Swift Joining 2 Arrays in Specific Order - arrays

Question

I wrote the following code in a playground and it works fine, but it seems a little messy. Is there a more concise way of writing this in Swift?
I want a string to look like the following:
1, 12, 2, 12, 3, 12, 4, 13, 5, 13, 6, 13, 7, 14, 8, 14
I will not know how many values in each array and the arrays may not be even multiples, but I will know the relationship of 3 array1 values for each array2 value
let array1 = [1,2,3,4,5,6,7,8] //sample 1st array
let array2 = [12,13,14] //sample 2nd array
let relationshipInterval = 3
let remainder = (array1.count % relationshipInterval)
let multiples = (array1.count - remainder)/relationshipInterval
var string = ""
var array1Start = 0
var array1End = relationshipInterval-1
var array2Value = 0
for _ in 1...multiples {
for array1value in array1[array1Start...array1End] {
string += "\(array1value), "
string += String(array2[array2Value])+", "
}
array1Start = array1End + 1
array1End = array1Start + relationshipInterval - 1
array2Value += 1
}
for array1value in array1[array1Start...array1Start+remainder-1] {
string += "\(array1value), "
string += String(array2[array2Value])+", "
}
print (string) //prints 1, 12, 2, 12, 3, 12, 4, 13, 5, 13, 6, 13, 7, 14, 8, 14

Answer 1

I'm not sure if this covers all of the use cases, but using zip, flatMap, and joined you do this in a single step!
let array1 = [1,2,3,4,5,6,7,8]
let array2 = [12, 13, 14]
let text = zip(array1, array2.flatMap({ [$0, $0, $0] }))
.flatMap({ [$0, $1] })
.map({ String($0) })
.joined(separator: ", ")
// text -> "1, 12, 2, 12, 3, 12, 4, 13, 5, 13, 6, 13, 7, 14, 8, 14"
Here's that one-liner broken into four steps so it's clearer what's happening at each stage:
// 1. create an array with each element in array2 is trippled:
let trippled = array2.flatMap({ item in
return [item, item, item]
})
// trippled -> [12, 12, 12, 13, 13, 13, 14, 14, 14]
// 2. zip array1 with the trippled array:
let zipped = zip(array1, trippled)
// zipped -> Zip2Sequence(_sequence1: [1, 2, 3, 4, 5, 6, 7, 8], _sequence2: [12, 12, 12, 13, 13, 13, 14, 14, 14])
// 3. flatten the zipped aray
let combined = zipped.flatMap({ leftItem, rightItem in
return [leftItem, rightItem]
})
// combined -> [1, 12, 2, 12, 3, 12, 4, 13, 5, 13, 6, 13, 7, 14, 8, 14]
// 4. tranform to a string
let text = combined.map({ item in
return "\(item)"
}).joined(separator: ", ")
// text -> "1, 12, 2, 12, 3, 12, 4, 13, 5, 13, 6, 13, 7, 14, 8, 14"