Arrays, dictionaries, and value copies

The idea

Swift arrays and dictionaries are value types. Assigning an array to a new variable gives it independent value semantics: changing the new array’s elements does not change the original array value. Implementations may share storage internally until mutation, but that optimization does not change the observable value model. Arrays hold ordered elements; dictionaries associate unique keys with values.

A dictionary lookup normally returns an optional because a key might be absent. The default subscript can supply a starting value for a counting operation. Collection mutating operations still have preconditions: removeFirst requires a nonempty array. Check emptiness or use an operation whose result explicitly represents absence before removing an item from a possibly empty queue.

var first = [2, 4]
var second = first
second.append(6)
var counts: [String: Int] = [:]
for tag in ["bug", "docs", "bug"] { counts[tag, default: 0] += 1 }
print(first.count)
print(second.count)
print(counts["bug", default: 0])

2
3
2

The appended element belongs to second’s array value. Dictionary updates begin missing counts at zero, then increment the selected key.