One expense is a value. A month of expenses is a collection. Pocket Budget needs three shapes of collection: an ordered list of amounts, a lookup from category to monthly limit, and a bag of unique tags. Swift gives you exactly those three, and all of them are typed, so an array of amounts can never accidentally contain a String.
import Foundation\n\nvar amounts: [Decimal] = [12.5, 4.25, 60]\namounts.append(18)\n\nprint(amounts.count) // 4\nprint(amounts[0]) // 12.5\nprint(amounts.first as Any) // Optional(12.5)[Decimal] reads “array of Decimal”. Indexes start at 0. Notice the difference between amounts[0] and amounts.first: the subscript trusts you and crashes with “Index out of range” on an empty array, while first returns an optional and lets you handle the empty case with the tools from the previous lesson.
Most of the time you don’t need indexes at all. A for-in loop visits every element:
var total: Decimal = 0\nfor amount in amounts {\n total += amount\n}\nprint(total) // 94.75\n\nfor (index, amount) in amounts.enumerated() {\n print(\"\\(index + 1). \\(amount)\")\n}enumerated() hands you pairs of position and value, which is the safe way to number rows. Ranges work with for-in too, in two flavours:
for day in 1...30 { } // closed range: 1 through 30\nfor i in 0..<amounts.count { } // half-open: stops before countThe closed range includes its upper bound; the half-open range stops one short, which is exactly what array indexes need.
\nA dictionary maps keys to values. Pocket Budget keeps a monthly limit per category:
\nlet limits: [String: Decimal] = [\"Food\": 300, \"Transport\": 120]\nvar spent: [String: Decimal] = [\"Food\": 280]\n\nlet foodLimit = limits[\"Food\"] // Decimal?, Optional(300)\nlet funLimit = limits[\"Fun\"] ?? 0 // Decimal, 0\n\nspent[\"Fun\", default: 0] += 15 // creates \"Fun\": 15Every lookup returns an optional, because the key might not be there. That’s the same safety idea again, now built into a collection. The default: subscript is the idiomatic way to accumulate totals: it reads the existing value or starts from your default, then writes the result back.
var tags: Set<String> = [\"work\", \"travel\"]\ntags.insert(\"work\") // already there, nothing changes\nprint(tags.count) // 2\nprint(tags.contains(\"travel\")) // trueA set has no order and no duplicates, and contains stays fast no matter how many items it holds. Use it for tags, selected filters and “have I seen this id already?” checks.
| You need | \nUse | \nPocket Budget example | \n
|---|---|---|
| Order, duplicates allowed, access by position | \nArray | \nThis month’s expenses, newest first | \n
| Look up a value by a key | \nDictionary | \nMonthly limit for each category | \n
| Membership and uniqueness, order irrelevant | \nSet | \nTags on one expense | \n
When in doubt, start with an array. It’s the collection SwiftUI lists display, and it’s the easiest to read in a debugger. Switch to a dictionary when you catch yourself searching an array for “the one with this name”, and to a set when you catch yourself checking for duplicates before every append.
\nAll three collections are values, not shared objects. var copy = amounts gives you an independent array: appending to copy leaves amounts untouched. That behaviour has a name, value semantics, and it gets its own lesson in the next section because it shapes how you design every model type.
switch in Swift is much more than a chain of equality checks. It matches ranges, binds values and adds conditions, and the compiler insists that every possible value is handled. Here is how Pocket Budget picks a status message from the percentage of a budget used:
func status(percentUsed: Int) -> String {\n switch percentUsed {\n case ..<0:\n return \"Check your numbers\"\n case 0..<50:\n return \"Plenty left\"\n case 50..<90:\n return \"On track\"\n case 90...100:\n return \"Nearly at your limit\"\n default:\n return \"Over budget\"\n }\n}\n\nprint(status(percentUsed: 95)) // Nearly at your limitTwo things differ from C-family languages. First, there is no break and no accidental fall-through: once a case matches, the switch is done. Second, remove the default and the build fails with “switch must be exhaustive”, because an Int has values your cases don’t cover. That error looks annoying with integers. With enums, in the next section, it becomes the most useful error in the language: add a new case to your model and the compiler lists every switch you forgot to update.
Cases can also bind the matched value and test it with where:
let used: Decimal = 15\nlet limit: Decimal = 10\n\nswitch used {\ncase 0:\n print(\"Nothing spent yet\")\ncase let amount where amount > limit:\n print(\"Over by \\(amount - limit)\")\ndefault:\n print(\"Within budget\")\n}Swift 5.9 also made switch and if usable as expressions, so let label = switch percentUsed { … } works when every case produces a single value. Use whichever form reads better; the exhaustiveness rule is the same.
