Is there a more elegant way to filter with an additional parameter (or map, reduce).
When I filter with a single parameter, we get a beautiful easy to ready syntax
let numbers = Array(1...10)
func isGreaterThan5(number:Int) -> Bool {
return number > 5
}
numbers.filter(isGreaterThan5)
However, if I need to pass an additional parameter to my function it turns out ugly
func isGreaterThanX(number:Int,x:Int) -> Bool {
return number > x
}
numbers.filter { (number) -> Bool in
isGreaterThanX(number: number, x: 8)
}
I would like to use something like
numbers.filter(isGreaterThanX(number: $0, x: 3))
but this gives a compile error annonymous closure argument not contained in a closure
You could change your function to return a closure which serves
as predicate for the filter method:
func isGreaterThan(_ lowerBound: Int) -> (Int) -> Bool {
return { $0 > lowerBound }
}
let filtered = numbers.filter(isGreaterThan(5))
isGreaterThan is a function taking an Int argument and returning
a closure of type (Int) -> Bool. The returned closure "captures"
the value of the given lower bound.
If you make the function generic then it can be used with
other comparable types as well:
func isGreaterThan<T: Comparable>(_ lowerBound: T) -> (T) -> Bool {
return { $0 > lowerBound }
}
print(["D", "C", "B", "A"].filter(isGreaterThan("B")))
In this particular case however, a literal closure is also easy to read:
let filtered = numbers.filter( { $0 > 5 })
And just for the sake of completeness: Using the fact that
Instance Methods are Curried Functions in Swift, this would work as well:
extension Comparable {
func greaterThanFilter(value: Self) -> Bool {
return value > self
}
}
let filtered = numbers.filter(5.greaterThanFilter)
but the "reversed logic" might be confusing.
Remark: In earlier Swift versions you could use a curried function
syntax:
func isGreaterThan(lowerBound: Int)(value: Int) -> Bool {
return value > lowerBound
}
but this feature has been removed in Swift 3.
I'm new to Swift. I was trying to implement a binary tree with recursive enumerations and generics:
enum BinaryTree<T> {
indirect case Node(T, BinaryTree<T>, BinaryTree<T>)
case Nothing
}
func inorder<T>(_ root: BinaryTree<T>) -> [T] {
switch root {
case .Nothing:
return []
case let .Node(val, left, right):
return inorder(left) + [val] + inorder(right)
}
}
Here's the error I got:
$ swift ADT.swift
ADT.swift:83:20: error: cannot convert value of type 'BinaryTree<T>' to expected argument type 'BinaryTree<_>'
return inorder(left) + [val] + inorder(right)
^~~~
However, this works:
func inorder<T>(_ root: BinaryTree<T>) -> [T] {
switch root {
case .Nothing:
return []
case let .Node(val, left, right):
let l = inorder(left)
let r = inorder(right)
return l + [val] + r
}
}
Is there any mistakes in my syntax? Thanks!
I'm using Swift 3.0.
Update
So I tried to condense the problem into minimal example code that fails to compile and asked a question myself and filed SR-4304. Turns out the answer is that this really is a bug in the compiler.
Original Answer
As far as I can tell, your syntax is perfectly valid. It seems that the Swift compiler’s type inference seems to need a nudge in the right direction which your second solution apparently provides. As I experienced several similar problems in the past, especially regarding the + operator, your question inspired me to try several other ways to join the arrays. These all work (I am just showing the return statements and supporting functions for the last three cases):
return (inorder(left) as [T]) + [val] + inorder(right)
return Array([inorder(left), [val], inorder(right)].joined())
return [inorder(left), [val], inorder(right)].reduce([], +)
return [inorder(left), [val], inorder(right)].flatMap { $0 }
func myjoin1<T>(_ arrays: [T]...) -> [T]
{
return arrays.reduce([], +)
}
return myjoin1(inorder(left), [val], inorder(right))
func myjoin2<T>(_ array1: [T], _ array2: [T], _ array3: [T]) -> [T]
{
return array1 + array2 + array3
}
return myjoin2(inorder(left), [val], inorder(right))
extension Array
{
func appending(_ array: [Element]) -> [Element]
{
return self + array
}
}
return inorder(left).appending([val]).appending(inorder(right))
Calling the operator as a function compiles, too:
return (+)(inorder(left), [val]) + inorder(right)
It would be great if somebody with more intimate knowledge of the Swift compiler could shed some light on this.
