Foundations

Variables and Constants

Use let for values that should not change and var for values that can be updated later.

Compute with stored values

unitPrice
variables.swift
Replay: real traced execution (multi-file project)
let unitPrice = 12
let quantity = 3
var total = unitPrice * quantity
total = total + 5
print("unit=\(unitPrice)")
print("total=\(total)")

let unitPrice = 8
let quantity = 3
var total = unitPrice * quantity
total = total + 5
print("unit=\(unitPrice)")
print("total=\(total)")

let unitPrice = 20
let quantity = 3
var total = unitPrice * quantity
total = total + 5
print("unit=\(unitPrice)")
print("total=\(total)")

  1. unitPrice ← 12, quantity ← 3, total ← 36

    1let unitPrice→ 12 = 12  //@unitPrice=8, 202let quantity→ 3 = 33var total→ 36 = unitPrice12 * quantity34total→ 41 = total + 55print("unit=\(unitPrice12)")6print("total=\(total41)")
    outputunit=12
total=41

  1. unitPrice ← 8, quantity ← 3, total ← 24

    1let unitPrice→ 8 = 82let quantity→ 3 = 33var total→ 24 = unitPrice8 * quantity34total→ 29 = total + 55print("unit=\(unitPrice8)")6print("total=\(total29)")
    outputunit=8
total=29

  1. unitPrice ← 20, quantity ← 3, total ← 60

    1let unitPrice→ 20 = 202let quantity→ 3 = 33var total→ 60 = unitPrice20 * quantity34total→ 65 = total + 55print("unit=\(unitPrice20)")6print("total=\(total65)")
    outputunit=20
total=65

Follow the Total

  1. unitPrice starts at 12.
  2. quantity starts at 3.
  3. var total = unitPrice * quantity makes 36.
  4. total = total + 5 changes it to 41.
  5. The program prints unit=12 and total=41. | unitPrice | initial total | after + 5 | | --- | --- | --- | | 8 | 24 | 29 | | 12 | 36 | 41 | | 20 | 60 | 65 |
let and var `let` creates a constant. `var` creates a variable that can be assigned again.

Exercise: variables.swift

Reproduce unit=12 and total=41, then try unitPrice 8 and 20 and predict each final total.