数组Map

gistfile1.txt

https://github.com/apple/swift/blob/master/stdlib/public/core/Collection.swift

  @_inlineable
  public func map<T>(_ transform: (Iterator.Element) throws -> T) rethrows -> [T] {
    // TODO: swift-3-indexing-model - review the following
    let count: Int = numericCast(self.count)
    if count == 0 {
      return []
    }

    var result = ContiguousArray<T>()
    result.reserveCapacity(count)

    var i = self.startIndex

    for _ in 0..<count {
      result.append(try transform(self[i]))
      formIndex(after: &i)
    }

    _expectEnd(of: self, is: i)
    return Array(result)
  }