How can I generate a random number within a range in Rust?

Question

Editor's note: This code example is from a version of Rust prior to 1.0 and is not syntactically valid Rust 1.0 code. Updated versions of this code produce different errors, but the answers still contain valuable information.

I came across the following example of how to generate a random number using Rust, but it doesn't appear to work. The example doesn't show which version of Rust it applies to, so perhaps it is out-of-date, or perhaps I got something wrong.

// http://static.rust-lang.org/doc/master/std/rand/trait.Rng.html  use std::rand; use std::rand::Rng;  fn main() {     let mut rng = rand::task_rng();     let n: uint = rng.gen_range(0u, 10);     println!("{}", n);     let m: float = rng.gen_range(-40.0, 1.3e5);     println!("{}", m); } 

When I attempt to compile this, the following error results:

test_rand002.rs:6:17: 6:39 error: type `@mut std::rand::IsaacRng` does not implement any method in scope named `gen_range` test_rand002.rs:6    let n: uint = rng.gen_range(0u, 10);                                    ^~~~~~~~~~~~~~~~~~~~~~ test_rand002.rs:8:18: 8:46 error: type `@mut std::rand::IsaacRng` does not implement any method in scope named `gen_range` test_rand002.rs:8    let m: float = rng.gen_range(-40.0, 1.3e5);                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

There is another example (as follows) on the same page (above) that does work. However, it doesn't do exactly what I want, although I could adapt it.

use std::rand; use std::rand::Rng;  fn main() {     let mut rng = rand::task_rng();     let x: uint = rng.gen();     println!("{}", x);     println!("{:?}", rng.gen::<(f64, bool)>()); } 

How can I generate a "simple" random number using Rust (e.g.: i64) within a given range (e.g.: 0 to n)?

Answer 1

This generates a random number between 0 (inclusive) and 100 (exclusive) using Rng::gen_range:

use rand::Rng; // 0.8.0  fn main() {     // Generate random number in the range [0, 99]     let num = rand::thread_rng().gen_range(0..100);     println!("{}", num); } 

Don't forget to add the rand dependency to Cargo.toml:

[dependencies] rand = "0.8" 

Answer 2

Editor's note: This answer is for a version of Rust prior to 1.0 and is not valid in Rust 1.0. See Manoel Stilpen's answer instead.

This has been changing a lot recently (sorry! it's all been me), and in Rust 0.8 it was called gen_integer_range (note the /0.8/ rather than /master/ in the URL, if you are using 0.8 you need to be reading those docs).

A word of warning: .gen_integer_range was entirely incorrect in many ways, the new .gen_range doesn't have incorrectness problems.

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Code for master (where .gen_range works fine):

use std::rand::{task_rng, Rng};  fn main() {     // a number from [-40.0, 13000.0)     let num: f64 = task_rng().gen_range(-40.0, 1.3e4);     println!("{}", num); }