Swift 2 issue with a closure for groupBy collection - group-by

Question

The code below is written in order to group journal publications by year of publication.
But I got the error "Cannot convert value of type '(Journal) -> Int' to expected argument type '(_) -> _'"
Here's the playground with a stripped down version of the real code for you to play around http://swiftlang.ng.bluemix.net/#/repl/1de81132cb2430962b248d2d6ff64922e2fe912b1480db6a7276c6a03047dd89
class Journal {
var releaseDate: Int = 0
static var journals = [Journal]()
class func groupedReduce<S: SequenceType, K: Hashable, U> (
source: S,
initial: U,
combine: (U, S.Generator.Element) -> U,
groupBy: (S.Generator.Element) -> K
)
-> [K:U]
{
var result: [K:U] = [:]
for element in source {
let key = groupBy(element)
result[key] = combine(result[key] ?? initial, element)
}
return result
}
class func groupBy() {
let byYear = { (journal: Journal) in
journal.releaseDate
}
let groupedJournals = groupedReduce(journals, initial: 0, combine:+, groupBy: byYear)
print("Grouped journals = \(groupedJournals)")
}
}
Journal.journals = [Journal(), Journal(), Journal(), Journal()]
for j in Journal.journals {
j.releaseDate = 1
}
Journal.groupBy()

Answer 1

Your code is overly complicated. Below is a groupBy function that group elements of an array according to criteria of your choice. Playground
import Foundation
class Journal {
var releaseDate: Int = 0
init(_ releaseDate: Int) {
self.releaseDate = releaseDate
}
}
extension Array {
func groupBy<T: Hashable>(f: Element -> T) -> [T: [Element]] {
var results = [T: [Element]]()
for element in self {
let key = f(element)
if results[key] == nil {
results[key] = [Element]()
}
results[key]!.append(element)
}
return results
}
func groupBy2<T: Hashable>(f: Element -> T) -> [T: [Element]] {
return self.reduce([T: [Element]]()) { (var aggregate, element) in
let key = f(element)
if aggregate[key] == nil {
aggregate[key] = [Element]()
}
aggregate[key]!.append(element)
return aggregate
}
}
}
let journals = [Journal(2015), Journal(2016), Journal(2015), Journal(2014)]
let groupedJournals = journals.groupBy {
$0.releaseDate
}
print(groupedJournals)