I'd like a function runningSum on an array of numbers a (or any ordered collection of addable things) that returns an array of the same length where each element i is the sum of all elements in A up to an including i.
Examples:
runningSum([1,1,1,1,1,1]) -> [1,2,3,4,5,6]
runningSum([2,2,2,2,2,2]) -> [2,4,6,8,10,12]
runningSum([1,0,1,0,1,0]) -> [1,1,2,2,3,3]
runningSum([0,1,0,1,0,1]) -> [0,1,1,2,2,3]
I can do this with a for loop, or whatever. Is there a more functional option? It's a little like a reduce, except that it builds a result array that has all the intermediate values.
Even more general would be to have a function that takes any sequence and provides a sequence that's the running total of the input sequence.
The general combinator you're looking for is often called scan, and can be defined (like all higher-order functions on lists) in terms of reduce:
extension Array {
func scan<T>(initial: T, _ f: (T, Element) -> T) -> [T] {
return self.reduce([initial], combine: { (listSoFar: [T], next: Element) -> [T] in
// because we seeded it with a non-empty
// list, it's easy to prove inductively
// that this unwrapping can't fail
let lastElement = listSoFar.last!
return listSoFar + [f(lastElement, next)]
})
}
}
(But I would suggest that that's not a very good implementation.)
This is a very useful general function, and it's a shame that it's not included in the standard library.
You can then generate your cumulative sum by specializing the starting value and operation:
let cumSum = els.scan(0, +)
And you can omit the zero-length case rather simply:
let cumSumTail = els.scan(0, +).dropFirst()
Swift 4
The general sequence case
Citing the OP:
Even more general would be to have a function that takes any sequence
and provides a sequence that's the running total of the input
sequence.
Consider some arbitrary sequence (conforming to Sequence), say
var seq = 1... // 1, 2, 3, ... (CountablePartialRangeFrom)
To create another sequence which is the (lazy) running sum over seq, you can make use of the global sequence(state:next:) function:
var runningSumSequence =
sequence(state: (sum: 0, it: seq.makeIterator())) { state -> Int? in
if let val = state.it.next() {
defer { state.sum += val }
return val + state.sum
}
else { return nil }
}
// Consume and print accumulated values less than 100
while let accumulatedSum = runningSumSequence.next(),
accumulatedSum < 100 { print(accumulatedSum) }
// 1 3 6 10 15 21 28 36 45 55 66 78 91
// Consume and print next
print(runningSumSequence.next() ?? -1) // 120
// ...
If we'd like (for the joy of it), we could condense the closure to sequence(state:next:) above somewhat:
var runningSumSequence =
sequence(state: (sum: 0, it: seq.makeIterator())) {
(state: inout (sum: Int, it: AnyIterator<Int>)) -> Int? in
state.it.next().map { (state.sum + $0, state.sum += $0).0 }
}
However, type inference tends to break (still some open bugs, perhaps?) for these single-line returns of sequence(state:next:), forcing us to explicitly specify the type of state, hence the gritty ... in in the closure.
Alternatively: custom sequence accumulator
protocol Accumulatable {
static func +(lhs: Self, rhs: Self) -> Self
}
extension Int : Accumulatable {}
struct AccumulateSequence<T: Sequence>: Sequence, IteratorProtocol
where T.Element: Accumulatable {
var iterator: T.Iterator
var accumulatedValue: T.Element?
init(_ sequence: T) {
self.iterator = sequence.makeIterator()
}
mutating func next() -> T.Element? {
if let val = iterator.next() {
if accumulatedValue == nil {
accumulatedValue = val
}
else { defer { accumulatedValue = accumulatedValue! + val } }
return accumulatedValue
}
return nil
}
}
var accumulator = AccumulateSequence(1...)
