Divide array into little arrays with the same elements - arrays

Question

let's say I have an array this way : [1,4,7,4,2,2,4,7,1,2].
I need a function that divides this array into arrays with the same elements so it shows a result as this in swift :
result = [[1,1],[4,4,4],[7,7],[2,2,2]] .
How to do that in swift ? Thanks in advance

Answer 1

You can use a helper dictionary to categorize the values of your array into the appropriate bins. E.g.:
let arr = [1, 4, 7, 4, 2, 2, 4, 7, 1, 2]
var dict: [Int: [Int]] = [:]
arr.forEach { dict[$0] = (dict[$0] ?? []) + [$0] }
let inBins = dict.map{ $1 }.sorted{ $0.first ?? 0 < $1.first ?? 0 }
print(inBins) // [[1, 1], [2, 2, 2], [4, 4, 4], [7, 7]]
Or, make use of the general Sequence extension for the categorising part, as described in the accepted answer in thread linked to by #Hamish:
How to group by the elements of an array in Swift
E.g.:
/* from https://stackoverflow.com/a/39388832/4573247:
#mientus's Swift 3 translation of #oisdk's accepted answer */
public extension Sequence {
func categorise<U : Hashable>(_ key: (Iterator.Element) -> U) -> [U:[Iterator.Element]] {
var dict: [U:[Iterator.Element]] = [:]
for el in self {
let key = key(el)
if case nil = dict[key]?.append(el) { dict[key] = [el] }
}
return dict
}
}
let arr = [1, 4, 7 ,4, 2, 2, 4, 7, 1, 2]
let inBins = arr.categorise{ $0 }.map{ $1 }.sorted{ $0.first ?? 0 < $1.first ?? 0 }
print(inBins) // [[1, 1], [2, 2, 2], [4, 4, 4], [7, 7]]
No need for the bins to be sorted (as above)? The two options above are then reduced to (simply dropping the last sortin part):
// ... first alternative above
let inBins = dict.map{ $1 }
// ... 2nd alternative above
let inBins = arr.categorise{ $0 }.map{ $1 }

Answer 2

Another option could be to create an NSCountedSet:
let array = [1,4,7,4,2,2,4,7,1,2]
let countedSet = NSCountedSet(array: array)
You could then easily get the count of each unique element:
let countForOne = countedSet.count(for: 1)
As far as I know there is no native Swift equivalent of NSCountedSet yet.