How do you make a piece of code run repeatedly without writing it again and again? Whether you’re printing a sequence of numbers, processing a list, or updating a game frame, you need a way to repeat instructions efficiently. This is when you might want to know about the Dart for loop.
A for loop in Dart allows you to execute a block of code multiple times based on a defined condition. It’s one of the most controlled looping structures in programming, letting you specify where the loop starts, when it should stop, and how it progresses after each iteration.
Syntax and Basic Usage
The for loop in Dart follows a clear and predictable structure. It consists of three parts: initialization, condition, and update, all of which are placed inside parentheses before the code block.
for (initialization; condition; update) {
// Code block to execute
}
Initialization: runs once before the loop begins, usually to set a counter variable.
Condition: checked before each iteration; if it’s true, the loop body executes.
Update: runs after every iteration, commonly used to increment or decrement the counter.
Here’s a simple example that prints numbers from 1 to 5:
void main() {
for (int i = 1; i <= 5; i++) {
print(i);
}
}This loop starts at 1, prints each number, and increases i by 1 on every iteration until the condition i <= 5 becomes false. Once the condition fails, the loop stops automatically.
Iterating Collections
In many Dart programs, you’ll often work with collections such as lists, sets, or maps. Instead of manually managing an index, Dart provides cleaner ways to loop through these elements using the for-in loop and the forEach() method.
The for-in loop lets you access each element in a collection directly:
void main() {
var fruits = ['apple', 'banana', 'cherry'];
for (var fruit in fruits) {
print(fruit);
}
}This approach eliminates index handling, making the code easier to read when you only need the values, not their positions.
You can also use the forEach() method, which applies a function to every element in the collection:
void main() {
var numbers = [1, 2, 3, 4, 5];
numbers.forEach((num) {
print('Square: ${num * num}');
});
}Or even shorter, using a function tear-off in modern Dart:
void main() {
var numbers = [1, 2, 3, 4, 5];
numbers.forEach(print); // Passes each element to print()
}Both techniques are common in Dart and preferred for readability when you don’t need fine-grained control over the iteration process.
Common Mistakes and Edge Cases
Even though for loops in Dart are straightforward, a few common issues can lead to incorrect behavior or inefficient code.
1. Infinite loops: If the loop condition never becomes false, it keeps running indefinitely. This usually happens when the update step is missing or incorrectly written.
for (int i = 1; i >= 1; i++) {
print(i);
}2. Off-by-one errors: Using <= instead of < can lead to one extra iteration, especially when working with list indexes.
for (int i = 0; i <= list.length; i++) { // Out of range
print(list[i]);
}3. Scope and variable reuse: Declaring loop variables outside the loop can lead to unexpected changes later. Always declare them inside the loop for isolation.
void main() {
int i;
for (i = 0; i < 3; i++) {
print(i);
}
print('After loop: $i'); // i is still accessible here
}4. Mutating collections during iteration: Avoid adding or removing items from a list while looping through it, as this can cause ConcurrentModificationError at runtime.
var items = [1, 2, 3];
for (var item in items) {
if (item == 2) items.remove(item); // Runtime error
}Wrapping Up
The for loop remains one of the most dependable tools for structured iteration in Dart. It gives you complete control over how many times a block of code runs and is ideal when the number of iterations is known in advance. Whether you’re counting values, iterating through lists, or managing simple automation tasks, the for loop keeps your logic predictable and efficient.
As a best practice, keep your loop variables scoped inside the loop, use clear conditions, and avoid unnecessary complexity in the loop header. When you’re iterating over collections, prefer the for-in loop or forEach() for cleaner and more readable code.