#Lecture 29 — break & continue

A loop so far runs every round until its condition turns false. break lets you leave a loop early, and continue lets you skip one round and carry on with the next.

Contents

Prerequisites · Connection · Objectives · Lesson · Code Examples · Common Mistakes · Best Practices · Mini Challenge · Practice Questions · Answers · Takeaways

Every error message in this lecture is real. Each one was produced by running the code shown on Dart 3.12.2, so you will recognize it when you meet it yourself.

#Prerequisites

Students should already understand, from Lectures 004–028:

• variables, string interpolation, and the operators ++, +=, %, ==, &&
• if and if-else (Lectures 022–023)
• for loops (Lecture 026)
• for-in loops over a list written in square brackets, such as [80, 45, 90] (Lecture 027)
• while and do-while (Lecture 028)

#Connection With Previous Lectures

Suppose Sara wants the first failing mark among 80, 92, 45, 70, 38. A for-in loop can find 45 — but then it keeps going and reports 38 too. And to ignore some values, the whole loop body has to sit inside an if. A loop needs a way to say "stop now" and "skip this one".

#Learning Objectives

By the end of this lecture you will be able to:

• Explain what break and continue do
• Use break to leave a loop as soon as the answer is found
• Use continue to skip the rest of one round
• Use both in for, for-in, and while loops
• Write a while (true) loop that ends with break
• Predict how break behaves inside nested loops
• Use a label such as outer: to leave or skip an outer loop
• Recognize the error messages Dart gives for misplaced break and continue

#1. Why Stop a Loop Early?

Many loops are searches: they check values one by one until they find the right one. After that, the remaining rounds are wasted work — and may even give wrong answers.

Analogy — you are looking for your keys in a row of drawers. When you find them in the second drawer, you stop (break). If a drawer is locked, you skip it and open the next one (continue).

#2. break Leaves the Loop

break ends the loop immediately, no matter what its condition says.

void main() {
  for (int i = 1; i <= 10; i++) {
    if (i == 4) {
      break;
    }
    print(i);
  }
}

1
2
3

The loop was written to count to 10, but when i reaches 4, break ends it. print(i) for 4 never runs.

#3. After break, the Program Continues Below the Loop

break ends only the loop, not the program. Execution jumps to the first line after the loop's closing brace.

void main() {
  for (int i = 1; i <= 10; i++) {
    if (i == 3) {
      break;
    }
    print('Round $i');
  }
  print('Loop finished');
}

Round 1
Round 2
Loop finished

#4. continue Skips One Round

continue stops the current round and jumps straight to the next one. The loop itself keeps going.

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

1
2
4
5

When i is 3, continue skips print(i). In a for loop, the update i++ still runs, so the next round has i equal to 4.

#5. break vs continue

#6. break and continue in a for-in Loop

They work the same way when the loop walks through a list. This solves Sara's problem from the start of the lecture:

void main() {
  for (int mark in [80, 92, 45, 70, 38]) {
    if (mark < 50) {
      print('First failing mark: $mark');
      break;
    }
  }
}

First failing mark: 45

38 is never checked — the loop stopped at 45.

#7. break and continue in a while Loop

In a while loop, break works exactly as before:

void main() {
  int balance = 100;
  while (balance > 0) {
    balance -= 30;
    if (balance < 50) {
      break;
    }
    print('Balance: $balance');
  }
  print('Stopped at $balance');
}

Balance: 70
Stopped at 40

continue needs care. A while loop has no separate update part, so continue jumps straight back to the condition. Change the counter before continue, or the loop never moves on:

void main() {
  int i = 0;
  while (i < 5) {
    i++; // update first
    if (i == 3) {
      continue;
    }
    print(i);
  }
}

1
2
4
5

#8. while (true) With break

Sometimes you don't know the stopping condition until you are inside the loop. Write while (true) — a loop that would run forever — and leave it with break when the moment comes.

void main() {
  int savings = 1000;
  int months = 0;
  while (true) {
    savings += 500;
    months++;
    if (savings >= 3000) {
      break;
    }
  }
  print('Reached $savings after $months months');
}

Reached 3000 after 4 months

Every while (true) loop must have a break that is sure to be reached. Without one, the program never ends.

