CODEWITHSUNDEEP
rust
Brian Oh
Brian Oh

Reputation: 10730

How Do I pass object "std::rand::task_rng()" to a function in Rust?

I have a test program where I need to generate a random number. I therefore did a test comparing using

"uVal = rand::task_rng().gen();"

each time a random number is generated compared to creating an object using eg. :

let mut oRandom = std::rand::task_rng()

and generating multiple random numbers. Using the object (oRandom) created is much faster, so I thought I should pass the object to the function generating the random number, however I haven't been able to find a way to do that. It's not critical, but I presume it can be done.

Example 1 : not using the object : (much slower than 2)

let mut uVal : u8;
for _ in range(1, iMax) {
    uVal = std::rand::task_rng().gen();

Example 2 : using the object : (much faster than 1)

let mut oRandom = std::rand::task_rng();
let mut uVal : u8;
for _ in range(1, iMax) {
    uVal = oRandom.gen();

Example 3 : my attempt to pass the object to the function :

12  let mut oRandom = std::rand::task_rng();
13  fTest03(iMax, &mut oRandom);    

53  fn fTest03(iMax : i64, oRandom : &mut std::rand::task_rng) {

This results in the following error :

    test_rand003.rs:53:38: 53:57 error: use of undeclared type name
        `std::rand::task_rng`
    test_rand003.rs:53 fn fTest03(iMax : i64, oRandom : &mut std::rand::task_rng) {

How can I pass the variable "oRandom" in line 13 above to a function?

Upvotes: 1

Views: 1442

Answers (2)

huon
huon

Reputation: 102126

There are two ways, either one can use the generic method, so it works with other RNGs, if you decide to stop using the task-local one:

fn fTest03<R: Rng>(iMax: i64, oRandom: &mut R) { ... }

Or, you can just use the return type of task_rng directly:

fn fTest03(iMax: i64, oRandom: @mut IsaacRng) { ... }

(You may have to import Rng/IsaacRng, or fully-qualify them e.g. std::rand::Rng.)

Either one should work with fTest03(10, std::rand::task_rng()).

Upvotes: 1

Ercan Erden
Ercan Erden

Reputation: 1926

task_rng() is not a type, it's a function that returns a task local random number generator (TaskRng).

The signature of task_rng function is:

pub fn task_rng() -> @mut TaskRng

So if you change line 53 to:

fn fTest03(iMax : i64, oRandom : &mut std::rand::Rng) {...

things should work nicely.

Upvotes: 0

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