I have got a tutorial from which I was studying Swift & about Subscript Options. The below is a simple code of documentation's example on this topic.
struct Matrix {
let rows: Int, columns: Int
var print: [Double]
init(rows: Int, columns: Int) {
self.rows = rows
self.columns = columns
print = Array(count: rows * columns, repeatedValue: 0.0)
}
subscript(row: Int, column: Int) -> Double {
get {
return print[(row * columns) + column]
}
set {
print[(row * columns) + column] = newValue
}
}
}
var mat = Matrix(rows: 3, columns: 3)
mat[0,0] = 1.0
mat[0,1] = 2.0
mat[1,0] = 3.0
mat[1,1] = 5.0
println("\(mat[0,0])")
println("\(mat[0,1])")
println("\(mat[1,0])")
println("\(mat[1,1])")
Can any one please explain me what is going on here. I am unable to make out about Subscript Options from this example. And might be there would be many like me. Thanks in advance.
Subscripts enable you to query instances of a type by writing one or
more values in square brackets after the instance name. Their syntax
is similar to both instance method syntax and computed property
syntax. You write subscript definitions with the subscript keyword,
and specify one or more input parameters and a return type, in the
same way as instance methods. Unlike instance methods, subscripts can
be read-write or read-only. This behavior is communicated by a getter
and setter in the same way as for computed properties
I think that Apple description is quit good.
If you like Ray Wenderlich - see here.
Related
In The Swift Programming Language, it says:
Functions can also take a variable number of arguments, collecting them into an array.
func sumOf(numbers: Int...) -> Int {
...
}
When I call such a function with a comma-separated list of numbers (`sumOf(1, 2, 3, 4), they are made available as an array inside the function.
Question: what if I already have an array of numbers that I want to pass to this function?
let numbers = [1, 2, 3, 4]
sumOf(numbers)
This fails with a compiler error, “Could not find an overload for '__conversion' that accepts the supplied arguments”. Is there a way to turn an existing array into a list of elements that I can pass to a variadic function?
Splatting is not in the language yet, as confirmed by the devs. Workaround for now is to use an overload or wait if you cannot add overloads.
Here's a work around that I found. I know it's not exactly what you want, but it seems to be working.
Step 1: Declare the function you'd like with an array instead of variadic arguments:
func sumOf(numbers: [Int]) -> Int {
var total = 0
for i in numbers {
total += i
}
return total
}
Step 2: Call this from within your variadic function:
func sumOf(numbers: Int...) -> Int {
return sumOf(numbers)
}
Step 3: Call Either Way:
var variadicSum = sumOf(1, 2, 3, 4, 5)
var arraySum = sumOf([1, 2, 3, 4, 5])
It seems strange, but it is working in my tests. Let me know if this causes unforeseen problems for anyone. Swift seems to be able to separate the difference between the two calls with the same function name.
Also, with this method if Apple updates the language as #manojid's answer suggests, you'll only need to update these functions. Otherwise, you'll have to go through and do a lot of renaming.
You can cast the function:
typealias Function = [Int] -> Int
let sumOfArray = unsafeBitCast(sumOf, Function.self)
sumOfArray([1, 2, 3])
You can use a helper function as such:
func sumOf (numbers : [Int]) -> Int { return numbers.reduce(0, combine: +) }
func sumOf (numbers : Int...) -> Int { return sumOf (numbers) }
I did this (Wrapper + Identity Mapping):
func addBarButtonItems(types: REWEBarButtonItemType...) {
addBarButtonItems(types: types.map { $0 })
}
func addBarButtonItems(types: [REWEBarButtonItemType]) {
// actual implementation
}
I know this response does not answer your exact question, but I feel its worth noting. I too was starting to play with Swift and immediately ran into a similar question. Manojlds answer is better for your question, I agree, but again, another workaround I came up with. I do happen to like Logan's better too.
