I am currently using `map`

property with a closure in Swift to extract linear factors from an array and calculate a list of musical frequencies spanning one octave.

```
let tonic: Double = 261.626 // middle C
let factors = [ 1.0, 1.125, 1.25, 1.333, 1.5, 1.625, 1.875]
let frequencies = factors.map { $0 * tonic }
print(frequencies)
// [261.62599999999998, 294.32925, 327.03249999999997, 348.74745799999994, 392.43899999999996, 425.14224999999999, 490.54874999999993]
```

I want to do this by making the closure extract two integers from a string and divide them to form each factor. The string comes from an SCL tuning file and might look something like this:

```
// C D E F G A B
let ratios = [ "1/1", "9/8", "5/4", "4/3", "3/2", "27/16", "15/8"]
```

Can this be done ?

**SOLUTION**

Thankfully, yes it can. In three Swift statements tuning ratios represented as fractions since before Ptolemy can be coverted into precise frequencies. A slight modification to the accepted answer makes it possible to derive the list of frequencies. Here is the code

```
import UIKit
class ViewController: UIViewController {
// Diatonic scale
let ratios = [ "1/1", "9/8", "5/4", "4/3", "3/2", "27/16", "15/8"]
// Mohajira scale
// let ratios = [ "21/20", "9/8", "6/5", "49/40", "4/3", "7/5", "3/2", "8/5", "49/30", "9/5", "11/6", "2/1"]
override func viewDidLoad() {
super.viewDidLoad()
_ = Tuning(ratios: ratios)
}
}
```

**Tuning Class**

```
import UIKit
class Tuning {
let tonic = 261.626 // frequency of middle C (in Hertz)
var ratios = [String]()
init(ratios: [String]) {
self.ratios = ratios
let frequencies = ratios.map { s -> Double in
let integers = s.characters.split(separator: "/").map(String.init).map({ Double($0) })
return (integers[0]!/integers[1]!) * tonic
}
print("// \(frequencies)")
}
}
```

And here is the list of frequencies in Hertz corresponding to notes of the diatonic scale

```
C D E F G A B
[261.626007, 294.329254, 327.032501, 348.834686, 392.439026, 441.493896, 490.548767]
```

It works for other scales with pitches not usually found on a black-and-white-note music keyboard
*Mohajira scale created by Jacques Dudon*

```
// D F G C'
let ratios = [ "21/20", "9/8", "6/5", "49/40", "4/3", "7/5", "3/2", "8/5", "49/30", "9/5", "11/6", "2/1"]
```

And here is a list of frequencies produced

```
// D F G C'
// [274.70729999999998, 294.32925, 313.95119999999997, 320.49185, 348.83466666666664, 366.27639999999997, 392.43899999999996, 418.60159999999996, 427.32246666666663, 470.92679999999996, 479.64766666666662, 523.25199999999995]
```

**Disclaimer**

Currently the closure only handles rational scales. To fully comply with Scala SCL format it must also be able to distinguish between strings with fractions and strings with a decimal point and interpret the latter using cents, i.e. logarithmic rather than linear factors.