#9. break in Nested Loops

When one loop sits inside another, break leaves only the innermost loop it is in. The outer loop carries on.

void main() {
  for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
      if (col == 2) {
        break;
      }
      print('row $row, col $col');
    }
  }
}

row 1, col 1
row 2, col 1
row 3, col 1

Each break ends the inner loop for that row. The outer loop still runs all three rows. continue behaves the same way — it skips to the next round of the innermost loop.

#10. Labels: Leaving the Outer Loop

To leave both loops at once, give the outer loop a label — a name followed by a colon — and write that name after break.

void main() {
  outer:
  for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
      if (row == 2 && col == 2) {
        break outer;
      }
      print('row $row, col $col');
    }
  }
  print('Done');
}

row 1, col 1
row 1, col 2
row 1, col 3
row 2, col 1
Done

break outer ends the loop labelled outer, which also ends the inner loop inside it.

#11. continue With a Label

continue outer abandons the rest of the inner loop and starts the next round of the outer loop.

void main() {
  outer:
  for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
      if (col == 2) {
        continue outer;
      }
      print('row $row, col $col');
    }
    print('end of row $row'); // never reached
  }
}

row 1, col 1
row 2, col 1
row 3, col 1

Unlike a plain break, continue outer also skips the rest of the outer body, so end of row never prints.

#12. A Real-World Example

An ATM gives Ali three tries to enter his PIN. A 0 means he pressed Enter without typing — that try doesn't count. As soon as the PIN is correct, the checking stops.

void main() {
  int correctPin = 4321;
  int attempts = 0;
  bool unlocked = false;

  for (int pin in [1111, 0, 4321, 2222]) {
    if (pin == 0) {
      continue; // empty entry, not counted
    }
    attempts++;
    if (pin == correctPin) {
      unlocked = true;
      break; // correct, stop checking
    }
    if (attempts == 3) {
      break; // out of tries
    }
  }

  print(unlocked ? 'Unlocked after $attempts tries' : 'Card blocked');
}

Unlocked after 2 tries

continue filters out the empty entry, break stops on success, and a second break enforces the limit. 2222 is never checked.

#Code Examples

#Example 1 — Stop at a Number

void main() {
  for (int i = 10; i >= 1; i--) {
    if (i == 7) {
      break;
    }
    print(i);
  }
}

10
9
8

#Example 2 — Skip a Number

void main() {
  for (int day = 1; day <= 7; day++) {
    if (day == 5) {
      continue;
    }
    print('Day $day');
  }
}

Day 1
Day 2
Day 3
Day 4
Day 6
Day 7

#Example 3 — Skipping Values in a for-in Loop

void main() {
  for (int n in [4, -2, 7, -5, 3]) {
    if (n < 0) {
      continue;
    }
    print(n);
  }
}

4
7
3

#Example 4 — First Multiple of 7

void main() {
  int n = 20;
  while (n <= 100) {
    if (n % 7 == 0) {
      print('First multiple of 7: $n');
      break;
    }
    n++;
  }
}

First multiple of 7: 21

#Example 5 — Doubling Until Over 100

void main() {
  int value = 3;
  while (true) {
    value *= 2;
    if (value > 100) {
      break;
    }
  }
  print(value);
}

192

#Example 6 — Finding a Pair With a Label

void main() {
  outer:
  for (int a = 1; a <= 5; a++) {
    for (int b = 1; b <= 5; b++) {
      if (a * b == 12) {
        print('Found: $a x $b = 12');
        break outer;
      }
    }
  }
}

Found: 3 x 4 = 12

Without the label, break would only end the inner loop, and 4 x 3 would be found as well.

#Example 7 — Student Marks

void main() {
  int total = 0;
  int counted = 0;
  for (int mark in [78, -1, 85, 90, -1, 66]) {
    if (mark == -1) {
      continue; // absent
    }
    total += mark;
    counted++;
  }
  print('Counted $counted marks, total $total');
}

Counted 4 marks, total 319

-1 means "absent", so continue keeps it out of both the total and the count.

