What Happens When You Modify an Array in Swift

When you modify an array in Swift by changing a value or appending a new element, several important things happen under the hood. Understanding this process is crucial for writing efficient Swift code.

Modifying an Existing Value

When you change a value at an existing index:

var fruits = ["Apple", "Orange", "Banana"]
fruits[1] = "Mango"  // Change "Orange" to "Mango"

Here's what happens:

Copy-on-Write Check: Swift checks if the array's storage is shared with any other variables

If Shared: Swift creates a complete copy of the array's storage before making the change

var fruits1 = ["Apple", "Orange", "Banana"]
var fruits2 = fruits1  // Both reference the same storage
fruits2[1] = "Mango"   // Copy-on-write triggers here
// Now fruits1 = ["Apple", "Orange", "Banana"]
// and fruits2 = ["Apple", "Mango", "Banana"]

If Not Shared: Swift modifies the array directly without creating a copy

var fruits = ["Apple", "Orange", "Banana"]
fruits[1] = "Mango"   // No copy needed, direct modification

Reference Count Management: The system updates reference counts as needed
Memory Layout: The array's internal storage remains the same size since you're just replacing an element

Appending a Value

When you append a new element:

var numbers = [1, 2, 3]
numbers.append(4)  // Add a new element

Here's what happens:

Copy-on-Write Check: Same as above, Swift checks if the storage is shared

Capacity Check: Swift checks if the current capacity can accommodate the new element

print(numbers.count)      // Elements currently in use
print(numbers.capacity)   // Total allocated space

If Capacity Sufficient: Swift adds the new element at the end without reallocating

// If capacity is 4 and count is 3
numbers.append(4)  // Just adds the element, no reallocation

If Capacity Insufficient: Swift allocates a new, larger storage buffer
```
// If capacity is 3 and count is 3
numbers.append(4)  // Triggers reallocation to larger capacity
```
The reallocation typically follows a growth factor (often around 2x):
- If capacity was 3, new capacity might be 6
- All existing elements are copied to the new storage
- The new element is added
- The old storage is deallocated
Memory Management: Reference counting ensures proper deallocation of the old buffer if needed

Performance Implications

For Changing Existing Values

Best Case: O(1) time complexity if not shared
Worst Case: O(n) if copy-on-write is triggered (where n is array size)

For Appending Elements

Amortized Time Complexity: O(1)
Individual Append
:
- Usually O(1) when capacity is sufficient
- O(n) when reallocation occurs
Multiple Appends: The occasional O(n) operation is amortized across many O(1) operations, resulting in O(1) average time per operation

Practical Tips

Reserve Capacity: For best performance when adding many elements:

var numbers = [Int]()
numbers.reserveCapacity(1000)  // Allocate space once
for i in 1...1000 {
    numbers.append(i)  // No reallocations during appends
}

Avoid Shared Copies when Modifying: If you know you'll modify an array, consider creating a unique copy first:

var uniqueCopy = originalArray  // Still shared
uniqueCopy.reserveCapacity(uniqueCopy.count)  // Forces a unique copy
// Now modifications won't trigger copy-on-write

Batch Operations: Prefer array transformations like map or filter over individual modifications when possible
Arrays vs Other Collections: Consider using other collection types if your usage pattern doesn't match array strengths:
- Set for frequent membership tests
- Dictionary for key-based lookup
- ContiguousArray for performance-critical code with value types

Understanding this process helps you write more efficient code and avoid unexpected performance bottlenecks when working with arrays in Swift.