Random Numbers, Dice and Games of Chance

Teach your program to roll dice and surprise you

Big Idea: A computer follows your instructions exactly, so how can it ever be "random"? The trick is a special number machine that spits out numbers you cannot easily guess. C++ gives us a modern toolbox called <random> for exactly this. Once you can make random numbers, you can build dice, coins, card shuffles and games.

By the end you will be able to:

say why programs use pseudo-random numbers,
use the three helpers from <random>: random_device, mt19937 and a distribution,
seed once and roll many times,
map numbers into a range like 1 to 6,
build a dice roller and a number-guessing game.

1. Why do we need random numbers?

Think about your favourite games. A board game needs dice. A card game needs a shuffle. A video game drops a surprise enemy or a lucky prize. Even a raffle needs to pick a winner. All of these need a number that nobody can predict in advance.

Randomness is not only for fun. Programmers use random numbers to test their code with lots of different inputs, and scientists use them to simulate things like weather or traffic. So learning to make good random numbers is a real, everyday skill, not just a toy.

A computer cannot flip a real coin. Instead it runs clever maths that looks random. We call the result a pseudo-random number - "pseudo" means "pretend", because it only pretends to be truly random.

2. The old way: `rand()` and its traps

For a long time C++ programmers used rand(), which lives in the header <cstdlib>. It hands back a whole number somewhere between 0 and a big value called RAND_MAX. It still works, but it has two famous traps.

Trap 1 - forgetting to seed. A seed is the starting point for the number machine. If you never set one, rand() starts from the same place every time, so your program prints the same "random" numbers on every run. To avoid that, older code calls srand(time(0)) once to seed from the clock.

#include <iostream>
#include <cstdlib>   // rand, srand
#include <ctime>     // time
using namespace std;

int main() {
    srand(time(0));          // seed once, using the clock
    int roll = rand() % 6 + 1;   // squeeze into 1..6
    cout << "You rolled: " << roll << endl;
    return 0;
}

Trap 2 - the % 6 trick is slightly unfair. Using rand() % 6 to get a small range makes some numbers come up a tiny bit more often than others. For a school project it is usually fine, but there is a better, fairer tool. That tool is the modern <random> library.

If you ever see a program that prints the exact same "random" numbers every single run, the most likely reason is a missing seed.

Random Numbers, Dice and Games of Chance