Swift Academy · Lesson

reduce and reduce(into:)

Aggregate collections into a single value.

Lesson 3 of 413 steps

reduce and reduce(into:) is a free Swift Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

What Is reduce?

reduce collapses a whole sequence into a single value by repeatedly combining an accumulator with each element.

You give a starting value
And a closure (accumulator, element) -> accumulator

Summing with reduce

The classic example — add everything up:

let nums = [1, 2, 3, 4]
let total = nums.reduce(0) { acc, x in acc + x }
print(total)  // 10

Operator Shorthand

When the closure is just an operator, you can pass the operator function directly:

let nums = [1, 2, 3, 4]
print(nums.reduce(0, +))   // 10
print(nums.reduce(1, *))   // 24

Reducing to a Different Type

The result type can differ from the element type:

let words = ["a", "b", "c"]
let joined = words.reduce("") { $0 + $1.uppercased() }
print(joined)  // "ABC"

Finding a Max with reduce

You can compute aggregates beyond sums:

let nums = [3, 9, 2, 7]
let maxVal = nums.reduce(Int.min) { max($0, $1) }
print(maxVal)  // 9

The Cost of reduce

Plain reduce creates a brand new accumulator each step. For value types like String or Array that means copying the whole accumulator on every element — O(n²) work for large inputs.

Enter reduce(into:)

reduce(into:) passes the accumulator as an inout parameter, so you mutate it in place instead of copying:

let nums = [1, 2, 2, 3, 3, 3]
let counts = nums.reduce(into: [Int: Int]()) { dict, n in
    dict[n, default: 0] += 1
}
print(counts)  // [1: 1, 2: 2, 3: 3]

Building an Array Efficiently

reduce(into:) shines when accumulating into a collection:

let nums = [1, 2, 3, 4, 5]
let evensSquared = nums.reduce(into: [Int]()) { result, n in
    if n % 2 == 0 { result.append(n * n) }
}
print(evensSquared)  // [4, 16]

reduce vs reduce(into:)

Same result, different performance characteristics:

let parts = ["a", "b", "c"]
let a = parts.reduce("") { $0 + $1 }            // copies each step
let b = parts.reduce(into: "") { $0 += $1 }    // mutates in place
print(a, b)  // abc abc

Grouping with reduce(into:)

A frequent pattern: bucket items by a key:

let names = ["Ann", "Al", "Bo", "Cy"]
let byInitial = names.reduce(into: [Character: [String]]()) { dict, name in
    dict[name.first!, default: []].append(name)
}
print(byInitial[Character("A")]!)  // ["Ann", "Al"]

Choosing Between Them

Use reduce for cheap scalar accumulators (Int, Double). Use reduce(into:) whenever the accumulator is a collection or other large value type to avoid copies.

let nums = [1, 2, 3]
print(nums.reduce(0, +))                          // scalar: plain reduce
print(nums.reduce(into: Set<Int>()) { $0.insert($1) }) // collection: into

Quick Check

Why prefer reduce(into:) when building a dictionary or array?

Answer it in the interactive lesson

Recap

You learned reduction:

reduce(initial) { acc, x in ... } collapses a sequence to one value
Operator shorthand: reduce(0, +)
reduce(into:) mutates an inout accumulator — use it for collections
Great for counts, grouping, and joins

Next: flatMap and chaining.

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Frequently Asked Questions

Is the “reduce and reduce(into:)” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “reduce and reduce(into:)”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “reduce and reduce(into:)”?

Aggregate collections into a single value. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

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