Chapter 5a - For Loops

A for loop is one of the most fundamental and versatile control structures in programming. It allows a program to execute a block of code repeatedly for a specified number of iterations or over a sequence of elements. For loops are a cornerstone of modern programming and are available in nearly every programming language, albeit with some syntactic variations.

What is a For Loop?

A for loop is a programming construct used for iterating over a sequence (like a range of numbers, elements of an array, or characters in a string). Each iteration performs a predefined set of actions before moving to the next element in the sequence.

There are different types of for loops, depending on the language and the context:

Range-Based For Loops: These loops iterate over a range of numbers or a sequence. For instance, in Python, you might use for i in range(10) to loop through numbers from 0 to 9. Similarly, Go, C++, and Zig provide constructs to iterate over ranges explicitly.
Collection-Based For Loops: These loops are designed to traverse elements in a collection such as an array, list, or map. Languages like Python use for element in collection, PHP uses foreach, Go uses for _, element := range collection, and Zig uses for (collection) |element| for this purpose.
Traditional For Loops: Commonly used in languages like C++, PHP, and Go, these loops consist of an initializer, a condition, and an iterator all defined in a single statement. For example, for (int i = 0; i < 10; i++) is a traditional for loop in C++, while Go uses for i := 0; i < 10; i++.
Nested For Loops: These are used when dealing with multi-dimensional data structures or performing repeated actions within each iteration of an outer loop. Nested loops are common in all languages and require careful design to avoid performance issues.
Specialized Iteration Constructs: Some languages provide additional loop structures tailored to specific needs, such as iterating over maps or generators, which can streamline certain operations.

Understanding the nuances of these different types of for loops will help you leverage their full potential in your programming tasks.

General Structure

Python:

for element in sequence:
    # Code block to execute

PHP:

foreach ($sequence as $element) {
    // Code block to execute
}

C++:

for (int i = 0; i < n; i++) {
    // Code block to execute
}

Go:

for _, element := range sequence {
    // Code block to execute
}

Zig:

for (sequence) |element| {
    // Code block to execute
}

Common Use Cases of For Loops

1. Iterating Over a Range of Numbers

For loops are commonly used to iterate through a range of numbers.

Python:

for i in range(5):
    print(i)  # Outputs: 0, 1, 2, 3, 4

PHP:

for ($i = 0; $i < 5; $i++) {
    echo $i . "\n";  // Outputs: 0, 1, 2, 3, 4
}

C++:

for (int i = 0; i < 5; i++) {
    std::cout << i << std::endl;  // Outputs: 0, 1, 2, 3, 4
}

package main

import "fmt"

func main() {
    for i := 0; i < 5; i++ {
        fmt.Println(i)  // Outputs: 0, 1, 2, 3, 4
    }
}

Zig:

const std = @import("std");
const stdout = std.io.getStdOut().writer();

for (0..5) |i| {
    stdout.print("{}\n", .{i});  // Outputs: 0, 1, 2, 3, 4
}

2. Iterating Over a Collection

For loops are useful for traversing arrays, lists, or other collections.

Python:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

PHP:

$fruits = ["apple", "banana", "cherry"];
foreach ($fruits as $fruit) {
    echo $fruit . "\n";
}

C++:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::string> fruits = {"apple", "banana", "cherry"};
    for (const auto& fruit : fruits) {
        std::cout << fruit << std::endl;
    }
    return 0;
}

package main

import "fmt"

func main() {
    fruits := []string{"apple", "banana", "cherry"}
    for _, fruit := range fruits {
        fmt.Println(fruit)
    }
}

Zig:

const std = @import("std");
const stdout = std.io.getStdOut().writer();

const fruits = [_][]const u8 {"apple", "banana", "cherry"};

for (fruits) |fruit| {
    stdout.print("{}\n", .{fruit});
}

3. Nested For Loops

For loops can be nested to iterate over multi-dimensional data structures or to perform repeated actions.

Python:

for i in range(3):
    for j in range(2):
        print(f"i={i}, j={j}")

PHP:

for ($i = 0; $i < 3; $i++) {
    for ($j = 0; $j < 2; $j++) {
        echo "i=$i, j=$j\n";
    }
}

C++:

for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 2; j++) {
        std::cout << "i=" << i << ", j=" << j << std::endl;
    }
}

package main

import "fmt"

func main() {
    for i := 0; i < 3; i++ {
        for j := 0; j < 2; j++ {
            fmt.Printf("i=%d, j=%d\n", i, j)
        }
    }
}

Zig:

const std = @import("std");
const stdout = std.io.getStdOut().writer();

for (0..3) |i| {
    for (0..2) |j| {
        stdout.print("i={}, j={}\n", .{i, j});
    }
}

4. Filtering Data

For loops can be used to filter data based on specific conditions.

Python:

numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        print(num)  # Outputs: 2, 4

package main

import "fmt"

func main() {
    numbers := []int{1, 2, 3, 4, 5}
    for _, num := range numbers {
        if num%2 == 0 {
            fmt.Println(num)  // Outputs: 2, 4
        }
    }
}

Key Considerations

Off-by-One Errors: When using loops that depend on a range of numbers, pay close attention to whether the upper bound is inclusive or exclusive.
Performance: Avoid unnecessary iterations, especially in nested loops, as they can lead to performance bottlenecks.
Infinite Loops: Ensure that the loop has a termination condition to prevent the program from running indefinitely.

Conclusion

For loops are indispensable tools for performing repetitive tasks in programming. By mastering their syntax and use cases, you can write efficient, readable, and effective code across various programming languages. Whether you’re iterating over arrays, processing data, or generating complex patterns, the for loop remains a reliable ally in every programmer’s toolkit.

Last modified: 26 January 2025