Loops

Loops allow you to execute code repeatedly. Here's how different languages handle for loops, while loops, and iteration.

For Loops

JavaScript

// Traditional for loop
for (let i = 0; i < 5; i++) {
  console.log(i);
}

// For...of (arrays)
const arr = [1, 2, 3];
for (const item of arr) {
  console.log(item);
}

// For...in (objects)
for (const key in obj) {
  console.log(key, obj[key]);
}

PHP

<?php
// Traditional for loop
for ($i = 0; $i < 5; $i++) {
  echo $i;
}

// Foreach (arrays)
$arr = [1, 2, 3];
foreach ($arr as $item) {
  echo $item;
}

// Foreach with keys
foreach ($arr as $key => $value) {
  echo "$key: $value";
}

Rust

// Range-based for loop
for i in 0..5 {
  println!("{}", i);
}

// Iterating over vector
let vec = vec![1, 2, 3];
for item in vec.iter() {
  println!("{}", item);
}

// With index
for (i, item) in vec.iter().enumerate() {
  println!("{}: {}", i, item);
}

Go

// Traditional for loop
for i := 0; i < 5; i++ {
  fmt.Println(i)
}

// Range-based for loop
arr := []int{1, 2, 3}
for index, value := range arr {
  fmt.Printf("%d: %d\n", index, value)
}

// Value only
for _, value := range arr {
  fmt.Println(value)
}

Python

# Range-based for loop
for i in range(5):
  print(i)

# Iterating over list
arr = [1, 2, 3]
for item in arr:
  print(item)

# With index
for i, item in enumerate(arr):
  print(f"{i}: {item}")

Zig

// Traditional for loop
var i: usize = 0;
while (i < 5) : (i += 1) {
  std.debug.print("{}\n", .{i});
}

// For loop (Zig style)
for (arr, 0..) |item, index| {
  std.debug.print("{}: {}\n", .{index, item});
}

C#

// Traditional for loop
for (int i = 0; i < 5; i++)
{
  Console.WriteLine(i);
}

// Foreach
int[] arr = {1, 2, 3};
foreach (int item in arr)
{
  Console.WriteLine(item);
}

// With index
foreach (var (index, item) in arr.Select((value, i) => (i, value)))
{
  Console.WriteLine($"{index}: {item}");
}

C++

// Traditional for loop
for (int i = 0; i < 5; i++) {
  std::cout << i << std::endl;
}

// Range-based for (C++11+)
std::vector<int> vec = {1, 2, 3};
for (int item : vec) {
  std::cout << item << std::endl;
}

// With index
for (size_t i = 0; i < vec.size(); i++) {
  std::cout << i << ": " << vec[i] << std::endl;
}

C

// Traditional for loop
for (int i = 0; i < 5; i++) {
  printf("%d\n", i);
}

// Iterating over array
int arr[] = {1, 2, 3};
int size = sizeof(arr) / sizeof(arr[0]);
for (int i = 0; i < size; i++) {
  printf("%d\n", arr[i]);
}

Java

// Traditional for loop
for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

// Enhanced for loop
int[] arr = {1, 2, 3};
for (int item : arr) {
  System.out.println(item);
}

// With index
for (int i = 0; i < arr.length; i++) {
  System.out.println(i + ": " + arr[i]);
}

Ruby

# Range-based for loop
(0...5).each do |i|
  puts i
end

# Iterating over array
arr = [1, 2, 3]
arr.each do |item|
  puts item
end

# With index
arr.each_with_index do |item, index|
  puts "#{index}: #{item}"
end

Swift

// Range-based for loop
for i in 0..<5 {
  print(i)
}

// Iterating over array
let arr = [1, 2, 3]
for item in arr {
  print(item)
}

// With index
for (index, item) in arr.enumerated() {
  print("(index): (item)")
}

While Loops

JavaScript

// While loop
let i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

// Do-while loop
let j = 0;
do {
  console.log(j);
  j++;
} while (j < 5);

PHP

<?php
// While loop
$i = 0;
while ($i < 5) {
  echo $i;
  $i++;
}

// Do-while loop
$j = 0;
do {
  echo $j;
  $j++;
} while ($j < 5);

Rust

// While loop
let mut i = 0;
while i < 5 {
  println!("{}", i);
  i += 1;
}

// Loop with break
loop {
  if i >= 5 {
      break;
  }
  println!("{}", i);
  i += 1;
}

Go

// While loop (for with condition)
i := 0
for i < 5 {
  fmt.Println(i)
  i++
}

// Infinite loop
for {
  if i >= 5 {
      break
  }
  fmt.Println(i)
  i++
}

Python

# While loop
i = 0
while i < 5:
  print(i)
  i += 1

# Infinite loop with break
while True:
  if i >= 5:
      break
  print(i)
  i += 1

Zig

// While loop
var i: usize = 0;
while (i < 5) {
  std.debug.print("{}\n", .{i});
  i += 1;
}

// While with continue
while (i < 10) : (i += 1) {
  if (i % 2 == 0) continue;
  std.debug.print("{}\n", .{i});
}

C#

// While loop
int i = 0;
while (i < 5)
{
  Console.WriteLine(i);
  i++;
}

// Do-while loop
int j = 0;
do
{
  Console.WriteLine(j);
  j++;
} while (j < 5);

C++

// While loop
int i = 0;
while (i < 5) {
  std::cout << i << std::endl;
  i++;
}

// Do-while loop
int j = 0;
do {
  std::cout << j << std::endl;
  j++;
} while (j < 5);

C

// While loop
int i = 0;
while (i < 5) {
  printf("%d\n", i);
  i++;
}

// Do-while loop
int j = 0;
do {
  printf("%d\n", j);
  j++;
} while (j < 5);

Java

// While loop
int i = 0;
while (i < 5) {
  System.out.println(i);
  i++;
}

// Do-while loop
int j = 0;
do {
  System.out.println(j);
  j++;
} while (j < 5);

Ruby

# While loop
i = 0
while i < 5
  puts i
  i += 1
end

# Until loop (opposite of while)
i = 0
until i >= 5
  puts i
  i += 1
end

Swift

// While loop
var i = 0
while i < 5 {
  print(i)
  i += 1
}

// Repeat-while (do-while equivalent)
var j = 0
repeat {
  print(j)
  j += 1
} while j < 5

Key Takeaways

• For loop styles -- C-style for (i=0; i<n; i++) in C/C++/Java/JavaScript; range-based in Python (for i in range(5)), Rust (0..5), Swift (0..<5); Go uses for for both.
• Range-based iteration -- for...of (JavaScript), foreach (PHP, C#), for...in/enumerate (Python), range (Go). Zig uses for (arr) |item|.
• While and iteration -- Go has no while keyword (use for condition); Ruby adds until; Rust offers loop with break; Swift uses repeat-while instead of do-while.