Appending to Array in Deeply Nested Dictionary of Arrays

Question

I am having the following problem in Swift. First, I declare a data structure, like this:

var books = [String : Dictionary<String, Dictionary<String, Dictionary<String, Array<Dictionary<String, String>>>>>]()

I later initialize the var like this:

books = [
    "Fiction" : [
        "Genre Fiction" : [
            "Mystery" : [
                "Classics" : [
                    ["Title" : "Ten Little Indians",
                     "Author" : "Agatha Christie",
                     "read" : "no"],
                    ["Title" : "A Study in Scarlet",
                     "Author" : "Arthur Conan Doyle",
                     "read" : "no"],
                ]
            ]
        ]
    ]
]

Note that the compiler does not complain about it. Later, I create a new dictionary which I would like to append to that innermost array, like this:

var bookDict = Dictionary<String, String>()
bookDict = ["title" : dict.valueForKey("title") as! String,
            "author": dict.valueForKey("author") as! String,
            "read"  : dict.valueForKey("read") as! String ]
books["category"]["genre"]["focus"]["set"].append(bookDict)

However, I get a compiler error that "Cannot invoke append with an argument list of type (Dictionary < String, String>)". This is confusing to me because the books data structure is declared such that that innermost array is an array of Dictionary< String, String>.

What am I missing?

hennes · Accepted Answer · 2015-08-15 08:40:16Z

1

It's not clear during compile time if the key is really present in the dictionary so dictionaries always return optional values. You have to check this optionals:

if let category = books["Fiction"] {
    if let genre = category["Genre Fiction"] {
        if let focus = genre["Mystery"] {
            if var set = focus["Classics"] {
                set.append(bookDict)
            }
        }
    }
}

Or you can do it also like this:

books["Fiction"]?["Genre Fiction"]?["Mystery"]?["Classics"]?.append(bookDict)

edited Aug 15, 2015 at 8:40

hennes

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answered Aug 15, 2015 at 6:27

Darko

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11 Comments

zeeple Over a year ago

This gets me closer! It no longer gives a compiler error, which is good. I have modified the append call to pull the subscripted strings from another dictionary, like this:

zeeple Over a year ago

bookage[dict.valueForKey("category") as! String]?[dict.valueForKey("genre") as! String]?[dict.valueForKey("focus") as! String]?[dict.valueForKey("set") as! String]?.append(bookDict)

Darko Over a year ago

So what is the problem now?

zeeple Over a year ago

But when I later println() the dictionary all of the new entries are not there. Is it not the case that when a dictionary key is not found, that it creates the key automatically? Seems like it worked that way in Obj-C.

Darko Over a year ago

You have named the key "Fiction" not "category", "Genre Fiction" not "genre", etc... you are using the wrong keys according to the code you posted. Please post the full code.

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