for, while & do-while Loops

Loops let you repeat a block of code multiple times without rewriting it. Whether you're printing a table, summing numbers, or processing a list, loops are the backbone of repetitive computation. Dart gives you three fundamental loop types: for, while, and do-while.

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🔹 The for Loop

The classic for loop is perfect when you know exactly how many times you want to repeat.

Syntax:

dart
for (initialization; condition; increment) {
  // body
}

dart
void main() {
  for (int i = 1; i <= 5; i++) {
    print('Count: $i');
  }
}

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

How It Works — Step by Step

Step	What Happens
1. Initialization	int i = 1 — runs once before the loop starts
2. Condition check	i <= 5 — checked before each iteration
3. Body	Code inside {} runs if condition is true
4. Increment	i++ — runs after each iteration
5. Repeat from 2	Until condition is false

Counting Down

dart
void main() {
  for (int i = 10; i >= 1; i--) {
    print(i);
  }
  print('Blast off!');
}

Stepping by More Than One

dart
void main() {
  // Even numbers from 0 to 20
  for (int i = 0; i <= 20; i += 2) {
    print(i);
  }
}

Loop Variable Scope

The variable declared in the for initialization (int i) is scoped to the loop and cannot be used after it:

dart
void main() {
  for (int i = 0; i < 3; i++) {
    print(i); // i is accessible here
  }
  // print(i); // ERROR! i is not defined here
}

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🔹 Nested for Loops

A loop inside another loop — the inner loop completes fully for each iteration of the outer loop:

dart
void main() {
  for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
      print('($i, $j)');
    }
  }
}

Output:

(1, 1)
(1, 2)
(1, 3)
(2, 1)
(2, 2)
(2, 3)
(3, 1)
(3, 2)
(3, 3)

Multiplication Table (Nested Loop Example)

dart
void main() {
  int n = 5; // table size

  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      // padLeft(4) for alignment
      String cell = (i * j).toString().padLeft(4);
      stdout.write(cell);
    }
    print(''); // newline after each row
  }
}

Output:

   1   2   3   4   5
   2   4   6   8  10
   3   6   9  12  15
   4   8  12  16  20
   5  10  15  20  25

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🔹 The while Loop

The while loop repeats as long as a condition is true. Use it when you don't know in advance how many times you'll need to loop.

Syntax:

dart
while (condition) {
  // body
}

The condition is checked before each iteration. If it's false from the start, the body never runs.

dart
void main() {
  int count = 1;

  while (count <= 5) {
    print('Count: $count');
    count++;
  }
}

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

When to Use while

while is most natural when the stopping condition depends on something that changes unpredictably:

dart
void main() {
  int number = 1;

  // Double until we exceed 100
  while (number <= 100) {
    number *= 2;
  }

  print(number);  // 128 (first power of 2 that exceeds 100)
}

The Infinite Loop Risk

If the condition never becomes false, the loop runs forever — this is a bug! Always ensure the loop body moves toward the exit condition:

dart
void main() {
  int i = 1;

  while (i <= 5) {
    print(i);
    i++; // Without this line, the loop runs forever!
  }
}

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🔹 The do-while Loop

The do-while loop is like while, but the condition is checked after the body runs. This guarantees the body runs at least once.

Syntax:

dart
do {
  // body (always runs at least once)
} while (condition);

dart
void main() {
  int count = 1;

  do {
    print('Count: $count');
    count++;
  } while (count <= 5);
}

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

When do-while Is Different from while

dart
void main() {
  int x = 10;

  // while — body does NOT run (10 is not <= 5)
  while (x <= 5) {
    print('while: $x');
  }

  // do-while — body runs ONCE even though 10 is not <= 5
  do {
    print('do-while: $x');
  } while (x <= 5);
}

Output:

do-while: 10

Perfect Use Case: Input Validation (conceptual)

dart
// Conceptual example — in a real app, input comes from a form or stdin
void main() {
  int attempts = 0;
  String password = '';

  // Always ask at least once
  do {
    attempts++;
    // Simulating: in a real app, you'd read from user input here
    password = attempts == 3 ? 'correct' : 'wrong'; // Simulate 3rd attempt succeeds
    print('Attempt $attempts: "$password"');
  } while (password != 'correct');

  print('Access granted after $attempts attempt(s).');
}

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🔹 Performance Considerations

• All three loops have O(n) complexity for a single loop — performance is comparable for simple use cases.
• Nested loops are O(n²) or worse — be careful with large datasets.
• For iterating over collections, prefer for-in (covered in the next lesson) which is cleaner and often slightly more efficient.
• Avoid doing heavy computation inside the condition of a while loop — compute it before if possible.

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🔹 Real-World Examples

Example 1: Sum of a List

dart
void main() {
  List<int> numbers = [10, 25, 3, 47, 8, 16];
  int sum = 0;

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

  print('Sum: $sum');  // Sum: 109
}

Example 2: Fibonacci Sequence

dart
void main() {
  int n = 10; // Print first 10 Fibonacci numbers
  int a = 0, b = 1;

  print('Fibonacci sequence:');
  for (int i = 0; i < n; i++) {
    stdout.write('$a ');
    int temp = a + b;
    a = b;
    b = temp;
  }
  print('');
}

Output: 0 1 1 2 3 5 8 13 21 34

Example 3: Palindrome Check

dart
bool isPalindrome(String word) {
  word = word.toLowerCase();
  int left = 0;
  int right = word.length - 1;

  while (left < right) {
    if (word[left] != word[right]) {
      return false;
    }
    left++;
    right--;
  }
  return true;
}

void main() {
  List<String> words = ['radar', 'hello', 'level', 'dart', 'racecar'];

  for (String word in words) {
    String result = isPalindrome(word) ? 'is a palindrome' : 'is NOT a palindrome';
    print('"$word" $result');
  }
}

Output:

"radar" is a palindrome
"hello" is NOT a palindrome
"level" is a palindrome
"dart" is NOT a palindrome
"racecar" is a palindrome

Example 4: Star Pattern (Nested Loop)

dart
void main() {
  int rows = 5;

  for (int i = 1; i <= rows; i++) {
    String row = '';
    for (int j = 1; j <= i; j++) {
      row += '* ';
    }
    print(row);
  }
}

Output:

* 
* * 
* * * 
* * * * 
* * * * * 

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Summary

Loop	Condition Checked	Minimum Runs	Best Used When
for	Before each iteration	0 (if condition false initially)	Known number of iterations
while	Before each iteration	0 (if condition false initially)	Condition-driven, unknown iterations
do-while	After each iteration	Always at least 1	Must execute at least once

Loops are fundamental to programming. Once you understand these three, you'll also want to know about for-in, break, and continue — which we cover in the very next lesson!