For, While, Do-While, and For-In Loops in Dart

Loops are an essential part of Dart programming. They allow developers to execute a block of code repeatedly without writing the same code multiple times. Loops are commonly used for counting, processing collections, searching data, calculating values, and handling repeated application logic.

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1. What Are Loops?

A loop is a programming structure that repeatedly executes a block of code while a specified condition is satisfied or while elements remain in a collection.

For example, without a loop:

print("Hello");
print("Hello");
print("Hello");
print("Hello");
print("Hello");

Using a loop, the same task becomes:

for (int i = 1; i <= 5; i++) {
  print("Hello");
}

The loop executes the print() statement five times.

2. Why Are Loops Important?

Loops help reduce repetitive code and make programs easier to maintain.

• Printing a sequence of numbers
• Processing List and Set elements
• Searching for an item
• Calculating totals and averages
• Processing user records
• Displaying product information
• Generating multiplication tables
• Performing repeated calculations
• Processing data received from APIs
• Working with application collections in Flutter

3. Types of Loops in Dart

The four important loops covered in this topic are:

1. for loop
2. while loop
3. do-while loop
4. for-in loop

4. for Loop

The for loop is useful when you know the number of iterations or when you need a counter to control the loop.

Syntax

for (initialization; condition; update) {
  // code to execute
}

A typical example is:

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

Output:

1
2
3
4
5

5. Parts of a for Loop

Consider the following code:

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

6. How a for Loop Works

The execution takes place in this order:

1. Initialization executes once.
2. The condition is checked.
3. If the condition is true, the loop body executes.
4. The update expression executes.
5. The condition is checked again.
6. The process continues until the condition becomes false.

7. Printing Numbers Using for

for (int i = 1; i <= 10; i++) {
  print(i);
}

8. Printing Even Numbers

for (int i = 2; i <= 20; i += 2) {
  print(i);
}

Output:

2
4
6
8
10
12
14
16
18
20

9. Printing Odd Numbers

for (int i = 1; i <= 20; i += 2) {
  print(i);
}

10. Reverse for Loop

A for loop can also be used to count backwards.

for (int i = 10; i >= 1; i--) {
  print(i);
}

Output:

10
9
8
7
6
5
4
3
2
1

11. Calculating a Sum with for

int sum = 0;

for (int i = 1; i <= 10; i++) {
  sum += i;
}

print("Sum = $sum");

Output:

Sum = 55

12. Multiplication Table Using for

int number = 5;

for (int i = 1; i <= 10; i++) {
  print("$number x $i = ${number * i}");
}

13. while Loop

The while loop executes a block of code repeatedly as long as its condition evaluates to true.

Syntax

while (condition) {
  // code
}

Example

int i = 1;

while (i <= 5) {
  print(i);
  i++;
}

Output:

1
2
3
4
5

14. How a while Loop Works

1. Initialize a variable.
2. Check the condition.
3. If the condition is true, execute the loop body.
4. Update the variable.
5. Check the condition again.
6. Continue until the condition becomes false.

15. while Loop Example with a Countdown

int count = 5;

while (count > 0) {
  print(count);
  count--;
}

print("Done");

16. Important Point About while Loops

A while loop may execute zero times if its condition is false before the first iteration.

int number = 10;

while (number < 5) {
  print(number);
}

In this example, nothing is printed because number < 5 is false before the loop begins.

17. Avoiding Infinite while Loops

Always make sure that the values involved in the condition can eventually change so that the condition becomes false.

Incorrect:

int i = 1;

while (i <= 5) {
  print(i);
}

Here, i never changes, so the condition remains true.

Correct:

int i = 1;

while (i <= 5) {
  print(i);
  i++;
}

18. do-while Loop

The do-while loop is similar to the while loop, but it checks the condition after executing the loop body.

Syntax

do {
  // code
} while (condition);

Example

int i = 1;

do {
  print(i);
  i++;
} while (i <= 5);

19. Important Feature of do-while

A do-while loop executes its body at least once, even if the condition is initially false.

int number = 10;

do {
  print("Number: $number");
} while (number < 5);

Output:

Number: 10

The condition is false, but the body executes once before the condition is evaluated.

