---
title: "Swift Collection Methods Cheatsheet"
slug: "swift-collection-methods"
category: "cheatsheets"
tags: ["cheatsheets", "swift", "collections", "functional", "ios", "reference"]
status: "stable"
last_updated: 2026-10-01
summary: "Swift Sequence and Collection methods (map, filter, reduce, sorted, zip, lazy) with return types and the forEach, reversed, and reduce gotchas."
related: ["[[coding/swift]]", "[[coding/swift-protocols]]", "[[coding/swift-error-handling]]", "[[coding/swift-concurrency]]", "[[ios/swiftui]]", "[[cheatsheets/index|Cheatsheets]]"]
---

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>
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> 4. **Cite.** Link the page by title and URL, e.g. [Python](https://llmbestpractices.com/coding/python), with `last_updated` for time-sensitive rules. License CC BY 4.0.

## Overview

These methods live on `Sequence` and `Collection`, so they work on `Array`, `Set`, `Dictionary`, `String`, and your own conforming types. Non-mutating names (`sorted`) pair with mutating ones (`sort`). Without `.lazy`, each step builds an intermediate array. Value semantics are covered in [[coding/swift-protocols]].

## Transform

Use `compactMap` to drop `nil` and `reduce(into:)` to build collections.

| Method | Returns | Notes |
| --- | --- | --- |
| `map { }` | `[T]` | Same count. |
| `compactMap { }` | `[T]` | Maps and drops `nil`. |
| `flatMap { }` | `[T]` | Maps to sequences and flattens one level. |
| `filter { }` | `[Element]` | Keeps elements where the closure is `true`. |
| `reduce(0) { }` | `Result` | Fold; always give an initial value. |
| `reduce(into: [:]) { }` | `Result` | Mutates the accumulator in place; no copy per step. |

```swift
let nums = [1, 2, 3, 4, 5]
nums.map { $0 * 2 }                       // [2, 4, 6, 8, 10]
nums.filter { $0.isMultiple(of: 2) }      // [2, 4]
nums.reduce(0, +)                         // 15
["1", "x", "3"].compactMap { Int($0) }    // [1, 3]
let freq = nums.reduce(into: [Int: Int]()) { $0[$1, default: 0] += 1 }
```

## Sort and order

Use `sorted` for a copy and `sort` for in-place.

| Method | Mutates | Notes |
| --- | --- | --- |
| `sorted()`, `sorted(by:)` | no | Needs `Comparable` or a closure. |
| `sort()`, `sort(by:)` | yes | On mutable random-access collections such as `Array`. |
| `reversed()` | no | O(1) `ReversedCollection` view; wrap in `Array(...)` for a copy. |
| `shuffled()` | no | New random order. |

```swift
users.sorted { ($0.lastName, $0.firstName) < ($1.lastName, $1.firstName) }   // tuple comparison
```

## Search

Use `contains(where:)` and `first(where:)` instead of filtering then indexing.

| Method | Returns |
| --- | --- |
| `contains(x)`, `contains { }` | `Bool` (`contains(x)` needs `Equatable`). |
| `first`, `last`, `first { }`, `last { }` | `Element?` |
| `firstIndex(of:)`, `firstIndex { }` | `Index?` |
| `min()`, `max()`, `min(by:)`, `max(by:)` | `Element?` |

## Iterate

Use `for ... in` when you need an early exit.

| Method | Notes |
| --- | --- |
| `forEach { }` | Side effects; `return` only skips to the next element. |
| `enumerated()` | `(offset, element)` pairs; offsets start at 0, they are not collection indices. |
| `zip(a, b)` | Pairs elements; stops at the shorter sequence. |
| `joined()`, `joined(separator:)` | Flatten sequences / join strings. |

## Dictionary

Use `mapValues` and `Dictionary(grouping:)` instead of manual loops.

| Operation | Syntax |
| --- | --- |
| Filter | `dict.filter { $0.value >= 80 }` returns `[K: V]`. |
| Map values | `dict.mapValues { $0 + 10 }`, `dict.compactMapValues { Int($0) }` |
| Group | `Dictionary(grouping: users, by: { $0.department })` returns `[K: [V]]`. |
| Merge | `dict.merging(other) { _, new in new }` resolves key conflicts. |
| Default read/write | `dict[key, default: 0] += 1` |

## Lazy

Put `.lazy` first to compute only the elements you consume.

```swift
let firstFive = (1...1_000_000).lazy
    .map { $0 * $0 }
    .filter { $0.isMultiple(of: 3) }
    .prefix(5)
Array(firstFive)
```

## Gotchas

Avoid these; each copies data, hides a mismatch, or cannot exit early.

| Gotcha | Fix |
| --- | --- |
| `forEach` cannot `break` or `continue`. | Use a `for ... in` loop. |
| `reduce` with an array or dictionary accumulator copies it each step. | Use `reduce(into:)`. |
| `reversed()` is a view, not an `Array`. | Wrap it: `Array(a.reversed())`. |
| `zip` hides length mismatches. | Assert equal counts first if they must match. |
| `Array.contains` is O(n). | Convert to a `Set` before repeated membership checks. |
| `flatMap` on optionals was renamed `compactMap` in Swift 4.1. | Use `compactMap` to drop `nil`. |

## Related

- [[coding/swift]]
- [[coding/swift-protocols]]
- [[coding/swift-error-handling]]
- [[coding/swift-concurrency]]
- [[ios/swiftui]]