#Example 8 — Shopping Total

void main() {
  int budget = 1000;
  int total = 0;
  for (int price in [250, 400, 300, 200]) {
    if (total + price > budget) {
      print('Cannot afford $price, stopping');
      break;
    }
    total += price;
  }
  print('Total: $total');
}

Cannot afford 200, stopping
Total: 950

250 + 400 + 300 is 950. Adding 200 would make 1150, so break stops the shopping.

#Example 9 — Bank Balance

void main() {
  int balance = 500;
  for (int amount in [120, 200, 300, 50]) {
    if (amount > balance) {
      print('Declined: $amount');
      continue;
    }
    balance -= amount;
    print('Paid $amount, left $balance');
  }
}

Paid 120, left 380
Paid 200, left 180
Declined: 300
Paid 50, left 130

continue, not break: one declined payment should not cancel the payments after it.

#Example 10 — Temperature Alert

void main() {
  int hour = 0;
  for (double temp in [28.5, 31.0, 999.0, 36.5, 38.0]) {
    hour++;
    if (temp == 999.0) {
      continue; // faulty sensor reading
    }
    if (temp > 35) {
      print('Heat alert at hour $hour: $temp');
      break;
    }
  }
}

Heat alert at hour 4: 36.5

999.0 is a sensor error, so continue ignores it. hour++ comes first, so the skipped reading still counts as an hour.

#Common Beginner Mistakes

1. Using break outside a loop

void main() {
  int marks = 45;
  if (marks < 50) {
    break;
  }
  print(marks);
}

Error: A break statement can't be used outside of a loop or switch statement.
Try removing the break statement.

break leaves a loop, not an if. The same rule applies to continue:

Error: A continue statement can't be used outside of a loop or switch statement.

2. Forgetting to update the counter before continue in a while loop

// ✗ never ends: when i is 3, continue skips i++
// int i = 0;
// while (i < 5) {
//   if (i == 3) {
//     continue;
//   }
//   print(i);
//   i++;
// }

This compiles, but prints 0 1 2 and then hangs forever. Move i++ above the continue.

3. Using a label that doesn't exist

void main() {
  for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
      if (col == 2) {
        break outer;
      }
    }
  }
}

Error: Can't break to 'outer'.
        break outer;
              ^^^^^

The label must be written in front of a loop that surrounds the break: outer: then for.

4. Writing code after break or continue

Lines after break in the same block can never run. dart analyze flags them as Dead code. Delete them or move them above the break.

#Best Practices

• Use break for searches: stop as soon as the answer is found.
• Use continue at the top of the loop body to filter out values you want to ignore.
• In a while loop, update the counter before any continue.
• Save while (true) for loops whose stopping point is only known inside the loop.
• Use labels only for nested loops, and keep the nesting shallow.

#Mini Challenge

Zayan is reading page numbers 1 to 20. He skips every page that is a multiple of 3, and he stops reading at the first page that is a multiple of 7. Print the pages he reads.

1
2
4
5

Try it yourself before opening the solution.

void main() {
  for (int page = 1; page <= 20; page++) {
    if (page % 3 == 0) {
      continue;
    }
    if (page % 7 == 0) {
      break;
    }
    print(page);
  }
}

1
2
4
5

#Practice Questions

Work through these in order — they run from beginner to application to challenge.

Beginner

1. Print the numbers 1 to 10, but stop the loop when the number is 6.
2. Print the numbers 1 to 10, skipping 4 and 8.
3. Loop over ['Ali', 'Sara', 'Zayan'] and stop when you reach 'Sara', printing only the names before her.

Application

4. Loop over the prices [120, 0, 75, 0, 300]. Skip the 0 prices and print the total of the rest as Total: ….
5. Using a while loop, start at n = 50 and find the first number divisible by both 4 and 6. Print Found: ….
6. Loop over the marks [66, 72, 49, 88]. Print All passed if no mark is below 50, or First fail: … otherwise. Stop at the first fail.
7. Using while (true), start a bank balance at 200 and add 75 each month. Stop when the balance is at least 500, then print the balance and the number of months.

