Welcome back to Collections in Julia! In this second lesson of our course, we build directly upon the array fundamentals you mastered previously, where you learned to create, access, and inspect arrays. Today marks an exciting progression as we transform from passive array users into active data manipulators, discovering Julia's powerful suite of array modification and operations that enable dynamic data transformation.

Throughout this lesson, you'll master the art of modifying arrays in place using Julia's mutating functions, understand the crucial distinction between operations that alter original data versus those that create copies, and explore advanced techniques for building, slicing, and querying arrays. These skills form the backbone of real-world data processing, where information constantly evolves, grows, and requires restructuring. By the end of this lesson, you'll possess the confidence to manipulate arrays dynamically, implementing the flexible data structures that power scientific computing and modern applications.

Before diving into specific array operations, let's explore a fundamental concept that sets Julia apart: the distinction between mutating and non-mutating functions. Arrays, unlike the immutable values we worked with in Julia Fundamentals (numbers, strings, booleans), represent mutable containers whose contents can change after creation. This mutability opens powerful possibilities for efficient data manipulation while requiring careful consideration of how operations affect our data.

Julia uses a brilliant convention to distinguish these operation types: mutating functions end with an exclamation mark (!), serving as a visual warning that the operation will modify the input directly. For instance, push! adds elements to an existing array, while a hypothetical push would create a new array with the addition. This naming convention prevents countless bugs by making mutation explicit and intentional. Non-mutating functions preserve original data by creating modified copies, trading memory for safety. As we explore various array operations throughout this lesson, pay attention to the presence or absence of the exclamation mark — it tells you whether your original data will survive the operation unchanged.

Let's begin our hands-on exploration by constructing arrays incrementally and observing how mutating functions transform our data structures:

This code demonstrates Julia's core array modification functions in action. Starting with Int64[], we create an empty array typed for 64-bit integers — like preparing a labeled container before filling it with specific items. The push! function adds elements one at a time to the array's end, perfect for scenarios where data arrives incrementally. Each call to push!(a, value) extends the array by one element, maintaining order and growing the collection efficiently.

The append! function takes this concept further by merging entire collections. When we call append!(a, [4, 5]), Julia extracts each element from the second array and adds them individually to our target array. This differs crucially from push!, which would treat [4, 5] as a single nested element. Finally, pop! showcases removal operations by extracting and returning the last element while simultaneously shrinking the array. The dual nature of pop! — both modifying the array and returning the removed value — enables elegant patterns where removed elements require further processing.

The output reveals the cumulative effect of these operations:

Building on basic operations, Julia provides sophisticated functions for targeted modifications and bidirectional array growth:

The splice! function provides surgical precision for element removal, targeting any valid index position rather than just the array's end. When we execute splice!(a, 4), Julia removes the element at position 4 and automatically shifts the remaining elements to maintain continuity. This operation proves invaluable for filtering data, implementing algorithms requiring position-based removal, or maintaining ordered collections where specific elements become obsolete.

The pushfirst! function complements push! by adding elements to the array's beginning, though this operation requires more computational work as existing elements must shift rightward. Similarly, popfirst! removes and returns the first element of the array, shifting the remaining elements leftward—useful for queue-like data structures or when processing elements in order. Our sorting example beautifully illustrates the mutating versus non-mutating paradigm: sort(arr) creates a new sorted array while preserving the original, whereas sort!(arr) transforms the original array directly. This distinction becomes crucial when deciding between memory efficiency and data preservation.

The results demonstrate these operations' effects:

Julia's elegant syntax for array creation and manipulation extends beyond individual element operations to powerful range-based techniques:

The expression [1:5;] demonstrates Julia's concise syntax for creating arrays from ranges. The semicolon performs vertical concatenation, converting the abstract range object into a concrete array containing [1, 2, 3, 4, 5]. This approach eliminates verbose loops for sequential data generation, proving especially valuable in mathematical computing, where numerical sequences frequently serve as algorithm inputs.

Array slicing unlocks powerful data extraction capabilities through intuitive range notation. The syntax range_arr[1:3] creates a new array containing elements from positions 1 through 3 inclusive, while range_arr[2:end] showcases Julia's convenient end keyword, which automatically represents the final valid index. These slicing operations create new arrays, ensuring that modifications to slices don't affect the original data. The in() function provides membership testing, returning true when the specified value exists within the array, while length() reports the total element count — both are essential for data validation and algorithm implementation.

Congratulations on completing this second lesson in Collections in Julia! You've successfully transformed from working with static arrays to becoming a master of dynamic array manipulation. Through mutating functions like push!, pop!, append!, splice!, and pushfirst!, you've learned to modify arrays in place efficiently. You've also discovered the critical distinction between mutating and non-mutating operations, mastered range-based array creation and slicing techniques, and explored essential array property functions.

These powerful array manipulation skills prepare you for real-world data processing challenges, where information constantly evolves and requires dynamic handling. The upcoming practice exercises will solidify these concepts through hands-on challenges that reinforce your understanding of array operations. As we continue our journey through Julia's collection system, these foundational skills will enable increasingly sophisticated data structures and algorithms!