user678392 - 4 months ago 10

iOS Question

I'm trying to learn about swift and closures.

I'm stuck on this example.

`numbers.map({`

(number: Int) -> Int in

let result = 3 * number

return result

})

What is (number: Int) -> Int? Is it a function? Where is it defined?

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/GuidedTour.html#//apple_ref/doc/uid/TP40014097-CH2-ID1

What does the keyword "in" do? The docs say to use "to separate the arguments and return type from the body". I'm not sure I understand this. Why isn't "in" used to separate "let result = 3 * number" from "return result".

Answer

A closure is just a function with the parameters moved inside the brackets, with the keyword `in`

to separate the parameters from the function body. The two following examples define equivalent functions:

```
func myFunc(number: Int) -> Int {
let result = 3 * number
return result
}
let myClosure = { (number: Int) -> Int in
let result = 3 * number
return result
}
```

You can actually call them both in exactly the same way:

```
let x = myFunc(2) // x == 6
let y = myClosure(2) // y == 6
```

Notice how the second example is exactly the same as the first, only in the first example, the parameters `(number: Int) -> Int`

are outside the brackets, and in the second example the parameters are inside the brackets, followed by the keyword `in`

.

`map`

works by taking an array (`numbers`

, in your example) and creating a new array that is the result of applying the closure function to each element in `numbers`

. So if `numbers`

is `[1, 2, 3]`

, the example above will start with `1`

. It will apply the closure function which will produce a `3`

(cuz all it does is multiply the element from the first array by 3). It does that for each element in `numbers`

, until it has produced a new array, `[3, 6, 9]`

.

If you wanted to, you could call `map`

using the names of either the above function or the above closure, or by writing it out explicitly inside of `map`

. All of the below examples are totally equivalent:

```
let numbers = [1, 2, 3]
// Example 1
let times3 = numbers.map(myFunc) // times3 == [3, 6, 9]
// Example 2
let timesThree = numbers.map(myClosure) // timesThree == [3, 6, 9]
// Example 3
let xThree = numbers.map({ (number: Int) -> Int in
let result = 3 * number
return result // xThree == [3, 6, 9]
})
```

Note that Example 3 is the same as Example 2, only in Example 3 the closure is spelled out explicitly inside of `map`

, whereas in Example 2 the closure has been assigned to a constant called `myClosure`

, and the constant has been supplied to `map`

.

Hope this helps - closures are fun, but confusing.