Create empty HashMap in Rust without type definition

HashMap new insert

In the other example we defined a HashMap and declared the type of the keys and the values then we used the HashMap::new function to actually create and initialize the HashMap. While people coming from some languages might think this is normal, other might dislike the duplicate mention of HashMap. Indeed in many cases we don't need to declare the type of the keys and those of the values. Rust can figure out the type by the first insert to the HashMap.

In this example we initialized the HashMap using the HashMap::new() call, but did not tell it the types. Then when the the Rust compiler encountered the first insert statement it could deduct the type of the keys and the values.

If you use an editor that provides inline type-hints (e.g. VS Code with the Rust-analyzer plugin) then you'd also see that Rust deducted that the invisible type declaration is HashMap<String, i32>.

You might notice that it is not exactly the same as we had in the other example where we explicitly define it as HashMap<String, u32>. That's because the default integer type of Rust is u23.

This, of course, might cause problems if additional data comes from some external source and the values don't fit in i32 only in a u32 and that's why sometimes you need to manually declare the type.

The code


use std::collections::HashMap;

fn main() {
    let mut length_of = HashMap::new();

    println!("{}", length_of.len());
    println!("{:#?}", length_of);

    length_of.insert(String::from("snake"), 320);
    length_of.insert(String::from("snail"), 4);
    println!("{}", length_of.len());
    println!("{:#?}", length_of);


The output


    "snake": 320,
    "snail": 4,

Related Pages

Initialize immutable HashMap with data
HashMap (hash, dictionary, associative array) in Rust
Create empty HashMap and insert key-value pairs


Gabor Szabo (szabgab)

Gabor Szabo, the author of the Rust Maven web site maintains several Open source projects in Rust and while he still feels he has tons of new things to learn about Rust he already offers training courses in Rust and still teaches Python, Perl, git, GitHub, GitLab, CI, and testing.

Gabor Szabo