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Rust collect function

last modified February 19, 2025

In this article we show how to use the collect function in Rust.

The collect function

The collect function is used to transform an iterator into a collection.

Simple example

The following is a simple Rust example with the collect function.

main.rs 
fn main() {
    let v = vec![1, 2, 3];
    let sum: i32 = v.into_iter().sum();

    println!("The sum is: {sum}");
}

In the example, we have a vector of integers. We use the into_iter method to get an iterator over the vector. We use the sum method to calculate the sum of the integers. 

$ cargo run -q
The sum is: 6

Transforming an iterator into a hash map

We can transform an iterator into a hash map with the collect function.

main.rs 
fn main() {
    let v = vec![("John Doe", 34), ("Roger Roe", 54)];
    let mut h = std::collections::HashMap::new();
    h.extend(v.into_iter());

    println!("{h:#?}");
}

In the example, we have a vector of tuples. We use the into_iter method to get an iterator over the vector. We use the extend method to populate a hash map. 

$ cargo run -q
HashMap {
    "John Doe": 34,
    "Roger Roe": 54,
}

Transforming an iterator into a vector

We can transform an iterator into a vector with the collect function.

main.rs 
fn main() {
    let v = vec![1, 2, 3];
    let vv: Vec = v.into_iter().filter(|x| x % 2 == 0).collect();

    println!("{vv:?}");
}

In the example, we have a vector of integers. We use the into_iter method to get an iterator over the vector. We use the filter method to filter out the odd numbers. We use the collect method to populate a new vector. 

$ cargo run -q
[2, 3]

The fold method

The fold method is a generalization of the reduce method.

main.rs 
fn main() {
    let v = vec![1, 2, 3];
    let sum: i32 = v.into_iter().fold(0, |acc, x| acc + x);

    println!("The sum is: {sum}");
}

In the example, we use the fold method to calculate the sum of the integers. 

$ cargo run -q
The sum is: 6

The author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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