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=={{header|Swift}}==

Data:

<lang swift>struct Place {

var name: String

var population: Double

}

let places = [

Place(name: "Lagos", population: 21.0),

Place(name: "Cairo", population: 15.2),

Place(name: "Kinshasa-Brazzaville", population: 11.3),

Place(name: "Greater Johannesburg", population: 7.55),

Place(name: "Mogadishu", population: 5.85),

Place(name: "Khartoum-Omdurman", population: 4.98),

Place(name: "Dar Es Salaam", population: 4.7),

Place(name: "Alexandria", population: 4.58),

Place(name: "Abidjan", population: 4.4),

Place(name: "Casablanca", population: 3.98)

]</lang>

===Using built-in methods===

<lang swift>guard let salaamI = places.firstIndex(where: { $0.name == "Dar Es Salaam" }) else {

fatalError()

}

print("Dar Es Salaam has index: \(salaamI)")

guard let lessThan5 = places.first(where: { $0.population < 5 }) else {

fatalError()

}

print("First city with less than 5mil population: \(lessThan5.name)")

guard let startsWithA = places.first(where: { $0.name.hasPrefix("A") }) else {

fatalError()

}

print("Population of first city starting with A: \(startsWithA.population)")</lang>

{{out}}

<pre>Dar Es Salaam has index: 6

First city with less than 5mil population: Khartoum-Omdurman

Population of first city starting with A: 4.58</pre>

===Custom method using Key Paths===

<lang swift>extension Collection {

func firstIndex<V: Equatable>(

withProperty prop: KeyPath<Element, V>,

_ op: (V, V) -> Bool,

_ val: V

) -> Index? {

for i in indices where op(self[i][keyPath: prop], val) {

return i

}

return nil

}

}

guard let salaamI = places.firstIndex(withProperty: \.name, ==, "Dar Es Salaam") else {

fatalError()

}

print("Dar Es Salaam has index: \(salaamI)")

guard let lessThan5I = places.firstIndex(withProperty: \.population, <, 5) else {

fatalError()

}

print("First city with less than 5mil population: \(places[lessThan5I].name)")

guard let aI = places.firstIndex(withProperty: \.name, { $0.hasPrefix($1) }, "A") else {

fatalError()

}

print("Population of first city starting with A: \(places[aI].population)")</lang>

{{out}}

Same as first method