Structs are values in Swift

Assignment copies; writers say mutating.

// Structs are value types: assignment copies, and a method that
// writes needs the mutating keyword.
struct Tally {
    var points = 0
    mutating func score(place: Int) {
        points += max(0, 10 - place)
    }
}

var kestrel = Tally()
kestrel.score(place: 1)
kestrel.score(place: 3)

var copyBefore = kestrel
kestrel.score(place: 2)

print("Kestrel: \(kestrel.points) points")
print("The copy stayed at \(copyBefore.points)")

How it works

1. struct Tally gets a memberwise-style default init.
2. mutating func score may write points.
3. The copy taken before the last score never sees it.

Keywords and builtins used here

• Int
• Tally
• func
• max
• mutating
• print
• score
• struct
• var

The run, in numbers

Lines

18

Characters to type

413

Tokens

89

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 76 seconds.

Type this snippet

Step 1 of 3 in Structs & classes, step 16 of 27 in Language basics.

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