原文:http://stackoverflow.com/questions/28012205/map-or-reduce-with-index-in-swift
You can useenumerate 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 toenumerate definition
Note that this isn't the same as getting theindexof 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 usezip :
let actualIndexAndNum: [String] = zip(numbers.indices,numbers).map { "($0): ($1)" }
print(actualIndexAndNum)
// ["0: 7",sans-serif"> When using an enumerated sequence withreduce ,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 yourreduce closure:
let summedProducts = numbers.enumerate().reduce(0) { (accumulate,current) in
return accumulate + current.0 * current.1
// ^ ^
// index element
}
print(summedProducts) // 56 (编辑:李大同)
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