std::rand(3)

💅 NAME 💅

std::rand — bestie's very first random number era ✨ (yes the NAME section just says std::rand - std::rand, we're keeping it real, no notes 💀)

This little icon lives in the C++ Standard Library (the manual name, folding it in like a cute lil cardigan) 🎀

---

✨ SYNOPSIS ✨

Defined in header <cstdlib> 🍓

int rand();

So basically: it hands you a pseudo-random integral value from the range [0, RAND_MAX]. That's the whole vibe 🦋

Now the seeding lore, because this part matters, no cap:

std::srand() seeds the pseudo-random number generator that rand() uses. And if you call rand() before any calls to std::srand()? It behaves as if it was seeded with std::srand(1). Like it just defaults to the same starting fit every time 🙈

Each time rand() is seeded with std::srand(), it must produce the same sequence of values on successive calls. Same seed, same sequence — that's the contract, fr fr 🌸

Other functions in the standard library may call rand. It is implementation-defined which functions do so. So like, you're not the only one in the group chat calling it 👀

It is implementation-defined whether rand() is thread-safe. Not gonna lie, that's a little scary but it is what it is 💀

---

🫶 PARAMETERS 🫶

(none) — takes literally nothing, zero arguments, main character who needs no input 💅

---

👑 RETURN VALUE 👑

A pseudo-random integral value between 0 and RAND_MAX. That's it. That's the gift 🎁

---

🌈 NOTES 🌈

Okay so real talk, and I need you to hear me on this one:

There are no guarantees as to the quality of the random sequence produced. None. Zero. The bar is on the floor 😭

In the past, some implementations of rand() have had serious shortcomings in the randomness, distribution and period of the sequence produced. And bestie, one well-known example: the low-order bit simply alternated between 1 and 0 between calls. Like literally 1, 0, 1, 0, that's not random that's a metronome 💀💀

🚨 rand() is not recommended for serious random-number generation needs. Periodt.

It is recommended to use C++11's random number generation facilities to replace rand(). 🧚✨

(since C++11)

---

🔥 EXAMPLE 🔥

// Run this code

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

int main()
{
    std::srand(std::time(nullptr)); // use current time as seed for random generator
    int random_value = std::rand();
    std::cout << "Random value on [0, " << RAND_MAX << "]: " << random_value << '\n';

    for (const int times = 8; const int sides : {2, 4, 6, 8})
    {
        std::cout << "Roll " << sides << "-sided dice " << times << " times: ";
        for (int n = 0; n != times; ++n)
        {
            int x = sides + 1;
            while (x > sides)
                x = 1 + std::rand() / ((RAND_MAX + 1u) / sides);
                    // Note: 1 + rand() % sides is biased
            std::cout << x << ' ';
        }
        std::cout << '\n';
    }
}

Obsessed with this example honestly — it seeds with std::time(nullptr) (current time as seed, iconic), prints the value on [0, RAND_MAX], then rolls dice of 2, 4, 6 and 8 sides, 8 times each 🎲 And clock the comment: 1 + rand() % sides is biased, so they do the division dance instead. Read the comment, bestie, iykyk 💜

---

⭐ POSSIBLE OUTPUT ⭐

Random value on [0, 2147483647]: 948298199
Roll 2-sided dice 8 times: 2 2 1 2 1 1 2 2
Roll 4-sided dice 8 times: 1 3 4 2 1 3 3 1
Roll 6-sided dice 8 times: 3 2 1 6 6 4 4 2
Roll 8-sided dice 8 times: 4 5 6 6 3 6 1 2

(That's possible output, not guaranteed output — remember the notes section, bestie 😭)

---

🦋 SEE ALSO 🦋

• uniform_int_distribution — produces integer values evenly distributed across a range (C++11) — (class template) 👑
• srand — seeds pseudo-random number generator — (function) 🌱
• RAND_MAX — maximum possible value generated by std::rand — (macro constant) 🔝
• randint — generates a random integer in the specified range — (function template) 🎀
• C documentation for rand — the crossover episode, go read it 💖

---

Okay that's the whole page, slay 💅✨ go forth and seed responsibly — and remember what we said about serious random-number needs (don't use this one, use the C++11 facilities, I'm begging 🙏).

Source/version and date, as promised in the cute sign-off: this page came from http://cppreference.com, version/date 2024.06.10, and yes, it's std::rand(3) — that's the footer, folded into a sign-off instead of copy-pasted, you're welcome 🫶🌸

bye bestie, happy rolling 🎲💖