CustomStringConvertible

Part of the Object Oriented Programming section of Coddy's Swift journey. Lesson 37 of 57.

When you print a value, Swift shows its type and properties by default, and for a class only the type name. Conforming to CustomStringConvertible and providing description controls the text:

struct Money {
    let cents: Int
}
struct Price: CustomStringConvertible {
    let cents: Int
    var description: String { "$\(cents / 100).\(cents % 100)" }
}

print(Money(cents: 1250))
print(Price(cents: 1250))

Output:

Money(cents: 1250)
$12.50

description is used by print, by string interpolation and by String(describing:), so one property controls every place the value is shown:

struct Point: CustomStringConvertible {
    let x: Int, y: Int
    var description: String { "(\(x), \(y))" }
}

let p = Point(x: 2, y: 5)
print("start at \(p)")
print([p, Point(x: 0, y: 0)])
print(String(describing: p).count)

Output:

start at (2, 5)
[(2, 5), (0, 0)]
6

Classes benefit most, because printing a class instance otherwise shows only its type name:

class Account {
    let owner: String
    var balance: Int
    init(owner: String, balance: Int) { self.owner = owner; self.balance = balance }
}

class NamedAccount: CustomStringConvertible {
    let owner: String
    var balance: Int
    init(owner: String, balance: Int) { self.owner = owner; self.balance = balance }
    var description: String { "\(owner): \(balance)" }
}

print(Account(owner: "Ada", balance: 5))
print(NamedAccount(owner: "Ada", balance: 5))

Output:

main.Account
Ada: 5

Keep description short and readable for people. It is not meant for parsing, and it should not change the value:

enum Status: CustomStringConvertible {
    case active, suspended
    var description: String {
        switch self {
        case .active: return "active"
        case .suspended: return "suspended"
        }
    }
}

Challenge

Easy

Make the struct Fraction (numerator, denominator) conform to CustomStringConvertible. Its description shows the fraction in lowest terms: 6/8 prints as 3/4, 4/2 as 2, 0/5 as 0, and a negative fraction keeps the sign in front (-1/2). Write a helper gcd(_:_:) for the reduction.

The supplied code reads fractions a/b, one per line, and prints each one with print.

Your code goes in Fraction.swift and Functions.swift. main.swift holds the supplied input/output code and cannot be edited.

Try it yourself

// Supplied input/output code: keep it as it is
var input: [String] = []
while let line = readLine() { input.append(line) }
for line in input {
    let p = line.split(separator: "/").map { Int($0)! }
    print(Fraction(numerator: p[0], denominator: p[1]))
}