is necessary to initialize it with srand(). srand() requires a seed to initialize the rand() function, and we want this to be as random as possible, so we utilize the time() function, which returns the amount of time that has passed on the system running the program.

Once we initialize it, we can call rand() to return a random integer. But if we just call it on its own, it will return a random number between 0 and RAND_MAX. If we want it to be within a specific range, we use the % operator.

Steps to use rand(): 1. Call srand( time(NULL)); 2. Call rand() and use % to get values within a range

Examples:¶

int v1 = rand() % 100; // v1 in the range 0 to 99
int v2 = rand() % 100 + 1; // v2 in the range 1 to 100
int v3 = rand() % 30 + 1985; // v3 in the range 1985-2014  

Example Code:¶

#include <iostream> //needed for cout/cin
#include <cstdlib> //needed for rand() and srand()
#include <ctime> //needed for time()

using namespace std;

int main()
{
    cout << "This program will print 10 random numbers between the range of 1 and 20\n";

    //initialize rand() with srand()
    srand(time(NULL));

    //the following loop will run the code of generating a random number between 1 and 20, 10 times
    for(int i = 0; i < 10; i++){
        int randomNum = rand() % 20 + 1;
        cout << "Random Number " << i+1 << ":" << randomNum << "\n";
    }

    return 0;
}

Output:

This program will print 10 random numbers between the range of 1 and 20
Random Number 1:11
Random Number 2:14
Random Number 3:2
Random Number 4:12
Random Number 5:19
Random Number 6:2
Random Number 7:9
Random Number 8:16
Random Number 9:18
Random Number 10:16