Rust Loop Keyword

What is the loop Keyword in Rust?

The loop keyword defines an infinite loop, making it a simple way to repeatedly execute a block of code until a specific condition is met. You control its termination manually using break, making it highly flexible for scenarios requiring complex exit conditions.

Syntax of the loop Keyword

loop {
// Code to execute
}
  • The loop starts executing the block of code immediately and continues indefinitely unless interrupted.
  • You can use break to exit the loop or continue to skip to the next iteration.

Key Features of the loop Keyword

  1. Infinite Execution: Runs the block of code forever unless explicitly terminated.
  2. Manual Termination: Requires a break statement to stop execution.
  3. Flexible Usage: Can be combined with conditions inside the loop for dynamic exits.

Example: Basic Infinite Loop

fn main() {
loop {
println!("This will print forever!");
}
}
  • The above code creates an infinite loop.
  • Pressing Ctrl + C (or equivalent) in your terminal is the only way to stop it unless a break statement is added.

Breaking the Loop

The break statement terminates the loop and transfers control to the code following the loop.

Example: Controlled Loop Termination

fn main() {
let mut counter = 0;

loop {
println!("Counter: {}", counter);
counter += 1;

if counter == 5 {
break;
}
}
}

Output:

Counter: 0  
Counter: 1
Counter: 2
Counter: 3
Counter: 4
  • The loop stops when counter reaches 5.

Returning Values from Loops

The loop construct in Rust allows returning a value using the break statement. This feature is particularly useful in scenarios like calculations, searches, or conditions where you want to exit the loop and use the result elsewhere.

Example: Returning a Value

fn main() {
let result = loop {
let mut num = 2;
num *= 2;

if num > 10 {
break num; // Return the value
}
};

println!("The result is: {}", result);
}

Output:

The result is: 16

Nested Loops with loop Keyword

Rust supports nested loops, and you can label loops for better control, especially when using break or continue to exit specific loops.

Example: Breaking a Specific Loop

fn main() {
'outer: loop {
println!("Entering outer loop");

loop {
println!("Inner loop");
break 'outer; // Break the outer loop
}
}

println!("Exited both loops");
}

Output:

Entering outer loop  
Inner loop
Exited both loops
  • The ‘outer label identifies the outer loop and break ‘outer exits it directly, skipping any remaining iterations.

Using continue with loop

The continue statement skips the remaining code in the current iteration and starts the next iteration.

Example: Skipping Iterations

fn main() {
let mut num = 0;

loop {
num += 1;

if num % 2 == 0 {
continue; // Skip even numbers
}

println!("Odd number: {}", num);

if num >= 10 {
break; // Exit loop when num reaches 10
}
}
}

Output:

Odd number: 1  
Odd number: 3
Odd number: 5
Odd number: 7
Odd number: 9

When to Use the loop Keyword?

  1. Complex Exit Conditions: Use loop when the exit condition is not straightforward or cannot be determined upfront.
  2. Server Applications: Ideal for tasks like server loops that should run indefinitely until an external event occurs.
  3. Polling and Waiting: Perfect for checking conditions continuously, such as waiting for user input or system state changes.

Advantages of Using the loop Keyword

  1. Simplicity: No need to define conditions at the start.
  2. Control: Full flexibility to exit or continue based on dynamic conditions.
  3. Efficiency: Can handle infinite or indefinite tasks effectively.

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