You can now hold a month of data and make decisions about it. The loops in this lesson work, but they’re longhand. Next you’ll package logic into functions and replace most of those loops with closures like filter, map and reduce.
first, last and for-in over manual indexes.","[object Object]","A set stores unique values with fast contains checks and no order.","A Swift switch must cover every possible value, has no implicit fallthrough, and can match ranges and where conditions."],"further":[{"title":"Collection Types","url":"https://docs.swift.org/swift-book/documentation/the-swift-programming-language/collectiontypes/"},{"title":"Control Flow","url":"https://docs.swift.org/swift-book/documentation/the-swift-programming-language/controlflow/"}],"quiz":[{"q":"let limits = ["Food": 300]. What is the type of limits["Fun"]?
Int, with the value 0","why":"Swift doesn’t invent a value for a missing key. You ask for a default explicitly with default:."},{"text":"Int?, with the value nil","why":"Correct. Every dictionary subscript returns an optional, because any key might be missing."},{"text":"It crashes because the key doesn’t exist.","why":"That’s arrays with a bad index. Dictionaries return nil for a missing key."}],"answer":1},{"q":"You switch on a String category and write cases for “Food” and “Transport” only. What does the compiler say?
default: case.","why":"Correct. A String has infinitely many values, so only default can cover the rest."}],"answer":2},{"q":"Which collection fits “the tags a user has applied to an expense, with no duplicates”?
","options":[{"text":"Set<String>","why":"Correct. A set ignores duplicate inserts and answers contains quickly; tag order doesn’t matter."},{"text":"[String]","why":"An array allows duplicates, so you would have to check before every append."},{"text":"[String: Bool]","why":"It works, but it’s a set in disguise. Use the type that says what you mean."},{"text":"[Int: String]","why":"Integer keys add nothing here; you’d be rebuilding an array with extra steps."}],"answer":0},{"q":"let amounts: [Decimal] = []. What does amounts[0] do at runtime?
nil.","why":"That’s what amounts.first does. Subscripting an array never returns an optional."},{"text":"It crashes with “Index out of range”.","why":"Correct. Array subscripts trust you; an empty array has no index 0."},{"text":"It returns 0.","why":"Arrays don’t fill gaps with default values."}],"answer":1}],"exercise":{"kind":"choice","options":["a","b","c","d"],"correct":["c"],"title":"Predict the monthly report","prompt":"Read this snippet from Pocket Budget’s monthly report and predict exactly what it prints. Work through each category in order, tracking what the dictionary lookups return.
\nlet limits: [String: Decimal] = [\"Food\": 300, \"Transport\": 120]\nvar spent: [String: Decimal] = [\"Food\": 280]\nspent[\"Fun\", default: 0] += 15\n\nfor category in [\"Food\", \"Transport\", \"Fun\"] {\n let used = spent[category] ?? 0\n let limit = limits[category] ?? 0\n switch used {\n case 0:\n print(\"\\(category): nothing yet\")\n case let amount where amount > limit:\n print(\"\\(category): over by \\(amount - limit)\")\n default:\n print(\"\\(category): ok\")\n }\n}spent has two keys. Which ones, and with what values?","A missing key with ?? 0 gives 0. That applies to both spent and limits.","“Fun” has no limit at all, so its limit is 0. Is 15 greater than 0?"],"explanation":"spent["Fun", default: 0] += 15 adds a “Fun” entry, so spent is ["Food": 280, "Fun": 15].
default prints “Food: ok”.spent, so used is 0 and the first case prints “Transport: nothing yet”.limits["Fun"] ?? 0 is 0. 15 > 0 prints “Fun: over by 15”.The last line is a real product question in disguise: should a category with no limit count as “over”? The code says yes because ?? 0 chose that default. Defaults are decisions.
case 0 first. And Fun has no limit, which the code treats as 0."},"b":{"text":"Food: ok / Transport: nothing yet / Fun: nothing yet","why":"The default: subscript on line 3 wrote 15 into “Fun”, so it isn’t 0 any more."},"c":{"text":"Food: ok / Transport: nothing yet / Fun: over by 15","why":"Correct. Transport’s missing key becomes 0, and Fun’s missing limit becomes 0, so 15 counts as over."},"d":{"text":"Food: over by 20 / Transport: nothing yet / Fun: over by 15","why":"280 is below Food’s 300 limit, so the where amount > limit case doesn’t match."}}},"words":738,"readMinutes":4}