let a: [Int: String] = [1: "a", 2: "b"] let b: [String: String] = Dictionary(uniqueKeysWithValues: a.map { ("\($0)", $1) })let a: [Int: String] = [1: "a", 2: "b"] let b: [String: String] = Dictionary(uniqueKeysWithValues: a.map { ("\($0)", $1) })// むりやり左から右の流れに・・・ let c: [String: String] = Optional(a.map { ("\($0)", $1) }).map(Dictionary.init)!