Lesson 11 of 20

5 mins read

Loops in Rust (loop, while, for)

In Plain English

Rust provides three loop constructs: loop (infinite loop that can return values via break expr), while (condition-based loop), and for (safe iteration over ranges and iterators).

The Mental Model

"loop is a conveyor belt that runs forever until you pull the emergency lever (break value). for is a delivery driver who visits every address on a list in order and stops when the list is done."

Deep Dive: How It Works

Returning Values from loop: break result; terminates loop and evaluates to result.

for in Range: for i in 0..5 iterates 0, 1, 2, 3, 4 (inclusive: 0..=5).

Iterator Safety: for element in &array eliminates index out-of-bounds panics completely.

Syntax Blueprint

let val = loop { if done { break 42; } }; | for i in 1..=3 { ... }

Use loop for retries that return values and for in ranges for clean counting.

Core Rules to Remember

loop Keyword: Infinite loop construct; break expr returns values directly.

while Loop: Runs as long as condition evaluates to true.

for..in Iteration: Idiomatic and safe iteration over ranges (0..5) and collections.

Common Beginner Traps & How to Fix Them

Mistake:Using 0..5 expecting it to include 5.

Why it happens: 0..5 is exclusive of the upper bound (yields 0 to 4).

How to fix: Use the inclusive range operator 0..=5 if 5 must be included.

Pro Tip: Label nested loops with single quotes ('outer: loop { break 'outer; }) to exit outer loops cleanly.

Live Interactive Example

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Returning Values from Loop and For Range

Rust 1.78 / rustc

Output Console

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How it works: Demonstrates break returning a value from loop and iterating over 1..=4.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Calculate factorial with a for loop

Calculate factorial of 5 (1 * 2 * 3 * 4 * 5 = 120).
Initialize let mut product = 1;
Use for i in 1..=5 { product *= i; }
Print "Factorial: 120".

Sandbox Output

Click "Run & Check" to test your solution.

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