Dictionary.updateValue() / Removing Entries / mapValues()

var scores: [String: Int] = ["Alice": 80, "Bob": 70, "Carol": 90]

// updateValue: returns the previous value before updating
if let oldValue = scores.updateValue(85, forKey: "Bob") {
    print("Bob のスコアを \(oldValue) から \(scores["Bob"]!)  に更新しました")
}

// Returns nil if the key does not exist yet
let result = scores.updateValue(75, forKey: "Dave")
print("Dave は新規追加: \(result == nil)")

// removeValue: returns the removed value
if let removed = scores.removeValue(forKey: "Carol") {
    print("Carol（\(removed)点）を削除しました")
}

// Remove a key by assigning nil via subscript
scores["Dave"] = nil
print("削除後: \(scores)")

// mapValues: transform all values
var doubled = scores.mapValues { $0 * 2 }
print("2倍: \(doubled)")

// filter: keep only pairs that match the condition
let highScores = scores.filter { $0.value >= 80 }
print("80点以上: \(highScores)")

// merge: merge dictionaries (duplicate keys are overwritten by the new value)
var extra: [String: Int] = ["Eve": 95, "Alice": 88]
scores.merge(extra) { _, new in new }
print("マージ後: \(scores)")

updateValue(_:forKey:) lets you update a value and check whether the key already existed at the same time. It returns the previous value as an Optional when updating an existing key, or nil when adding a new key.

mapValues(_:) transforms all values while keeping the keys unchanged. The original dictionary is not modified — a new dictionary is returned instead. The closure passed to merge(_:uniquingKeysWith:) receives two arguments — the existing value and the new value — and returns whichever value should be kept.

For the basics of dictionaries, see Dictionary Basics / dictionary.keys / dictionary.values.