Introduction

The count function in Rust is an iterator adapter that consumes the iterator and returns the number of elements it contains. This function is useful when you need to determine the size of an iterator's output.

Syntax


iterator.count()

The count function takes no arguments and returns a usize indicating the number of elements in the iterator.

Example Usage

Example 1: Counting Elements in a Vector

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];
    let count = numbers.iter().count();
    println!("{}", count); // Output: 5
}

Example 2: Counting Elements in a Range

fn main() {
    let range = 1..10;
    let count = range.count();
    println!("{}", count); // Output: 9
}

Example 3: Counting Elements in a Filtered Iterator

fn main() {
    let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
    let count = numbers.iter().filter(|&&x| x % 2 == 0).count();
    println!("{}", count); // Output: 5
}

Example 4: Counting Elements in a String Vector

fn main() {
    let words = vec!["apple", "banana", "cherry"];
    let count = words.iter().count();
    println!("{}", count); // Output: 3
}

Example 5: Counting Characters in a String

fn main() {
    let s = "hello";
    let count = s.chars().count();
    println!("{}", count); // Output: 5
}

Considerations

The count function consumes the iterator, meaning it cannot be used again after calling count.
Using count on an infinite iterator will result in an infinite loop.
For some collections (e.g. linked lists), counting elements may be an O(n) operation as it needs to iterate through each element.