While Loops

Learn how to use while loops for conditional iteration in Dart

Overview

While loops execute a block of code repeatedly as long as a condition remains true. Unlike for loops, while loops are ideal when you don’t know in advance how many iterations are needed.

Basic Syntax

while (condition) {
  // Code to execute
}

The condition is checked before each iteration. If the condition is false initially, the loop body never executes.

Incrementing While Loop

A basic while loop that increments a counter:

int contador = 1;
print('\nCiclo While Incremento');

while (contador <= 5) {
  print('Contador: $contador');
  contador++;
}
// Output: Contador: 1
//         Contador: 2
//         Contador: 3
//         Contador: 4
//         Contador: 5

Decrementing While Loop

You can also decrement the counter:

print('\nCiclo While Decremento');
int contador = 5;

while (contador >= 1) {
  print('Contador: $contador');
  contador--;
}
// Output: Contador: 5
//         Contador: 4
//         Contador: 3
//         Contador: 2
//         Contador: 1

Complete Example

void main() {
  int contador = 1;
  print('\nCiclo While Incremento');
  
  while (contador <= 5) {
    print('Contador: $contador');
    contador++;
  }

  // Decrement
  print('\nCiclo While Decremento');
  contador = 5;
  
  while (contador >= 1) {
    print('Contador: $contador');
    contador--;
  }
}

When to Use While Loops

1. Unknown iteration count

   Use while loops when the number of iterations depends on runtime conditions rather than a fixed count.

2. Condition-based execution

   While loops are ideal when you need to continue until a specific condition is met.

3. Reading input

   Useful for reading user input or processing data until a sentinel value is encountered.

While vs For Loops

Use For Loop When:

• You know the exact number of iterations
• You’re iterating over a range of numbers
• You’re iterating over a collection

Use While Loop When:

• The number of iterations is unknown
• The loop depends on external conditions
• You’re waiting for a specific state to occur

Common Patterns

Counting up

int i = 0;
while (i < n) {
  // Code
  i++;
}

Counting down

int i = n;
while (i > 0) {
  // Code
  i--;
}

Condition-based

while (someCondition) {
  // Code that may change someCondition
}

Loop Control

Use break to exit the loop early and continue to skip the current iteration:

int contador = 0;
while (contador < 10) {
  contador++;
  
  if (contador == 5) {
    continue; // Skip 5
  }
  
  if (contador > 7) {
    break; // Exit at 8
  }
  
  print(contador);
}

Infinite Loops

Be careful to avoid infinite loops! Always ensure the condition will eventually become false:

// DANGER: Infinite loop!
int x = 1;
while (x > 0) {
  print(x);
  x++; // x will never be <= 0
}

Best Practices

• Always initialize the loop variable before the loop
• Ensure the condition will eventually become false
• Update the loop variable inside the loop body
• Use meaningful variable names
• Consider using for loops when the iteration count is known
• Add safety mechanisms (like maximum iteration counts) for potentially infinite loops

Practical Example

Here’s a practical example of reading values until a condition is met:

void main() {
  int sum = 0;
  int count = 1;
  
  // Sum numbers from 1 to 10
  while (count <= 10) {
    sum += count;
    count++;
  }
  
  print('Sum of numbers 1-10: $sum');
  // Output: Sum of numbers 1-10: 55
}