Types and Strings

Type Annotations

When the intended type should be explicit, write the type after a colon.

Declare the type you want

width
type_annotations.swift
Replay: real traced execution (multi-file project)
let width: Int = 8
let height: Int = 3
let title: String = "panel"
let area: Int = width * height
let summary: String = "\(title): \(area)"

print(summary)

let width: Int = 5
let height: Int = 3
let title: String = "panel"
let area: Int = width * height
let summary: String = "\(title): \(area)"

print(summary)

let width: Int = 12
let height: Int = 3
let title: String = "panel"
let area: Int = width * height
let summary: String = "\(title): \(area)"

print(summary)

  1. width ← 8, height ← 3, title ← panel, area ← 24, summary ← panel: 24

    1let width→ 8: Int = 8  //@width=5, 122let height→ 3: Int = 33let title→ panel: String = "panel"4let area→ 24: Int = width8 * height35let summary→ panel: 24: String = "\(titlepanel): \(area24)"67print(summarypanel: 24)
    outputpanel: 24

  1. width ← 5, height ← 3, title ← panel, area ← 15, summary ← panel: 15

    1let width→ 5: Int = 52let height→ 3: Int = 33let title→ panel: String = "panel"4let area→ 15: Int = width5 * height35let summary→ panel: 15: String = "\(titlepanel): \(area15)"67print(summarypanel: 15)
    outputpanel: 15

  1. width ← 12, height ← 3, title ← panel, area ← 36, summary ← panel: 36

    1let width→ 12: Int = 122let height→ 3: Int = 33let title→ panel: String = "panel"4let area→ 36: Int = width12 * height35let summary→ panel: 36: String = "\(titlepanel): \(area36)"67print(summarypanel: 36)
    outputpanel: 36

Follow the Area

  1. width starts at 8.
  2. height is 3.
  3. title is panel.
  4. area = width * height becomes 24.
  5. summary becomes panel: 24, and the program prints it. | width | height | area | summary | | --- | --- | --- | --- | | 5 | 3 | 15 | panel: 15 | | 8 | 3 | 24 | panel: 24 | | 12 | 3 | 36 | panel: 36 |
annotation `let name: Type = value` tells Swift the exact type expected for the value.

Exercise: type_annotations.swift

Reproduce panel: 24, then use width 5 and 12 to predict each summary.