LearnLoops: standard for, for-in, while & do-while

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Lesson 08 of 59

5 mins read

Loops: standard for, for-in, while & do-while

In Plain English

Dart offers versatile looping constructs for repetitive iteration. Standard `for (var i = 0; i < n; i++)` handles index-based counters; `for (final item in collection)` iterates cleanly over any `Iterable`; `while` executes while a condition holds true; and `do-while` guarantees at least one execution before testing the predicate.

The Mental Model

"A `for-in` loop is like inspecting each item in a grocery bag one by one until the bag is completely empty."

Deep Dive: How It Works

`for-in` Iteration: The most idiomatic loop in Dart for collections (Lists, Sets, Maps keys/values).

`while` vs `do-while`: `while` checks upfront; `do-while` executes the body first before checking.

Closure Capture in for loops: Dart creates a new variable binding per iteration, making async closures inside `for` loops safe and predictable.

Syntax Blueprint

for (var i = 0; i < 5; i++) { ... }
for (final item in items) { ... }
while (hasMore) { ... }
do { ... } while (condition);

Core iteration patterns across indexed ranges and iterable collections.

Core Rules to Remember

Idiomatic for-in: Cleanest way to iterate through List, Set, or Map entries.

Safe Loop Closures: Each loop iteration creates a fresh binding for the index variable.

do-while Guarantee: Guarantees the loop body runs at least once.

Common Beginner Traps & How to Fix Them

Mistake:Modifying a List (adding/removing items) while iterating over it with `for-in`.

Why it happens: Mutating an active iterator triggers a `ConcurrentModificationError`.

How to fix: Iterate over a copy (`for (final item in [...list])`) or use `list.removeWhere()`.

Pro Tip: Use `forEach()` for simple single-expression callbacks, but prefer `for-in` when using `await` or `break`.

Live Interactive Example

Hit Run Code to see it live

Collection and Counter Iteration

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: Iterated through the languages list with for-in and calculated accumulator sum with standard for.

Your Turn: Micro Challenge

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

Micro Exercise

Calculate Factorial with While Loop

Compute the factorial of `int n = 5;` using a while loop.
Print `Factorial: $fact`.

Sandbox Output

Click "Run & Check" to test your solution.

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