In my case I just wanted to pass an array:
func sumOf(numbers: Array<Int>) -> Int {
var sum = 0
for number in numbers {
sum += number
}
return sum
}
var someNums = [8,7,2,9,12]
sumOf(someNums)
sumOf([10, 15, 20])
Just wanted to share, in case anyone else was thinking like me. Most of the time I would prefer pass the array like this, but I don't think the "Swiftly" yet. :)
Swift 5
This is an approach with #dynamicCallable feature that allows to avoid overloading or unsafeBitCast but you should make a specific struct to call:
#dynamicCallable
struct SumOf {
func dynamicallyCall(withArguments args: [Int]) -> Int {
return args.reduce(0, +)
}
}
let sum = SumOf()
// Use a dynamic method call.
sum(1, 2, 3) // 6
// Call the underlying method directly.
sum.dynamicallyCall(withArguments: [1, 2, 3]) // 6
I am attempting to sort a mutable array in Swift 3.1.1, but get the same error every time:
No 'sort' candidates produce the expected contextual result type 'NSMutableArray'.
Is there a way to sort a mutable array (Ints only) in ascending order?
In my code, elements from options are being removed. Removed (the array) is adding the removed elements. At the end of the code I am attempting to add the elements from the removed array back to options and sort it.
// set up tiles
var options = NSMutableArray()
var removed = NSMutableArray()
for i in 1...49 {
options.add(i as Int)
print("options\(options.count)")
}
for i in 1...49 {
print("i = \(i)")
options.remove(i)
let tilea: Int = options[Int(arc4random_uniform(UInt32(options.count)))] as! Int
options.remove(tilea)
let tileb: Int = options[Int(arc4random_uniform(UInt32(options.count)))] as! Int
options.remove(tileb)
removed.add([i, tilea, tileb])
print(options.count)
if options.count < 20 {
options.add(removed)
options = options.sort {
$0 < $1
}
}
}
As already mentioned, in Swift you should really be using the Array<T> for this (aka, [T]) instead of NSMutableArray. For instance:
var options = [Int]()
when adding elements to it, use append (and, by the way, you can drop the type cast as well):
options.append(i)
options.append(contentsOf: [i, j, k])
finally, when sorting the array, use the sort function (it doesn't return a value; the array is sorted in-place):
options.sort()
and you don't need even to provide a comparation function since integers conform to the Comparable protocol.
NSMutableArray, among other Objective C types, was implicitly bridged to/from its Swift counterparts. In a move to lessen peoples (usually unnecessary) reliance on these Objective C types, this implicit bridging has been changed in Swift 3, and now needs an explicit type coercion (e.g nsArray as [Int])
I am using a 3rd party C library in my iOS application, which I am in the process of converting from Objective-C to Swift. I hit an obstacle when attempting to read one of the structs returned by the C library in Swift.
The struct looks similar to this:
typedef unsigned int LibUint;
typedef unsigned char LibUint8;
typedef struct RequestConfiguration_ {
LibUint8 names[30][128];
LibUint numberNames;
LibUint currentName;
} RequestConfiguration;
Which is imported into Swift as a Tuple containing 30 Tuples of 128 LibUint8 values. After a long time of trial and error using nested withUnsafePointer calls, I eventually began searching for solutions to iterating a Tuple in Swift.
What I ended up using is the following functions:
/**
* Perform iterator on every children of the type using reflection
*/
func iterateChildren<T>(reflectable: T, #noescape iterator: (String?, Any) -> Void) {
let mirror = Mirror(reflecting: reflectable)
for i in mirror.children {
iterator(i.label, i.value)
}
}
/**
* Returns a String containing the characters within the Tuple
*/
func libUint8TupleToString<T>(tuple: T) -> String {
var result = [CChar]()
let mirror = Mirror(reflecting: tuple)
for child in mirror.children {
let char = CChar(child.value as! LibUint8)
result.append(char)
// Null reached, skip the rest.
if char == 0 {
break;
}
}
// Always null terminate; faster than checking if last is null.
result.append(CChar(0))
return String.fromCString(result) ?? ""
}
/**
* Returns an array of Strings by decoding characters within the Tuple
*/
func libUint8StringsInTuple<T>(tuple: T, length: Int = 0) -> [String] {
var idx = 0
var strings = [String]()
iterateChildren(tuple) { (label, value) in
guard length > 0 && idx < length else { return }
let str = libUint8TupleToString(value)
strings.append(str)
idx++
}
return strings
}
Usage
func handleConfiguration(config: RequestConfiguration) {
// Declaration types are added for clarity
let names: [String] = libUint8StringsInTuple(config.names, config.numberNames)
let currentName: String = names[config.currentName]
}
My solution uses reflection to iterate the first Tuple, and reflection to iterate the second, because I was getting incorrect strings when using withUnsafePointer for the nested Tuples, which I assume is due to signage. Surely there must be a way to read the C strings in the array, using an UnsafePointer alike withUsafePointer(&struct.cstring) { String.fromCString(UnsafePointer($0)) }.
