Same deterministic code on same platform produces different outputs on different runs

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1

Hi.
Please try the following code on the playground and hit "run" successively.
In stdout you'll get rstu some times, and xyzu some other times.
I expected it to return rstu every time.
This is concerning. Any advice and explanations are appreciated. Thank you.

use std::collections::HashMap;
fn main() {
    let mut hm:HashMap<&str,Vec<&str>> = HashMap::new();
    hm.insert("a",vec!["r","s","t"]);
    hm.insert("b",vec!["x","y","z"]);
    let mut hm_iter = hm.into_iter();
    let entry1:(&str,Vec<&str>) = hm_iter.next().unwrap(); 
    let entry2:(&str,Vec<&str>) = hm_iter.next().unwrap(); 
    let mut vec_entry1 = entry1.1;
    vec_entry1.push("u");
    for e in vec_entry1 {println!("{e}");}
}

(Playground)

2

The code is not deterministic. Every time you create a HashMap, it gets a new randomly-chosen hash function and therefore a random ordering of elements.

6 Likes
3

HashMap is randomly seeded.

Perhaps you want a hash map with a stable algorithm, or that can be explicitly seeded. rustc uses one (but I don't know how suitable it is for general use).

2 Likes
4

I think you can use the std hashmap and override the hasher.

1 Like
5

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