// Consume and print accumulated values less than 100
while let accumulatedSum = accumulator.next(),
accumulatedSum < 100 { print(accumulatedSum) }
// 1 3 6 10 15 21 28 36 45 55 66 78 91
The specific array case: using reduce(into:_:)
As of Swift 4, we can use reduce(into:_:) to accumulate the running sum into an array.
let runningSum = arr
.reduce(into: []) { $0.append(($0.last ?? 0) + $1) }
// [2, 4, 6, 8, 10, 12]
By using reduce(into:_:), the [Int] accumulator will not be copied in subsequent reduce iterations; citing the Language reference:
This method is preferred over reduce(_:_:) for efficiency when the
result is a copy-on-write type, for example an Array or a
Dictionary.
See also the implementation of reduce(into:_:), noting that the accumulator is provided as an inout parameter to the supplied closure.
However, each iteration will still result in an append(_:) call on the accumulator array; amortized O(1) averaged over many invocations, but still an arguably unnecessary overhead here as we know the final size of the accumulator.
Because arrays increase their allocated capacity using an exponential
strategy, appending a single element to an array is an O(1) operation
when averaged over many calls to the append(_:) method. When an array
has additional capacity and is not sharing its storage with another
instance, appending an element is O(1). When an array needs to
reallocate storage before appending or its storage is shared with
another copy, appending is O(n), where n is the length of the array.
Thus, knowing the final size of the accumulator, we could explicitly reserve such a capacity for it using reserveCapacity(_:) (as is done e.g. for the native implementation of map(_:))
let runningSum = arr
.reduce(into: [Int]()) { (sums, element) in
if let sum = sums.last {
sums.append(sum + element)
}
else {
sums.reserveCapacity(arr.count)
sums.append(element)
}
} // [2, 4, 6, 8, 10, 12]
For the joy of it, condensed:
let runningSum = arr
.reduce(into: []) {
$0.append(($0.last ?? ($0.reserveCapacity(arr.count), 0).1) + $1)
} // [2, 4, 6, 8, 10, 12]
Swift 3: Using enumerated() for subsequent calls to reduce
Another Swift 3 alternative (with an overhead ...) is using enumerated().map in combination with reduce within each element mapping:
func runningSum(_ arr: [Int]) -> [Int] {
return arr.enumerated().map { arr.prefix($0).reduce($1, +) }
} /* thanks #Hamish for improvement! */
let arr = [2, 2, 2, 2, 2, 2]
print(runningSum(arr)) // [2, 4, 6, 8, 10, 12]
The upside is you wont have to use an array as the collector in a single reduce (instead repeatedly calling reduce).
Just for fun: The running sum as a one-liner:
let arr = [1, 2, 3, 4]
let rs = arr.map({ () -> (Int) -> Int in var s = 0; return { (s += $0, s).1 } }())
print(rs) // [1, 3, 6, 10]
It does the same as the (updated) code in JAL's answer, in particular,
no intermediate arrays are generated.
The sum variable is captured in an immediately-evaluated closure returning the transformation.
If you just want it to work for Int, you can use this:
func runningSum(array: [Int]) -> [Int] {
return array.reduce([], combine: { (sums, element) in
return sums + [element + (sums.last ?? 0)]
})
}
If you want it to be generic over the element type, you have to do a lot of extra work declaring the various number types to conform to a custom protocol that provides a zero element, and (if you want it generic over both floating point and integer types) an addition operation, because Swift doesn't do that already. (A future version of Swift may fix this problem.)
Assuming an array of Ints, sounds like you can use map to manipulate the input:
let arr = [0,1,0,1,0,1]
var sum = 0
let val = arr.map { (sum += $0, sum).1 }
print(val) // "[0, 1, 1, 2, 2, 3]\n"
I'll keep working on a solution that doesn't use an external variable.
I thought I'd be cool to extend Sequence with a generic scan function as is suggested in the great first answer.