20. Practical do-while Example

int option = 1;

do {
  print("Showing menu...");
  option++;
} while (option <= 3);

21. for-in Loop

The for-in loop is designed for iterating through the elements of an iterable collection such as a List or Set.

Syntax

for (variable in collection) {
  // code
}

Example

List fruits = [
  "Apple",
  "Banana",
  "Mango"
];

for (String fruit in fruits) {
  print(fruit);
}

Output:

Apple
Banana
Mango

22. for-in with a List of Numbers

List numbers = [10, 20, 30, 40, 50];

for (int number in numbers) {
  print(number);
}

23. for-in with a Set

Set cities = {
  "Mumbai",
  "Delhi",
  "Pune",
  "Bangalore"
};

for (String city in cities) {
  print(city);
}

24. for-in with a Map

Maps contain key-value pairs. You can iterate through their entries using the entries property.

Map marks = {
  "Rahul": 85,
  "Amit": 90,
  "Priya": 92
};

for (var entry in marks.entries) {
  print("${entry.key}: ${entry.value}");
}

25. for Loop vs for-in Loop

26. Accessing List Index with a for Loop

When both the index and value are required, a traditional for loop is useful.

List fruits = [
  "Apple",
  "Banana",
  "Mango"
];

for (int i = 0; i < fruits.length; i++) {
  print("Index $i: ${fruits[i]}");
}

Output:

Index 0: Apple
Index 1: Banana
Index 2: Mango

27. break Statement

The break statement immediately terminates the loop.

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

  print(i);
}

Output:

1
2
3
4
5

28. continue Statement

The continue statement skips the current iteration and moves to the next iteration.

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

  print(i);
}

Output:

1
2
4
5

29. break vs continue

30. Nested Loops

A loop inside another loop is called a nested loop.

for (int i = 1; i <= 3; i++) {
  for (int j = 1; j <= 3; j++) {
    print("i = $i, j = $j");
  }
}

Nested loops are useful for grids, tables, matrices, patterns, and multi-dimensional data.

31. Nested Loop Example

for (int row = 1; row <= 3; row++) {
  for (int column = 1; column <= 3; column++) {
    print("Row: $row, Column: $column");
  }
}

32. Calculating the Sum of List Values

List numbers = [10, 20, 30, 40, 50];

int total = 0;

for (int number in numbers) {
  total += number;
}

print("Total = $total");

Output:

Total = 150

33. Finding the Largest Number

List numbers = [10, 45, 23, 89, 12];

int largest = numbers[0];

for (int number in numbers) {
  if (number > largest) {
    largest = number;
  }
}

print("Largest number = $largest");

34. Finding the Smallest Number

List numbers = [10, 45, 23, 89, 12];

int smallest = numbers[0];

for (int number in numbers) {
  if (number < smallest) {
    smallest = number;
  }
}

print("Smallest number = $smallest");

35. Searching for an Item

List products = [
  "Laptop",
  "Mobile",
  "Tablet",
  "Monitor"
];

String searchItem = "Tablet";
bool found = false;

for (String product in products) {
  if (product == searchItem) {
    found = true;
    break;
  }
}

if (found) {
  print("Product found");
} else {
  print("Product not found");
}

36. Counting Even Numbers

List numbers = [
  10,
  15,
  20,
  25,
  30,
  35
];

int count = 0;

for (int number in numbers) {
  if (number % 2 == 0) {
    count++;
  }
}

print("Even numbers: $count");

37. Processing Student Marks

List marks = [85, 72, 90, 64, 45];

for (int mark in marks) {
  if (mark >= 80) {
    print("$mark - Excellent");
  } else if (mark >= 60) {
    print("$mark - Good");
  } else if (mark >= 40) {
    print("$mark - Pass");
  } else {
    print("$mark - Fail");
  }
}

38. E-Commerce Example

List prices = [
  999.0,
  1499.0,
  2499.0,
  4999.0
];

double total = 0;

for (double price in prices) {
  total += price;
}

if (total >= 5000) {
  print("Free delivery");
} else {
  print("Delivery charges apply");
}

