In programming, a dictionary is a general-purpose data structure, used to store a group of objects, in a “real” dictionary-like lemma-definition approach - or as group of key-value pairs, properly put.
In this chapter, we’ll explore most of Swift’s dictionary-related capabilities and learn how to create new ones, add/remove keys, merge them, and move on to even more advanced topics, such as handling property lists.
9.1 Add key-value pair to dictionary
Problem
I want to add a key-value pair to an existing dictionary
Solution
Let’s initialize our test dictionary.
OK, let’s add some more info.
user["nationality"] = "US"
And print the result...
print("User: (user)")
How It Works
A dictionary is a collection of key-value pairs. In order to add another key with a value, we can do it pretty much like changing an already existing key, that is: by using the subscript along with the value’s key and assign it the desired value.
User: ["name": "John", "surname": "Doe", "email": "[email protected]", "nationality": "US"]
9.2 Check if dictionary is empty
Problem
I want to check if a dictionary is empty.
Solution
First, we initialize our example dictionary with some data.
let a = [
"name" : "John",
"surname" : "Doe"
]
Now, let’s see...
if a.isEmpty {
print("Our dictionary is empty :(")
} else {
print("Of course it's not empty - we just created it!")
}
How It Works
To check if a dictionary is empty, we can use its isEmpty property.
Of course it's not empty - we just created it!
9.3 Check if key exists in dictionary
Problem
I want to check if a specific key exists in a given dictionary.
Solution
First, we initialize our example dictionary.
Now, let’s see...
if d["address"] != nil {
print("Yes, address found!")
} else {
print("Ooops, no address found!")
}
How It Works
To check if a given dictionary contains a specific key, we may use the dictionary’s subscript.
Ooops, no address found!
9.4 Check if object is dictionary
Problem
I want to check if a given object is of type dictionary.
Solution
First, we initialize our example “object” – let’s make it a dictionary, as expected.
var a : Any = [
"name" : "John",
"surname" : "Doe"
]
Now, let’s see...
if a is [String:String] {
print("Yes, it's a String:String dictionary. Well done!")
} else {
print("Oh, no, this is not a dictionary!")
}
How It Works
To check if an object is of type dictionary, we can use an X is a dictionary or X is [type:type] statement.
Yes, it's a String:String dictionary. Well done!
9.5 Check if two dictionaries are equal
Problem
I want to check if two different dictionaries are equal.
Solution
First, we initialize our dictionaries.
let a = ["name": "John", "surname": "Doe"]
let b = ["name": "Jane", "surname": "Doe"]
Let’s see...
if a == b {
print("Yep, the dictionaries are equal")
} else {
print("Nope, they are different dictionaries")
}
How It Works
To compare two dictionaries and check if they are equal, we can use the == comparison operator.
Nope, they are different dictionaries
9.6 Convert dictionary to array of tuples
Problem
I want to convert a given dictionary to an array of key-value tuples.
Solution
First, we initialize an example dictionary.
let fruit = [
"banana" : "yellow",
"apple" : "red",
"watermelon" : "green"
]
Let’s map it to an array of tuples.
let fruitArray = fruit.map { ($0,$1) }
And... print the result.
print("Fruit: (fruitArray)")
How It Works
If we have a dictionary, we can convert it into an array, by mapping its key-value pairs to tuples using the dictionary’s map method
.
Fruit: [("watermelon", "green"), ("apple", "red"), ("banana", "yellow")]
9.7 Convert dictionary to JSON string
Problem
I want to convert a dictionary object to its corresponding JSON string representation.
Solution
import Foundation
First, let’s create an example dictionary.
let dict : [String:Any] = [
"name" : "John",
"surname" : "Doe",
"age" : 69
]
do {
// Convert our dictionary to Json data
let json = try JSONSerialization.data(withJSONObject: dict, options: .prettyPrinted)
// Convert our Json data to string
let str = String(data: json, encoding: .utf8)
// And if all went well, print it out
print(str ?? "Ooops... Error converting Json to string!")
}
catch let error
{
print("Error: (error)")
}
How It Works
In order to convert a Swift dictionary to JSON, we may use the JSONSerialization
class.
{
"name" : "John",
"surname" : "Doe",
"age" : 69
}
9.8 Convert JSON string to dictionary
Problem
I want to convert an object represented as a JSON string to a dictionary object.
Solution
import Foundation
First, let’s set some test JSON.
let json = "{"name": "John", "surname": "Doe", "age": 69 }"
Then, we convert it to a Data object.
if let data = json.data(using: .utf8) {
// if everything went fine,
// it's time to convert our Data object to a Dictionary
let result = try? JSONSerialization.jsonObject(with: data, options: []) as! [String:Any]
// And finally, what about printing out our dictionary?
print(result ?? "Ooops... Error converting Json!")
}
How It Works
In order to convert a JSON string to a Swift dictionary, we may use the JSONSerialization class.
["name": John, "surname": Doe, "age": 69]
9.9 Create a dictionary with literal
Problem
I want to create a dictionary object from a dictionary literal.
