Rust — For Loops

Rust's for loop iterates — it doesn't count

let fruits = ["apple", "banana", "mango"];

for fruit in fruits {
    println!("{}", fruit);
}

No index bookkeeping, no bounds checks at runtime — the compiler guarantees you can't fall off the end. This is why idiomatic Rust prefers for over while whenever a collection exists.

Ranges replace counting loops

for i in 0..5 {          // 0 1 2 3 4     (end EXCLUSIVE)
    print!("{} ", i);
}

for i in 0..=5 {         // 0 1 2 3 4 5   (..= = inclusive)
    print!("{} ", i);
}

for i in (1..10).step_by(2) {   // 1 3 5 7 9
}

for i in (1..4).rev() {  // 3 2 1  countdown
}

Syntax	Produces
0..5	0,1,2,3,4
0..=5	0..5 including 5
(1..10).step_by(2)	odd numbers
(0..n).rev()	n-1 → 0

Looping collections

let scores = vec![90, 85, 77];

for s in &scores {              // borrow: read each item
    println!("{}", s);
}

for name in ["Ada", "Bo"] { … } // arrays too

let mut total = 0;
for s in &scores { total += s; }

The & matters — for s in scores would move the vector, making it unusable afterwards. Borrow with & to keep ownership (ownership lesson explains deeply).

enumerate — index + value together

for (i, fruit) in fruits.iter().enumerate() {
    println!("{}. {}", i + 1, fruit);
}
// 1. apple · 2. banana · 3. mango

Never write for i in 0..list.len() just to index — enumerate is safer and expresses intent.

Nested loops

for row in 1..=3 {
    for col in 1..=3 {
        print!("{} ", row * col);
    }
    println!();
}

break/continue work here too

for n in 1..100 {
    if n % 13 == 0 { break; }
    if n % 2 == 0 { continue; }
    print!("{} ", n);
}

Labels ('outer:) apply identically to nested for loops.

The index trap this design eliminates

Other languages:

for (let i = 0; i <= list.length; i++) …   // 💥 off-by-one crash

Rust's iterator-based for makes that bug class unrepresentable — there is no manual bound to get wrong.

Rule: iterate collections with for x in &coll; count with ranges; reserve while for condition-driven repetition.

Mini Practice

1. FizzBuzz over 1..=30.
2. Sum even numbers of a vector using for … in &.
3. Print a 5×5 multiplication grid (nested).
4. enumerate() a todo list starting at 1.
5. Countdown from 10 with .rev(); skip multiples of 3 via continue.

Next: functions →