To be clear, I'm looking for the fastest way to read these C strings in Swift, even if that involves using Reflection.
Here is a possible solution:
func handleConfiguration(var config: RequestConfiguration) {
let numStrings = Int(config.numberNames)
let lenStrings = sizeofValue(config.names.0)
let names = (0 ..< numStrings).map { idx in
withUnsafePointer(&config.names) {
String.fromCString(UnsafePointer<CChar>($0) + idx * lenStrings) ?? ""
}
}
let currentName = names[Int(config.currentName)]
print(names, currentName)
}
It uses the fact that
LibUint8 names[30][128];
are 30*128 contiguous bytes in memory. withUnsafePointer(&config.names)
calls the closure with $0 as a pointer to the start of that
memory location, and
UnsafePointer<CChar>($0) + idx * lenStrings
is a pointer to the start of the idx-th subarray. The above code requires
that each subarray contains a NUL-terminated UTF-8 string.
The solution suggested by Martin R looks good to me and, as far as I can see from my limited testing, does work. However, as Martin pointed out, it requires that the strings be NUL-terminated UTF-8. Here are two more possible approaches. These follow the principle of handling the complexity of C data structures in C instead of dealing with it in Swift. Which of these approaches you choose depends on what specifically you are doing with RequestConfiguration in your app. If you are not comfortable programming in C, then a pure Swift approach, like the one suggested by Martin, might be a better choice.
For the purposes of this discussion, we will assume that the 3rd party C library has the following function for retrieving RequestConfiguration:
const RequestConfiguration * getConfig();
Approach 1: Make the RequestConfiguration object available to your Swift code, but extract names from it using the following C helper function:
const unsigned char * getNameFromConfig(const RequestConfiguration * rc, unsigned int nameIdx)
{
return rc->names[nameIdx];
}
Both this function's signature and the RequestConfiguration type must be available to the Swift code via the bridging header. You can then do something like this in Swift:
var cfg : UnsafePointer<RequestConfiguration> = getConfig()
if let s = String.fromCString(UnsafePointer<CChar>(getNameFromConfig(cfg, cfg.memory.currentName)))
{
print(s)
}
This approach is nice if you need the RequestConfiguration object available to Swift in order to check the number of names in multiple places, for example.
Approach 2: You just need to be able to get the name at a given position. In this case the RequestConfiguration type does not even need to be visible to Swift. You can write a helper C function like this:
const unsigned char * getNameFromConfig1(unsigned int idx)
{
const RequestConfiguration * p = getConfig();
return p->names[idx];
}
and use it in Swift as follows:
if let s = String.fromCString(UnsafePointer<CChar>(getNameFromConfig1(2)))
{
print(s)
}
This will print the name at position 2 (counting from 0). Of course, with this approach you might also want to have C helpers that return the count of names as well as the current name index.
Again, with these 2 approaches it is assumed the strings are NUL-terminated UTF-8. There are other approaches possible, these are just examples.
Also please note that the above assumes that you access RequestConfiguration as read-only. If you also want to modify it and make the changes visible to the 3rd party library C code, then it's a different ballgame.
I have a method that I'm using to create an array w/ n of a specific type of object in it:
func many(count: Int) -> [Cube] {
var cubes: [Cube] = []
for i in 0...count {
let cube = CubeFactory.single()
cubes.append(cube)
}
return cubes
}
This works but I'm sure there's a more swift like way to do this. Any one know how this can be refactored?
Does CubeFactory.single() actually do anything other than return the same instance every time?
If it does active stuff, return map(0..<count) { _ in CubeFactory.single() } (or numerous variants) will give you an array of them.
