Position

Often in Rust we call iter() to get an iteration over a slice of elements. We may need to find an element that matches a condition.

And with position, and rposition, we can get the first matching element's index. This gives us the ability to use the index, not just the value.

Example

To begin we need a slice of some integers, so we declare 5 integers in a vector. Then we act upon this vector in the remainder of the program.

fn main() {
    let values = vec![10, 20, 30, 40, 50];

    // Step 1: compute position for a value.
    if let Some(twenty_position) = values.iter().position(|&x| x == 20) {
        println!("POSITION FOR 20: {twenty_position}");
    }

    // Step 2: this item does not exist.
    if values.iter().position(|&x| x == 100).is_none() {
        println!("POSITION FOR 100 IS NONE");
    }

    // Step 3: try a greater than or equal to expression.
    if let Some(high_value) = values.iter().position(|&x| x >= 40) {
        println!("POSITION FOR HIGH VALUE: {high_value}");
    }
}
POSITION FOR 20: 1
POSITION FOR 100 IS NONE
POSITION FOR HIGH VALUE: 3

Rposition

Sometimes we want to begin searching from the end of the slice to the beginning. This is a rightward search, and rposition helps here.

fn main() {
    let values = vec!["dog", "dog"];

    // Find index of rightmost dog.
    if let Some(rightmost) = values.iter().rposition(|&v| v == "dog") {
        println!("RIGHTMOST: {}", rightmost);
    }
}
RIGHTMOST: 1

Reference error

In Rust we must usually use the ampersand operator on the argument to position. This fixes an error where Rust cannot compare a reference with a value.

fn main() {
    let values = vec![0i32];
    if let Some(example) = values.iter().position(|x| x == 20) {
    }
}
error[E0277]: can't compare `&i32` with `{integer}`
 --> src/main.rs:3:57
 ...
  = help: the trait `PartialEq<{integer}>` is not implemented for `&i32`
  = help: the trait `PartialEq` is implemented for `i32`

For more information about this error, try `rustc --explain E0277`.

When we need to get the index of a matching element within a slice, the position (and rposition) functions are ideal. We must first call iter() before using position.