swift map reduce 获取下标(index)的方法

原文:http://stackoverflow.com/questions/28012205/map-or-reduce-with-index-in-swift

You can use enumerate to convert a sequence (Array, String, etc.) to a sequence of tuples with an integer counter and and element paired together. That is:

let numbers = [7, 8, 9, 10]
let indexAndNum: [String] = numbers.enumerate().map { (index, element) in
    return "\(index): \(element)"
}
print(indexAndNum)
// ["0: 7", "1: 8", "2: 9", "3: 10"]

Link to enumerate definition

Note that this isn't the same as getting the index of the collection—enumerate gives you back an integer counter. This is the same as the index for an array, but on a string or dictionary won't be very useful. To get the actual index along with each element, you can use zip:

let actualIndexAndNum: [String] = zip(numbers.indices, numbers).map { "\($0): \($1)" }
print(actualIndexAndNum)
// ["0: 7", "1: 8", "2: 9", "3: 10"]

When using an enumerated sequence with reduce, you won't be able to separate the index and element in a tuple, since you already have the accumulating/current tuple in the method signature. Instead, you'll need to use .0 and .1 on the second parameter to your reduce closure:

let summedProducts = numbers.enumerate().reduce(0) { (accumulate, current) in
    return accumulate + current.0 * current.1
    //                          ^           ^
    //                        index      element
}
print(summedProducts)   // 56