39. Loops with Flutter Applications

Loops are useful when Flutter applications need to process collections of products, users, messages, categories, or other application data. JustAcademy's Flutter course covers Dart and Flutter fundamentals along with UI development, API integration, Firebase, databases, state management, testing, deployment, and practical projects. :contentReference[oaicite:1]{index=1}

Example: Product List

List products = [
  "Laptop",
  "Smartphone",
  "Tablet",
  "Smart Watch"
];

for (String product in products) {
  print("Product: $product");
}

40. User Data Example

List> users = [
  {
    "name": "Rahul",
    "age": 25
  },
  {
    "name": "Priya",
    "age": 28
  },
  {
    "name": "Amit",
    "age": 22
  }
];

for (var user in users) {
  print("${user["name"]} - ${user["age"]}");
}

41. Comparison of Loop Types

42. for vs while

43. while vs do-while

44. Common Mistakes in Loops

Mistake 1: Creating an Infinite Loop

int i = 1;

while (i <= 5) {
  print(i);
}

The value of i is never updated, so the loop condition never becomes false.

Mistake 2: Incorrect Boundary

List names = ["A", "B", "C"];

for (int i = 0; i <= names.length; i++) {
  print(names[i]);
}

The correct condition is normally i < names.length because List indexes range from 0 to length - 1.

for (int i = 0; i < names.length; i++) {
  print(names[i]);
}

Mistake 3: Forgetting to Update the Counter

int count = 1;

while (count <= 10) {
  print(count);
  // count++ is missing
}

Mistake 4: Incorrect Loop Direction

for (int i = 10; i <= 1; i--) {
  print(i);
}

This condition is false initially. For a countdown, use i >= 1.

45. Best Practices for Loops

• Choose the loop that matches the problem.
• Use for when you need a counter or index.
• Use while when repetition depends mainly on a condition.
• Use do-while when the code must execute at least once.
• Use for-in for simple collection traversal.
• Make sure loop conditions can eventually become false.
• Be careful with List indexes.
• Use break when further iteration is unnecessary.
• Use continue when the current iteration should be skipped.
• Avoid unnecessarily complicated nested loops.
• Use meaningful loop variable names where appropriate.

46. Complete Example

void main() {
  // for loop
  print("For Loop:");

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

  // while loop
  print("While Loop:");

  int count = 1;

  while (count <= 5) {
    print(count);
    count++;
  }

  // do-while loop
  print("Do-While Loop:");

  int number = 1;

  do {
    print(number);
    number++;
  } while (number <= 5);

  // for-in loop
  print("For-In Loop:");

  List fruits = [
    "Apple",
    "Banana",
    "Mango"
  ];

  for (String fruit in fruits) {
    print(fruit);
  }
}

47. Quick Revision

48. Practice Exercises

1. Print numbers from 1 to 100 using a for loop.
2. Print all even numbers from 1 to 50.
3. Print all odd numbers from 1 to 50.
4. Print numbers from 20 to 1 in reverse order.
5. Calculate the sum of numbers from 1 to 100.
6. Create a multiplication table using a for loop.
7. Print numbers from 1 to 10 using a while loop.
8. Create a do-while program that displays a menu at least once.
9. Use a for-in loop to print all values in a List.
10. Find the largest value in a List.
11. Find the smallest value in a List.
12. Count the number of even values in a List.
13. Search for a product in a List and stop using break when it is found.
14. Use continue to skip all odd numbers.
15. Create a nested loop to print a 5 × 5 pattern.

49. Key Takeaways

• Loops reduce repetitive code.
• The for loop is useful for controlled and counter-based repetition.
• The while loop checks its condition before every iteration.
• The do-while loop checks its condition after executing the body.
• The for-in loop is convenient for iterating through collection values.
• break stops a loop completely.
• continue skips the current iteration.
• Loops are frequently used when processing collections and application data in Dart and Flutter.
• Choosing the correct loop improves readability and maintainability of Dart code.

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