Use of Random

ericlang · December 23, 2025, 2:03pm

I am afraid the std.Random.zig is unreadable for me. No useful comments provided. Strange anytype pointers..
How can I generate some random numbers 0 <= r <= 9?

vulpesx · December 23, 2025, 2:22pm

const std = @import("std");

pub fn main() !void {
    // userland rng implementation
    var prng = std.Random.DefaultPrng.init(seed: {
        var seed: u64 = undefined;
        // get random seed from OS
        try std.posix.getrandom(std.mem.asBytes(&seed));
        break :seed seed;
    });
    // get interface
    const rand = prng.random();
    const n = rand.intRangeAtMost(u8, 0, 9);
    std.debug.print("{}", .{n});
}

dimdin · December 23, 2025, 2:29pm

Some Alternatives

Use crypto to generate the seed:

const seed = std.crypto.random.int(u64);
var prng = std.Random.DefaultPrng.init(seed);
const rand = prng.random();
_ = rand.uintAtMost(u4, 9);

When testing there is std.testing.random_seed:

var prng = std.Random.DefaultPrng.init(std.testing.random_seed);
const rand = prng.random();
_ = rand.uintAtMost(u4, 9);

Note that prng must be var to hold the random generator state.

ericlang · December 23, 2025, 2:35pm

Thanks!
Writing Zig is easier then reading.

matklad · December 23, 2025, 4:25pm

We also have PRNG module in TigerBeetle:

github.com/tigerbeetle/tigerbeetle

src/stdx/prng.zig

522c0b9d1

//! TigerBeetle standard Pseudo Random Number generator.
//!
//! Import qualified and use `prng` for field/variable name:
//!
//! ```
//! prng: *stdx.PRNG
//! ```
//!
//! The implementation matches Zig's `std.Random.DefaultPrng`, but we avoid using that directly in
//! order to:
//! - remove floating point from the API, to ensure determinism
//! - isolate our test suite from stdlib API churn
//! - isolate TigerBeetle from the churn in the PRNG algorithms
//! - simplify and extend the API
//! - remove dynamic-dispatch indirection (a minor bonus).

const std = @import("std");
const stdx = @import("stdx.zig");
const assert = std.debug.assert;
const math = std.math;

This file has been truncated. show original

This is mostly to avoid stdlib churn, but I also cleaned up the API to my taste somewhat (e.g.), and we have a second fun implementation of the same API here (and a third one, finite rng, is on my todo list…)

SolusBellator · May 7, 2026, 6:13pm

Heyo, sorry for having to bring this back from the dead, but with Zig being an extremely fast-changing language (which is a good thing, don’t get me wrong), sometimes solutions become outdated.

I’ve been trying to achieve the same goal for a while today: Generate a random number min < n < max

I’ve managed to figure out (vial trial and error… ) how to get me a Random that I can use, but I fail to find a proper way of seeding it.

Here’s why I can’t get a proper seed value:

std.posix does not contain the getrandom member (anymore?)
std.crypto does not contain the random member anymore
all timestamp functions in std.Io.Clock return signed integers, while the Random expects a u64

So really, it’s about getting a proper u64 seed for my random, either form a timestamp or from system entropy, but I haven’t figured out either so far, and asking the (awesome!) community is my best bet right now.

Hope y’all can help! Maybe I’m just missing something extremely obvious…

alanza · May 7, 2026, 6:15pm

if you have 64 bits of something from std.Io.Clock, why not @bitCast to get a u64?

SolusBellator · May 7, 2026, 6:19pm

Tried that just now, maybe I did something wrong but I’m getting:

error: cannot @bitCast from ‘Io.Timestamp’; struct does not have a guaranteed in-memory layout

I also remember trying @intCast but that was futile (I’ very recently picked up Zig, and despite having lots of previous experience especially in C and partially C++ it’s some new stuff for me)

SolusBellator · May 7, 2026, 6:20pm

Nvm I’m an idiot I tried to cast the Timestamp. Actually using the toSeconds or toMicroseconds functions does exactly what it should do.