Challenge

8. Using a while loop with a counter starting at 0, print the numbers 1 to 10 that are not multiples of 3. Use continue.
9. Using nested for loops (a and b from 1 to 9), find the first pair where a + b == 10 and a * b == 21. Print it and leave both loops with a label.
10. Print a table for row 1 to 4. Inside it, loop col from 1 to 4 and print row x col = product. If the product is greater than 6, move on to the next row with continue outer.

#Practice Answers

#Question 1 — Solution

void main() {
  for (int i = 1; i <= 10; i++) {
    if (i == 6) {
      break;
    }
    print(i);
  }
}

1
2
3
4
5

#Question 2 — Solution

void main() {
  for (int i = 1; i <= 10; i++) {
    if (i == 4 || i == 8) {
      continue;
    }
    print(i);
  }
}

1
2
3
5
6
7
9
10

#Question 3 — Solution

void main() {
  for (String name in ['Ali', 'Sara', 'Zayan']) {
    if (name == 'Sara') {
      break;
    }
    print(name);
  }
}

Ali

#Question 4 — Solution

void main() {
  int total = 0;
  for (int price in [120, 0, 75, 0, 300]) {
    if (price == 0) {
      continue;
    }
    total += price;
  }
  print('Total: $total');
}

Total: 495

#Question 5 — Solution

void main() {
  int n = 50;
  while (true) {
    if (n % 4 == 0 && n % 6 == 0) {
      break;
    }
    n++;
  }
  print('Found: $n');
}

Found: 60

#Question 6 — Solution

void main() {
  int? firstFail;
  for (int mark in [66, 72, 49, 88]) {
    if (mark < 50) {
      firstFail = mark;
      break;
    }
  }
  print(firstFail == null ? 'All passed' : 'First fail: $firstFail');
}

First fail: 49

firstFail stays null unless a failing mark is found.

#Question 7 — Solution

void main() {
  int balance = 200;
  int months = 0;
  while (true) {
    balance += 75;
    months++;
    if (balance >= 500) {
      break;
    }
  }
  print('Balance $balance after $months months');
}

Balance 500 after 4 months

#Question 8 — Solution

void main() {
  int i = 0;
  while (i < 10) {
    i++;
    if (i % 3 == 0) {
      continue;
    }
    print(i);
  }
}

1
2
4
5
7
8
10

#Question 9 — Solution

void main() {
  outer:
  for (int a = 1; a <= 9; a++) {
    for (int b = 1; b <= 9; b++) {
      if (a + b == 10 && a * b == 21) {
        print('a = $a, b = $b');
        break outer;
      }
    }
  }
}

a = 3, b = 7

#Question 10 — Solution

void main() {
  outer:
  for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= 4; col++) {
      int product = row * col;
      if (product > 6) {
        continue outer;
      }
      print('$row x $col = $product');
    }
  }
}

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
1 x 4 = 4
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
3 x 1 = 3
3 x 2 = 6
4 x 1 = 4

#Quick Revision

1. What is the difference between break and continue?
2. Where does the program continue after a break?
3. In a for loop, does the update part (i++) run after continue?
4. Why can continue cause an endless while loop?
5. What must every while (true) loop contain?
6. In nested loops, which loop does a plain break leave?
7. How do you write a label, and how do you use it with break?

#Key Takeaways

• break ends the loop immediately; the program carries on after the loop.
• continue skips the rest of the current round and moves to the next one.
• Both work in for, for-in, while, and do-while loops.
• In a for loop, the update still runs after continue; in a while loop, update the counter before continue.
• while (true) plus a reachable break handles loops whose stopping point is found inside.
• In nested loops, a plain break or continue affects only the innermost loop.
• A label (outer:) lets break outer leave, and continue outer skip to the next round of, an outer loop.
• break and continue outside a loop are compile errors.

#What's Next?

Lecture 030 — Introduction to Functions. With break and continue, Category 5 is complete: you can choose which code runs and repeat it exactly as long as you need. But our programs are now long lists of statements inside main(), and the same logic gets copied from place to place. Next we learn functions — how to give a piece of code a name and run it whenever we need it.