Ben Morrow - 5 months ago 24

Swift Question

Swift 3 recently added

`joined()`

`let array = [[1,2,3],[4,5,6],[7,8,9]]`

let j = Array(array.joined())

let f = array.flatMap{$0}

They both flatten the nested

`array`

`[1, 2, 3, 4, 5, 6, 7, 8, 9]`

Answer

When it comes just to flattening 2D arrays (without any transformations or separators applied, see @dfri's answer for more info about that aspect), `array.flatMap{$0}`

and `Array(array.joined())`

are both conceptually the same and have similar performance.

The main difference between `flatMap(_:)`

and `joined()`

(note that this isn't a new method, it has just been renamed from `flatten()`

) is that `joined()`

is always lazily applied (for arrays, it returns a special `FlattenBidirectionalCollection<Base>`

).

Therefore in terms of performance, it makes sense to use `joined()`

over `flatMap(_:)`

in situations where you only want to iterate over part of a flattened sequence (without applying any transformations). For example:

```
let array2D = [[2, 3], [8, 10], [9, 5], [4, 8]]
if array2D.joined().contains(8) {
print("contains 8")
} else {
print("doesn't contain 8")
}
```

Because `joined()`

is lazily applied & `contains(_:)`

will stop iterating upon finding a match, only the first two inner arrays will have to be 'flattened' to find the element `8`

from the 2D array. Although, as @dfri correctly notes below, you are also able to lazily apply `flatMap(_:)`

through the use of a `LazySequence`

/`LazyCollection`

– which can be created through the `lazy`

property. This would be ideal for lazily applying both a transformation & flattening a given 2D sequence.

In cases where `joined()`

is iterated fully through, it is conceptually no different from using `flatMap{$0}`

. Therefore, these are all valid (and conceptually identical) ways of flattening a 2D array:

```
array2D.joined().map{$0}
```

```
Array(array2D.joined())
```

```
array2D.flatMap{$0}
```

In terms of performance, `flatMap(_:)`

is documented as having a time-complexity of:

O(m + n), where m is the length of this sequence and n is the length of the result

This is because its implementation is simply:

```
public func flatMap<SegmentOfResult : Sequence>(
_ transform: (${GElement}) throws -> SegmentOfResult
) rethrows -> [SegmentOfResult.${GElement}] {
var result: [SegmentOfResult.${GElement}] = []
for element in self {
result.append(contentsOf: try transform(element))
}
return result
}
}
```

As `append(contentsOf:)`

has a time-complexity of O(n), where n is the length of sequence to append, we get an overall time-complexity of O(m + n), where m will be total length of all sequences appended, and n is the length of the 2D sequence.

When it comes to `joined()`

, there is no documented time-complexity, as it is lazily applied. However, the main bit of source code to consider is the implementation of `FlattenIterator`

, which is used to iterate over the flattened contents of a 2D sequence (which will occur upon using `map(_:)`

or the `Array(_:)`

initialiser with `joined()`

).

```
public mutating func next() -> Base.Element.Iterator.Element? {
repeat {
if _fastPath(_inner != nil) {
let ret = _inner!.next()
if _fastPath(ret != nil) {
return ret
}
}
let s = _base.next()
if _slowPath(s == nil) {
return nil
}
_inner = s!.makeIterator()
}
while true
}
```

Here `_base`

is the base 2D sequence, `_inner`

is the current iterator from one of the inner sequences, and `_fastPath`

& `_slowPath`

are hints to the compiler to aid with branch prediction.

Assuming I'm interpreting this code correctly & the full sequence is iterated through, this also has a time complexity of O(m + n), where m is the length of the sequence, and n is the length of the result. This is because it goes through each outer iterator and each inner iterator to get the flattened elements.

So, performance wise, `Array(array.joined())`

and `array.flatMap{$0}`

both have the same time complexity.

Although what's interesting is that running a quick benchmark reveals that:

```
import QuartzCore
func benchmark(repeatCount:Int = 1, name:String? = nil, closure:() -> ()) {
let d = CACurrentMediaTime()
for _ in 0..<repeatCount {
closure()
}
let d1 = CACurrentMediaTime()-d
print("Benchmark of \(name ?? "closure") took \(d1) seconds")
}
let arr = [[Int]](repeating: [Int](repeating: 0, count: 1000), count: 1000)
benchmark {
_ = arr.flatMap{$0} // 0.3377s
}
benchmark {
_ = Array(arr.joined()) // 0.514s
}
benchmark {
_ = arr.joined().map{$0} // 1.573s
}
```

`flatMap(_:)`

appears to be the fastest by a fraction of a second. I suspect that `joined()`

being slower could be due to the branching that occurs within the `FlattenIterator`

(although the hints to the compiler minimise this cost) – although just why `map(_:)`

is so slow, I'm not too sure. Would certainly be interested to know if anyone else knows more about this.