Chapter 5b - While Loops

Loops are fundamental building blocks in programming, and the "while" loop is one of the most versatile types. A while loop repeatedly executes a block of code as long as its condition remains true. Unlike for-loops, which often iterate over a fixed range or collection, while loops are more flexible and are usually employed when the number of iterations isn’t known beforehand. In this article, we will dive deep into what while loops are, how they work, and explore their variations with practical examples in Python, PHP, C++, Zig, and Go.

What Are While Loops?

A while loop allows you to execute a block of code as long as a specified condition evaluates to true. Each iteration checks the condition before executing the loop body. This makes while loops particularly useful for scenarios where you want to keep looping until a dynamic condition changes (e.g., user input, sensor data, or unpredictable states).

Key Variants of While Loops

Basic While Loop: This is the simplest form, where the condition is checked at the start of each iteration.
Infinite While Loop: Runs indefinitely unless explicitly broken out of, often used for event-driven programming or servers.
While-Else Loop (in languages like Python): Executes an additional block of code if the loop ends naturally without a break.
Do-While Loop: A variation where the condition is checked after the loop body, guaranteeing at least one execution. This form is available in PHP, C++, Zig, and Go but not Python.

Let’s break down these types with examples.

Basic While Loops

Python:

counter = 0
while counter < 5:
    print(f"Counter is {counter}")
    counter += 1

PHP:

$counter = 0;
while ($counter < 5) {
    echo "Counter is $counter\n";
    $counter++;
}

C++:

#include <iostream>

int main() {
    int counter = 0;
    while (counter < 5) {
        std::cout << "Counter is " << counter << "\n";
        counter++;
    }
    return 0;
}

Zig:

const std = @import("std");

pub fn main() void {
    var counter: usize = 0;
    while (counter < 5) : (counter += 1) {
        std.debug.print("Counter is {d}\n", .{counter});
    }
}

package main

import "fmt"

func main() {
    counter := 0
    for counter < 5 { // Go doesn't have a 'while', but 'for' can act as one
        fmt.Printf("Counter is %d\n", counter)
        counter++
    }
}

Infinite While Loops

Infinite loops occur when the condition always evaluates to true. These loops are often controlled by external logic, like user input or event handlers.

Python:

while True:
    user_input = input("Enter 'quit' to stop: ")
    if user_input == 'quit':
        break

PHP:

while (true) {
    $userInput = readline("Enter 'quit' to stop: ");
    if ($userInput === "quit") {
        break;
    }
}

C++:

#include <iostream>
#include <string>

int main() {
    std::string input;
    while (true) {
        std::cout << "Enter 'quit' to stop: ";
        std::cin >> input;
        if (input == "quit") {
            break;
        }
    }
    return 0;
}

Zig:

const std = @import("std");

pub fn main() void {
    var input = "";
    while (true) {
        std.debug.print("Enter 'quit' to stop: ", .{});
        input = std.io.getLine();
        if (input == "quit") break;
    }
}

package main

import "fmt"

func main() {
    for {
        var input string
        fmt.Print("Enter 'quit' to stop: ")
        fmt.Scanln(&input)
        if input == "quit" {
            break
        }
    }
}

While-Else Loops (Python)

Python provides a unique "else" block with while loops. This block runs only if the loop exits naturally without hitting a break statement.

Python Example:

counter = 0
while counter < 3:
    print(f"Counter is {counter}")
    counter += 1
else:
    print("Loop ended naturally")

Do-While Loops

Unlike the standard while loop, a do-while loop guarantees at least one execution of the loop body.

PHP:

$counter = 0;
do {
    echo "Counter is $counter\n";
    $counter++;
} while ($counter < 5);

C++:

#include <iostream>

int main() {
    int counter = 0;
    do {
        std::cout << "Counter is " << counter << "\n";
        counter++;
    } while (counter < 5);
    return 0;
}

Zig:

const std = @import("std");

pub fn main() void {
    var counter: usize = 0;
    while (true) : (counter += 1) {
        std.debug.print("Counter is {d}\n", .{counter});
        if (counter >= 5) break;
    }
}

package main

import "fmt"

func main() {
    counter := 0
    for { // Emulating a do-while loop
        fmt.Printf("Counter is %d\n", counter)
        counter++
        if counter >= 5 {
            break
        }
    }
}

Common Use Cases for While Loops

Waiting for a Condition: While loops are ideal for scenarios where you wait for some external condition, like user input or network data.
- Example: Monitoring a file system for changes.
Processing Streams: Continuously process data until a stream ends.
- Example: Reading a file line by line.
Simulations: Repeating actions in physics engines, games, or other simulations until a condition is met.

Key Considerations

Avoid Infinite Loops: Ensure your loop has a way to exit to avoid unresponsive programs.
Optimize Conditions: Make conditions simple and efficient to evaluate.
Debugging: Use print statements or debuggers to trace issues in complex while loops.

With this understanding of while loops, you can harness their power to create efficient and dynamic programs across various languages. Experiment with these examples and adapt them to your needs!

Last modified: 26 January 2025