Get all values into dictionary and create a String with a specific format

Question

I have a dictionary with this structure:

a: [1,2]
b: [3,4]
c: [5,6]

and I need to return a string with this structure.

a,b,c\n1,3,5\n2,4,6

I solved the first part of the string. But to get the rest of the String. I try to iterate into my dictionary to get the first elements for each key in my dictionary and then get the rest for each value into the array.

Is there an easier way to get this?

Answer 1

Once you know what's the order of the keys (alpha ?), you can use this:

let dict: [String: [Int]] = ["a": [1,2], "b": [3, 4], "c": [5, 6]]
let keys = dict.keys.sorted() //Or do whatever you want here to get your target order
var matrix: [[String]] = []
keys.forEach {
    guard let arrayAsInt = dict[$0] else { return }
    let arrayAsString = arrayAsInt.map{ "\($0)" }
    matrix.append( [$0] + arrayAsString)
}
print("Matrix: \(matrix)")
let transposed = matrix.transposed()
print("Transposed Matrix: \(transposed)")
let output = transposed.map { $0.joined(separator: ",")}.joined(separator: "\n")
print(output)

The outputs:

$>Matrix: [["a", "1", "2"], ["b", "3", "4"], ["c", "5", "6"]]
$>Transposed Matrix: [["a", "b", "c"], ["1", "3", "5"], ["2", "4", "6"]]
$>a,b,c
1,3,5
2,4,6

Obvisouly the "\n" might be invisible and be an actual new line

a,b,c
1,3,5
2,4,6

Being

a,b,c\n1,3,5\n2,4,6

What's the idea behind that? Create a matrix and use the transpose (it's used in maths with matrix, it's one of the basic modification of a matrix).

First transform the [String: [Int]] into a [[String]], where each element would be key followed by its values. I transformed it there as String for simpler code later.

Why doing that? Because the matrix value is easy to get from your initial dict. the transposed value is harder (not impossible) to get from dict but easier from matrix, and the transposed is quickly transformed into your format.
So my thinking was the reverse: Get a structure from your output, then how to get it, it's a transpose, so I need to get the initial input as it, etc.

With the help of a code for Transpose Matrix (that accept String elements).

extension Collection where Self.Iterator.Element: RandomAccessCollection {
    // PRECONDITION: `self` must be rectangular, i.e. every row has equal size.
    func transposed() -> [[Self.Iterator.Element.Iterator.Element]] {
        guard let firstRow = self.first else { return [] }
        return firstRow.indices.map { index in
            self.map{ $0[index] }
        }
    }
}

Any code (there a various) working ones, should the trick. I took it from here.

As pointed by @Leo Dabus, you can remove the Self.Iterator.Element from the extension code (twice). I just wanted to it as such, not modifying the initial answer since it's not mind.

Answer 2

What you are looking for, besides composing the final string, is how to transpose a collection (this would work with collections of different sizes as well):

extension Sequence {
    func max<T: Comparable>(_ predicate: (Element) -> T)  -> Element? {
        self.max(by: { predicate($0) < predicate($1) })
    }
}

---

extension Collection where Element: RandomAccessCollection, Element.Indices == Range<Int> {
    func transposed() -> [[Element.Element]] {
        (0..<(max(\.count)?.count ?? .zero)).map {
            index in compactMap { $0.indices ~= index ? $0[index] : nil }
        }
    }
}

---

let dict = ["a": [1,2,3],
            "b": [4,5,6],
            "c": [7,8,9]]

let sorted = dict.sorted(by: {$0.key < $1.key})
let result = sorted.map(\.key).joined(separator: ",") + "\n" +
             sorted.map(\.value).transposed().map {
                 $0.map(String.init).joined(separator: ",")
             }.joined(separator: "\n")

result // "a,b,c\n1,4,7\n2,5,8\n3,6,9"

Answer 3

A dictionary is an unordered collection so you need to sort it according to any specific key. Here I sort the dictionary according to the key if you don't care about an order you can just remove sort.

let dict: [String: Any] = ["a": [1,2], "b": [3,4], "c": [5,6]]

let sortedKey = dict.keys.sorted(by: <)
let key = sortedKey.joined(separator: ",")

var firstValues: [String] = []
var secondValues: [String] = []

sortedKey.forEach { (key) in
    if let dictValue = dict[key] as? [Int],
       let firstValue = dictValue.first,
       let secondValue = dictValue.last {
        
        firstValues.append("\(firstValue)")
        secondValues.append("\(secondValue)")
    }
}

let finalString = key + "\n" + firstValues.joined(separator: ",") + "\n" + secondValues.joined(separator: ",")

print(finalString) // "a,b,c\n1,3,5\n2,4,6"