Iterating over Arrays

Goal of This Episode

Use a for loop to walk through every element in an array.

Main Text

Last episode we learned to fetch values one at a time with arr[0], arr[1]. But if an array has 100 elements, you can’t write 100 lines, right? That’s when we use a for loop to iterate over the whole array.

Basic Syntax

fn main() {
    let arr = [1, 2, 3, 4, 5];

    for x in arr {
        println!("{}", x);
    }
}

for x in arr means: “Take the elements of arr out one by one, putting each into x, then run the code in the curly braces.”

Computing with the Elements

fn main() {
    let scores = [80, 95, 72, 88, 100];

    for score in scores {
        if score >= 90 {
            println!("{} points → Excellent!", score);
        } else {
            println!("{} points → Keep it up!", score);
        }
    }
}

Summing All the Elements

fn main() {
    let arr = [1, 2, 3, 4, 5];
    let mut total = 0;

    for x in arr {
        total += x;
    }

    println!("Total: {}", total);
}

First create a mutable accumulator with let mut total = 0;, then add each value onto it in the loop.

for in a Range vs for in an Array

The for i in 0..5 from Chapter 1 iterates over a range of numbers. This episode’s for x in arr iterates over an array. Same syntax — only the thing after in differs:

fn main() {
    let arr = [10, 20, 30];

    // Iterating over a range: i is 0, 1, 2 in turn
    for i in 0..3 {
        println!("Index {}: {}", i, arr[i]);
    }

    // Iterating over the array: x is 10, 20, 30 in turn
    for x in arr {
        println!("Value: {}", x);
    }
}

When walking an array, for x in arr is cleaner, safer, and possibly faster than using indices — no worrying about going out of bounds. For when you need both the index and the value, we’ll learn a better way later.

Recap

• for x in arr { ... } iterates over each element of the array.
• Inside the loop you can compute, test, and accumulate with each element.
• for x in arr (over an array) and for i in 0..n (over a range) share the same syntax; only the thing after in differs.
• When iterating over an array, for x in arr beats indexing: cleaner, safer, possibly faster.