Given this extension, you can get the running sum of an array like this: [1,2,3].scan(0, +)
But you can also get other interesting things…
Running product: array.scan(1, *)
Running max: array.scan(Int.min, max)
Running min: array.scan(Int.max, min)
Because the implementation is a function on Sequence and returns a Sequence, you can chain it together with other sequence functions. It is efficient, having linear running time.
Here's the extension…
extension Sequence {
func scan<Result>(_ initialResult: Result, _ nextPartialResult: #escaping (Result, Self.Element) -> Result) -> ScanSequence<Self, Result> {
return ScanSequence(initialResult: initialResult, underlying: self, combine: nextPartialResult)
}
}
struct ScanSequence<Underlying: Sequence, Result>: Sequence {
let initialResult: Result
let underlying: Underlying
let combine: (Result, Underlying.Element) -> Result
typealias Iterator = ScanIterator<Underlying.Iterator, Result>
func makeIterator() -> Iterator {
return ScanIterator(previousResult: initialResult, underlying: underlying.makeIterator(), combine: combine)
}
var underestimatedCount: Int {
return underlying.underestimatedCount
}
}
struct ScanIterator<Underlying: IteratorProtocol, Result>: IteratorProtocol {
var previousResult: Result
var underlying: Underlying
let combine: (Result, Underlying.Element) -> Result
mutating func next() -> Result? {
guard let nextUnderlying = underlying.next() else {
return nil
}
previousResult = combine(previousResult, nextUnderlying)
return previousResult
}
}
One solution using reduce:
func runningSum(array: [Int]) -> [Int] {
return array.reduce([], combine: { (result: [Int], item: Int) -> [Int] in
if result.isEmpty {
return [item] //first item, just take the value
}
// otherwise take the previous value and append the new item
return result + [result.last! + item]
})
}
I'm very late to this party. The other answers have good explanations. But none of them have provided the initial result, in a generic way. This implementation is useful to me.
public extension Sequence {
/// A sequence of the partial results that `reduce` would employ.
func scan<Result>(
_ initialResult: Result,
_ nextPartialResult: #escaping (Result, Element) -> Result
) -> AnySequence<Result> {
var iterator = makeIterator()
return .init(
sequence(first: initialResult) { partialResult in
iterator.next().map {
nextPartialResult(partialResult, $0)
}
}
)
}
}
extension Sequence where Element: AdditiveArithmetic & ExpressibleByIntegerLiteral {
var runningSum: AnySequence<Element> { scan(0, +).dropFirst() }
}
As an exercise, I'm trying to extend Array in Swift to add a sum() member function. This should be type safe in a way that I want a call to sum() to compile only if the array holds elements that can be added up.
I tried a few variants of something like this:
extension Array {
func sum<U : _IntegerArithmeticType where U == T>() -> Int {
var acc = 0
for elem in self {
acc += elem as Int
}
return acc
}
}
The idea was to say, “OK, this is a generic function, the generic type must be something like an Int, and must also be the same as T, the type of the elements of the array”. But the compiler complains: “Same-type requirement make generic parameters U and T equivalent”. That's right, and they should be, with the additional contraint T : _IntegerArithmeticType.
Why isn't the compiler letting me do this? How can I do it?
(I know that I should later fix how things are added up and what the return type exactly is, but I'm stuck at the type constraint for now.)
As per Martin R's comment, this is not currently possible. The thing I'm tempted to use in this particular situation would be an explicit passing of a T -> Int conversion function:
extension Array {
func sum(toInt: T -> Int?) -> Int {
var acc = 0
for elem in self {
if let i = toInt(elem) {
acc += i
}
}
return acc
}
}
Then I can write stuff like this:
func itself<T>(t: T) -> T {
return t
}
let ss = ["1", "2", "3", "4", "five"].sum { $0.toInt() }
let si = [1, 2, 3, 4].sum(itself)
An explicit function has to be passed, though. The (itself) part can of course be replaced by { $0 }. (Others have called the itself function identity.)
Note that an A -> B function can be passed when A -> B? is needed.