Swift - 高阶函数介绍（map、flatMap、filter、reduce） 原文出自：www.hangge.com 转载请保留原文链接：http://www.hangge.com/blog/

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let prices = [20,30,40]

let strs = prices.map({ "$\($0)" })

print(strs)

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let values = [4, 6, 9]

let squares = values.map({ $0 * $0 })

print(squares)

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let array = ["Apple", "Orange", "Grape", ""]

let arr1 = array.map { a -> Int? in

    let length = a.count

    guard length > 0 else { return nil }

    return length

}

print("arr1:\(arr1)")

let arr2 = array.flatMap { a-> Int? in

    let length = a.count

    guard length > 0 else { return nil }

    return length

}

print("arr2:\(arr2)")

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let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr1 = array.map{ $0 }   // [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr2 = array.flatMap{ $0 } // [1, 2, 3, 4, 5, 6, 7, 8, 9]

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let prices = [20,30,40]

let result = prices.filter({ $0 > 25 })

print(result)

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let prices = [20,30,40]

let sum = prices.reduce(0) { $0 + $1 }

print(sum)

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let sum = prices.reduce(0, +)

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let array = ["Apple", "Orange", "Grape"]

let str = array.reduce("", {

    return $0 == "" ? $1 : $0 + "、" + $1

})

print(str)

//上面等效与

let str = array.joined(separator: "、")

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let collections = [[5,2,7],[4,8],[9,1,3]]

let onlyEven = collections.flatMap {

    $0.filter { $0 % 2 == 0 }

}

print(onlyEven)

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let collections = [[5,2,7],[4,8],[9,1,3]]

let sums = collections.map({ $0.reduce(0, +) })

print(sums)

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let marks = [4,5,8,2,9,7]

let totalPass = marks.filter{$0 > 7}.reduce(0,+) //17

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let numbers = [4,5,8,2,9,7]

let evenSquares = numbers.map{$0 * $0}.filter{$0 % 2 == 0} //[16, 64, 4]