Category: 01. Basic

Swift Collection Protocols

Arrays, Sets and Dictionaries conform to Sequence and Collection, which provide common APIs like count and isEmpty.

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Common APIs

Use count and isEmpty to check collection size.

Example

let arr = [1, 2, 3]
print(arr.count)
print(arr.isEmpty)
let s: Set<Int> = [1, 2, 3]
print(s.contains(2))
print(s.isEmpty)

This example shows shared operations exposed via collection protocols.

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Indices

Use the indices property to iterate valid positions.

Example

let arr = [10, 20, 30]
for i in arr.indices {
  print("index: \(i), value: \(arr[i])")
}

Mutability (let vs var)

Use let for constants and var for variables.

Arrays and dictionaries declared with var can be modified in-place.

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Mutable vs Immutable

Declare with var when you intend to add/remove elements; use let to prevent mutations.

Example

let fixed = [1, 2]
print(fixed.count)
var bag = [1, 2]
bag.append(3)
print(bag.count)

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Dictionary Mutability

Mutate dictionaries declared with var by inserting or updating keys.

Example

var user = ["name": "Kai"]
user["city"] = "Oslo"
print(user.count)

Swift Sorting

Sort arrays with sorted() (returns new) or sort() (in-place). Provide a closure for custom order.

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Sort

Sort ascending using sorted() and descending in-place using sort(by:).

Example

var nums = [3, 1, 2]
let ascending = nums.sorted()
print(ascending)      // [1, 2, 3]
nums.sort(by: >)
print(nums)           // [3, 2, 1]

This example sorts ascending using sorted() and descending in-place using sort(by:).

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Case-Insensitive Sort

Provide a custom closure to sort strings without regard to case.

Example

let names = ["bob", "Alice", "dave"]
let caseInsensitive = names.sorted { $0.lowercased() < $1.lowercased() }
print(caseInsensitive) // ["Alice", "bob", "dave"]

Swift map, filter, reduce

Transform and aggregate collections with map, filter, and reduce.

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Transform and aggregate with map/filter/reduce

Use map to transform elements, filter to select a subset, and reduce to combine into a single result.

Example

let nums = [1, 2, 3, 4]
let doubled = nums.map { $0 * 2 }
print(doubled)
let evens = nums.filter { $0 % 2 == 0 }
print(evens)
let sum = nums.reduce(0, +)
print(sum)

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Convert and Sum

Use compactMap to convert valid strings to numbers, then reduce to sum.

Example

let raw = ["1", "x", "2", "3"]
let ints = raw.compactMap { Int($0) }   // [1, 2, 3]
let total = ints.reduce(0, +)
print(total)

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Swift Dictionaries

Dictionaries store key-value pairs.

Lookups are fast by key.

Use subscripting to read and write values.

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Basics

Example

var ages: [String: Int] = ["Kai": 30]
ages["Elisabeth"] = 25
print(ages["Kai"] ?? 0)

This example adds and reads values using dictionary subscripting.

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Iterate Keys and Values

Loop through a dictionary to access keys and values.

Example

let ages = ["Kai": 30, "Elisabeth": 25]
for k in ages.keys.sorted() {
  print("\(k): \(ages[k]!)")
}

Swift Sets

Sets store unique values with no defined order.

Test membership with contains, and use union/intersection operations.

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Deduplicate Values

Use sets to deduplicate values from an array.

Example

var letters: Set<Character> = ["a", "b", "a"]
print(letters.contains("a"))

This example creates a Set which deduplicates values and checks membership.

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Set Operations

Combine or compare sets using union, intersection and subtracting.

Example

let a: Set<Int> = [1, 2, 3]
let b: Set<Int> = [3, 4]
print(a.union(b).sorted())         // [1, 2, 3, 4]
print(a.intersection(b).sorted())  // [3]
print(a.subtracting(b).sorted())   // [1, 2]

Swift Collections

Use arrays, dictionaries, and sets to store ordered lists, key-value pairs, and unique items.

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Arrays

Arrays store ordered lists of values.

Syntax: [Type] for type, append with .append, count with .count, access with array[index].

Example

var nums: [Int] = [1, 2, 3]
nums.append(4)
print(nums.count)    // 4
print(nums[0])       // 1

This example creates an [Int] array, appends a value, reads the count, and accesses the first element.

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Dictionaries

Dictionaries store key-value pairs.

Syntax: [Key: Value] for type, assign with dict[key] = value, read with dict[key] ?? default.

Example

var ages: [String: Int] = ["Kai": 30]
ages["Elisabeth"] = 25
print(ages["Kai"] ?? 0)

This example defines a [String: Int] dictionary, inserts a key, and reads with nil-coalescing.

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Sets

Sets store unique items.

Syntax: Set<Element> or literal, test membership with .contains.

Example

var letters: Set<Character> = ["a", "b", "a"]
print(letters.contains("a"))

Swift Break/Continue

Use break to exit a loop early, and continue to skip to the next iteration.

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break

Stop the loop immediately when a condition is met.

Example

for i in 1...10 {
  if i == 4 { break }
  print(i)
}

This example stops printing when i reaches 4.

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continue

Skip the rest of the current iteration but keep looping.

Example

for i in 1...5 {
  if i % 2 == 0 { continue }
  print(i) // only odd numbers
}

Swift For Loop (Real-Life)

Use loops to process arrays, paginate data, or aggregate results.

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Process arrays and aggregate results

Loop over a collection and accumulate values to compute totals like sums or averages.

Example

let numbers = [1,2,3,4,5]
var sum = 0
for n in numbers { sum += n }
print(sum)

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Filter Even Numbers

Filter a collection to keep only the values you need.

Example

let numbers = [1,2,3,4,5]
let evens = numbers.filter { $0 % 2 == 0 }
print(evens)

Swift For-Each Loop

Use forEach to iterate sequences with a closure.

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Iterate with forEach closures

Pass a closure to forEach to process each element of a sequence.

Example

["A","B","C"].forEach { print($0) }

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Enumerated forEach

Use enumerated() with forEach to get index and value.

Example

let items = ["A","B","C"]
items.enumerated().forEach { print("\($0.offset): \($0.element)") }