Fill

Often in programs we want to set each value in an array to a specific number. A for-loop could iterate over each element and assign it.

But with fill, we can avoid the for-loop and keep out code cleaner and simpler. This may help reduce bugs. And fill() is easier to read again when we forget about the logic.

Fill example

To begin, we specify an array of arrays—this is like a 2D array. There are 200 arrays, and each nested array has 200 elements. We then fill each nested array.

Tip For a 1D array, we would not need a for-loop at all. But this program has 200 separate arrays.

use std::time::*;

fn main() {
    // 200 by 200 array.
    let mut data = [[0; 200]; 200];

    // Fill each array in the 2D array with a value.
    for i in 0..data.len() {
        data[i].fill(6);
    }

    // Test that the array was filled.
    println!("SAMPLE FOR FILL: {}", data[100][100]);
}
SAMPLE FOR FILL: 6

Benchmark

Does fill() perform faster compared to a for-loop? Sometimes functions that act upon many elements at once are more optimized.

Version 1 This version of the code calls the fill function repeatedly on the array. It fills it with the value 5.

Version 2 Here we use a for-loop to assign each element in the array to the value 5. The effect is the same as the fill function.

Result The fill() function is the same speed. The two functions were likely optimized to the same code.

use std::time::*;

fn main() {
    // Data to fill.
    let mut array = [0; 15];

    if let Ok(max) = "10000".parse::<usize>() {

        // Version 1: use fill.
        let t0 = Instant::now();
        let mut sum = 0;
        for _ in 0..max {
            for i in 0..max {
                array.fill(5);
            }
        }
        println!("{} ms", t0.elapsed().as_millis());

        // Version 2: use for-loop.
        let t1 = Instant::now();
        for _ in 0..max {
            for i in 0..max {
                for q in 0..array.len() {
                    array[q] = 5;
                }
            }
        }
        println!("{} ms", t1.elapsed().as_millis());
        println!("{}", array[0]);
    }
}783 ms    fill()
781 ms
5

Summary

The standard library in Rust provides many functions like fill() that can simplify programs. They reduce the size of the source text, which sometimes makes programs easier to maintain.