Create Dictionary<String, [SomeStruct]> from [SomeStruct] source-array - ios

Question

var sourceEntries: [Entry] = [entry1, ..., entry14]
var myDict: Dictionary<String, [Entry]> = [:]
for entry in sourceEntries {
if var array = myDict[entry.attribute1] { theArray.append(entry) }
else { myDict[entry.attribute1] = [entry] }
}
I am intending to create a Dictionary, which matches all the objects of the struct "Eintrag" with the same attribute from the source-Array "alleEinträge" to a String containing the value of the shared attribute. For some reason my final Dictionary just matches Arrays of one element to the Strings, although some Arrays ought to contain up to four elements.

Answer 1

The problem is that the array is passed by value (i.e. "copied"), so the array you are writing to when you say array.append is not the array that is "inside" the dictionary. You have to write back into the dictionary explicitly if you want to change what's in it.
Try it in a simple situation:
var dict = ["entry":[0,1,2]]
// your code
if var array = dict["entry"] { array.append(4) }
// so what happened?
println(dict) // [entry: [0, 1, 2]]
As you can see, the "4" never got into the dictionary.
You have to write back into the dictionary explicitly:
if var array = dict["entry"] { array.append(4); dict["entry"] = array }
FURTHER THOUGHTS: You got me thinking about whether there might be a more elegant way to do what you're trying to do. I'm not sure whether you will think this is "more elegant", but perhaps it has some appeal.
I will start by setting up a struct (like your Entry) with a name attribute:
struct Thing : Printable {
var name : String
var age : Int
var description : String {
return "{\(self.name), \(self.age)}"
}
}
Now I will create an array like your sourceEntries array, where some of the structs share the same name (like your shared attribute attribute1):
let t1 = Thing(name: "Jack", age: 40)
let t2 = Thing(name: "Jill", age: 38)
let t3 = Thing(name: "Jill", age: 37)
let arr = [t1,t2,t3]
And of course I will prepare the empty dictionary, like your myDict, which I call d:
var d = [String : [Thing]]()
Now I will create the dictionary! The idea is to use map and filter together to do all the work of creating key-value pairs, and then we just build the dictionary from those pairs:
let pairs : [(String, [Thing])] = arr.map {
t in (t.name, arr.filter{$0.name == t.name})
}
for pair in pairs { d[pair.0] = pair.1 }