(the _ in really oughtn’t to be necessary, but Swift’s type inference gets a bit flummoxed without it and you’ll get an error about half-open intervals – the other possibility for the overloaded ..< operator – not conforming to SequenceType)
If it’s just inert and every cube is identical, return Array(count: count, repeatedValue: Cube.single()) is all you need.
In Swift 2, this is
let array = (0..<30).map { _ in CubeFactory.single() }
Not sure if it's exactly what you're looking for, but you can use:
let array = map(0..<30) { _ in CubeFactory.single() }
Swift 4.2
struct Cube {
let id:Int
}
...
let array = (0..<24).compactMap {
return Cube(id: $0)
}
Today I was just going through some basic swift concepts and was working with some examples to understand those concepts. Right now I have completed studying tuples.
I have got one doubt i.e, what is the need of using tuples ? Ya I did some digging on this here is what I got :
We can be able to return multiple values from a function. Ok but we can also do this by returning an array.
Array ok but we can return an multiple values of different types. Ok cool but this can also be done by array of AnyObject like this :
func calculateStatistics (scores:[Int])->[AnyObject]
{
var min = scores[0]
var max = scores[0]
var sum = 0
for score in scores
{
if score > max{
max = score
}
else if score < min{
min = score
}
sum += score
}
return [min,max,"Hello"]
}
let statistics = calculateStatistics([25,39,78,66,74,80])
var min = statistics[0]
var max = statistics[1]
var msg = statistics[2] // Contains hello
We can name the objects present in the tuples. Ok but I can use a dictionary of AnyObject.
I am not saying that Why to use tuples when we have got this . But there should be something only tuple can be able to do or its easy to do it only with tuples. Moreover the people who created swift wouldn't have involved tuples in swift if there wasn't a good reason. So there should have been some good reason for them to involve it.
So guys please let me know if there's any specific cases where tuples are the best bet.
Thanks in advance.
Tuples are anonymous structs that can be used in many ways, and one of them is to make returning multiple values from a function much easier.
The advantages of using a tuple instead of an array are:
multiple types can be stored in a tuple, whereas in an array you are restricted to one type only (unless you use [AnyObject])
fixed number of values: you cannot pass less or more parameters than expected, whereas in an array you can put any number of arguments
strongly typed: if parameters of different types are passed in the wrong positions, the compiler will detect that, whereas using an array that won't happen
refactoring: if the number of parameters, or their type, change, the compiler will produce a relevant compilation error, whereas with arrays that will pass unnoticed
named: it's possible to associate a name with each parameter
assignment is easier and more flexible - for example, the return value can be assigned to a tuple:
let tuple = functionReturningTuple()
or all parameters can be automatically extracted and assigned to variables
let (param1, param2, param3) = functionReturningTuple()
and it's possible to ignore some values
let (param1, _, _) = functionReturningTuple()
similarity with function parameters: when a function is called, the parameters you pass are actually a tuple. Example:
// SWIFT 2
func doSomething(number: Int, text: String) {
println("\(number): \(text)")
}
doSomething(1, "one")
// SWIFT 3
func doSomething(number: Int, text: String) {
print("\(number): \(text)")
}
doSomething(number: 1, text: "one")
(Deprecated in Swift 2) The function can also be invoked as:
let params = (1, "one")
doSomething(params)
This list is probably not exhaustive, but I think there's enough to make you favor tuples to arrays for returning multiple values
For example, consider this simple example:
enum MyType {
case A, B, C
}
func foo() -> (MyType, Int, String) {
// ...
return (.B, 42, "bar")
}
let (type, amount, desc) = foo()
Using Array, to get the same result, you have to do this:
func foo() -> [Any] {
// ...
return [MyType.B, 42, "bar"]
}
let result = foo()
let type = result[0] as MyType, amount = result[1] as Int, desc = result[2] as String
Tuple is much simpler and safer, isn't it?
Tuple is a datastructure which is lighter weight than heterogeneous Array. Though they're very similar, in accessing the elements by index, the advantage is tuples can be constructed very easily in Swift. And the intention to introduce/interpolate this(Tuple) data structure is Multiple return types. Returning multiple data from the 'callee' with minimal effort, that's the advantage of having Tuples. Hope this helps!
A tuple is ideally used to return multiple named data from a function for temporary use. If the scope of the tuple is persistent across a program you might want to model that data structure as a class or struct.