Introduction

The flat_map function in Rust is a powerful iterator adaptor that applies a function to each item of an iterator and flattens the result. This function is useful when you have an iterator of iterators and you want to process the inner elements as a single sequence.

Syntax

The syntax for using flat_map is as follows:

iterator.flat_map(|item| some_function(item))

Example Usage

Example 1: Flattening a Vector of Vectors

fn main() {
    let v = vec![vec![1, 2, 3], vec![4, 5, 6], vec![7, 8, 9]];
    let flattened: Vec<i32> = v.into_iter().flat_map(|x| x.into_iter()).collect();
    println!("{:?}", flattened); // Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]
}

Example 2: Mapping and Flattening Option Values

fn main() {
    let v = vec![Some(1), None, Some(2), Some(3), None];
    let flattened: Vec<i32> = v.into_iter().flat_map(|x| x).collect();
    println!("{:?}", flattened); // Output: [1, 2, 3]
}

Example 3: Using flat_map with Strings

fn main() {
    let words = vec!["hello", "world"];
    let chars: Vec<char> = words.into_iter().flat_map(|word| word.chars()).collect();
    println!("{:?}", chars); // Output: ['h', 'e', 'l', 'l', 'o', 'w', 'o', 'r', 'l', 'd']
}

Example 4: Combining flat_map with filter

fn main() {
    let v = vec![vec![1, 2, 3], vec![4, 5, 6], vec![7, 8, 9]];
    let even_numbers: Vec<i32> = v.into_iter().flat_map(|x| x.into_iter()).filter(|&x| x % 2 == 0).collect();
    println!("{:?}", even_numbers); // Output: [2, 4, 6, 8]
}

Example 5: Processing Nested Iterators

fn main() {
    let nested_iter = vec![0..3, 4..7, 8..10];
    let flattened: Vec<i32> = nested_iter.into_iter().flat_map(|it| it).collect();
    println!("{:?}", flattened); // Output: [0, 1, 2, 4, 5, 6, 8, 9]
}

See Also

• map - Applies a function to each element of an iterator.
• filter - Creates an iterator which uses a predicate to determine which elements to return.
• collect - Transforms an iterator into a collection.