Loops | Hacking with Flutter

Print the numbers 1 to 100.

Without a loop, that is 100 print statements. With a loop, it is two lines.

Loops repeat code. They keep going until you tell them to stop.

Think of a chore: wash each dish in the pile, one at a time. Keep going until the pile is empty, or stop after washing five. A loop does the same thing with code.

for

The for loop is the classic way to repeat something a fixed number of times.

void main() {
  for (int i = 1; i <= 5; i++) {
    print(i);
  }
}

Run it. You will see 1, 2, 3, 4, 5 printed one per line.

The for loop has three parts, separated by semicolons:

Part	Example	What it does
Initialize	`int i = 1`	Sets the starting value
Condition	`i <= 5`	Keeps looping while `true`
Increment	`i++`	Runs after each iteration

Dart checks the condition before every run. When i reaches 6, the condition i <= 5 becomes false. The loop stops.

for-in

The for loop above requires you to count. But often you do not care about counting. You just want to do something with each item in a collection.

That is what for-in is for.

void main() {
  var names = ['Ali', 'Sara', 'John'];
  for (var name in names) {
    print(name); // prints each name in order
  }
}

No index. No counter. For each name in names, the body runs.

It works the same way with a Set:

void main() {
  var fruits = {'apple', 'banana', 'orange'};
  for (var fruit in fruits) {
    print(fruit);
  }
}

Iterating over a Map requires a method call we have not learned yet. We will cover that in Lesson 13.

while

A for loop is for when you know the count in advance. A while loop is for when you do not.

void main() {
  var count = 1;
  while (count <= 5) {
    print(count);
    count++;
  }
}

The condition is checked before the body runs. If it is true, the body runs. Then Dart checks again. It keeps going until the condition is false.

do-while

The do-while loop is like while, but the body always runs first. The condition is checked after.

void main() {
  var count = 1;
  do {
    print(count);
    count++;
  } while (count <= 5);
}

The body runs at least once. Even if the condition is false from the very start.

When does that matter? When you need to do something first, then decide whether to keep going.

break

You already saw break in switch. It works in loops too.

break exits the loop immediately. No matter what the condition says.

void main() {
  for (int i = 1; i <= 10; i++) {
    if (i == 4) {
      break; // stop the loop here
    }
    print(i);
  }
}

Output:

1
2
3

The loop never reaches 4. break exits it the moment the condition is met.

continue

continue skips the rest of the current iteration and jumps to the next one.

void main() {
  for (int i = 1; i <= 5; i++) {
    if (i == 3) {
      continue; // skip 3
    }
    print(i);
  }
}

Output:

3 is skipped. The loop keeps going.

Infinite loops

A loop whose condition never becomes false runs forever.

Infinite loop	Noun. A loop that never stops because its condition is always `true`.

void main() {
  while (true) { // always true, never stops
    print('stuck');
  }
}

Do not run that. It will freeze your program.

This is a common beginner mistake. If your program hangs, check your loop condition. It is probably always true.

Summary

Concept	What it does
`for`	Repeats a fixed number of times using initialize, condition, and increment
`for-in`	Repeats once for each item in a collection
`while`	Repeats as long as a condition is `true`, checked before each run
`do-while`	Like `while`, but runs the body at least once before checking
`break`	Exits the loop immediately
`continue`	Skips the current iteration and moves to the next one

for