如何根据存储在另一个数组中的索引获取数组的项

Question

I have two arrays:

let a = ["apple","banana","orange","pomelo","kiwi","melon"]
let b = [1, 2, 4]

a contains all the items and b contains the indexes of the ones I'm interested in.
So I would like to create a function to extract the items at the indexes specified in array b .

I can do tis with for loops:

for i in 0...a.count-1{
    if i == b[i]{
    print(a[i])
  }
}

To make it clear, the desired output would be:

banana orange kiwi

The problem is that with big numbers the for loop would be too slow.
I would like to know if there exists something with a lower complexity.

Answer 1

You can simply map the indices and return the associated elements:

---

let aa = ["apple","banana","orange","pomelo","kiwi","melon"]
let bb = [1, 2, 4]

let elements = bb.map { aa[$0] }
print(elements)    // ["banana", "orange", "kiwi"]

---

Or extending RandomAccessCollection protocol:

---

extension RandomAccessCollection {
    func elements(at indices: [Index]) -> [Element] { indices.map { self[$0] } }
}

---

let a = ["apple","banana","orange","pomelo","kiwi","melon"]
let b = [1, 2, 4]

let elements = a.elements(at: b)    // ["banana", "orange", "kiwi"]