Solution
This is a dictionary, with its type automatically inferred.
let a = [
"Country" : "Spain",
"Capital" : "Madrid"
]
This is the same one, but this time let’s set a type, for example, keys of String type, with values of String type.
let b : [String:String] = [
"Country" : "Spain",
"Capital" : "Madrid"
]
Let’s create a mixed dictionary with different types of elements. And different types of keys. This is rather tricky.
This time we have to explicitly say it’s of type [AnyHashable:Any]. Otherwise, the compiler will complain.
let c : [AnyHashable:Any] = [
1 : "one",
"two" : 2
]
Let’s see what we’ve managed...
print("a: (a)")
print("b: (b)")
print("c: (c)")
How It Works
Creating a dictionary from a dictionary literal is as simple as listing your key-value pairs (with a : between the two), separated by commas, surrounded by a pair of square brackets.
a: ["Capital": "Madrid", "Country": "Spain"]
b: ["Capital": "Madrid", "Country": "Spain"]
c: [AnyHashable(1): "one", AnyHashable("two"): 2]
9.10 Create an empty dictionary
Problem
I want to create an empty dictionary object .
Solution
First approach: Use the [:] empty dictionary expression.
let a : [String:String] = [:]
Second approach: Use a dictionary constructor.
let b = [String:Any]()
Third approach: “Enforce” its type with as.
let c = [:] as [String:Int]
Let’s print our three... empty dictionaries.
print("a: (a), b: (b), c: (c)")
How It Works
In order to create an empty dictionary, it works pretty much as with the arrays ; the only thing you have to specify is the type of items (keys and values) it’s going to contain.
a: [:], b: [:], c: [:]
9.11 Create NSDictionary from dictionary
Problem
I want to create an NSDictionary object from a dictionary.
Solution
import Foundation
First, we initialize our example array with some values.
Let’s convert it to an NSDictionary.
let b = NSDictionary(dictionary: dict)
Let’s try using the NSDictionary’s hash property
(not available for Swift pure dictionaries) to make sure we made it.
print("Final dictionary's hash: (b.hash)")
print("Yep, it's an NSDictionary!")
How It Works
In order to convert/bridge a dictionary to an NSDictionary, for example, when you need to access APIs that expect data in an NSDictionary instance, we can use the NSDictionary(dictionary:) initializer.
Final dictionary's hash: 3
Yep, it's an NSDictionary!
9.12 Get array of keys in dictionary
Problem
I want to get a given dictionary’s keys as an array.
Solution
Let’s initialize our test dictionary.
Then, we get its keys into an array.
let keys = Array(dict.keys)
Let’s print the result...
print("Our dictionary keys: (keys)")
How It Works
A dictionary is a collection of key-value pairs. But what if we want to get just the keys? For that, we can use the dictionary’s keys property.
Our dictionary keys: ["name", "surname", "email"]
9.13 Get dictionary value for key
Problem
I want to get the value of a specific key from a given dictionary.
Solution
Let’s initialize our test dictionary.
Then, we get the user’s name.
Careful: This is optional; and it is not guaranteed that we’ll find this particular key in the dictionary.
if let userName = user["name"] {
// Let's print the result
print("Our user's name is: (userName)")
}
How It Works
A dictionary is a collection of key-value pairs. In order to retrieve a specific value, we may use the subscript along with the value’s key.
Our user's name is: John
9.14 Get size of dictionary
Problem
I want to get the size of a given dictionary.
Solution
First, we initialize our example dictionary with some date.
Now, let’s count how many key-value pairs we’ve got...
print("The dictionary contains (a.count) elements")
How It Works
To check the size of a dictionary, we can use its count property.
The dictionary contains 3 elements
9.15 Loop through a Dictionary
Problem
I want to loop through a dictionary, keeping track of each key-value pair.
Solution
Let’s initialize our example dictionary.
Let’s iterate through our dictionary...
for (item,value) in userInfo {
print("(item) = (value)")
}
How It Works
Looping through a dictionary is just as easy as looping through an array. Only we have to keep track of both parts of each pair: key and value.
9.16 Merge two dictionaries
Problem
I want to merge two different dictionaries into a new one.
Solution
First, we initialize our example dictionaries.
Note
The dictionary that is going to change must be declared as a variable (in this case, dictB will be merged into dictA).
var dictA = [ "1" : "one", "2" : "two", "4" : "four" ]
let dictB = [ "1" : "uno", "3" : "tres" ]
Let’s loop through our dictB.
for (key,value) in dictB {
// if key doesn't exist, create it
// if it does exist, update it
dictA[key] = value
}
Now, let’s print our new dictionary...
print("Merged: (dictA)")
How It Works
If we have two dictionaries and want to merge them, that is: to add one dictionary’s items to another one, updating keys if necessary; there is no built-in method, but we can loop and update the keys/values accordingly.
Merged: ["3": "tres", "2": "two", "1": "uno", "4": "four"]
9.17 Read contents of Plist from file into dictionary
Problem
I want to read the contents of a Property List (Plist) file into a dictionary object.
For example:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Books</key>
<array>
<dict>
<key>author</key>
<string>Fyodor Dostoyevsky</string>
<key>title</key>
<string>Crime and Punishment</string>
</dict>
<dict>
<key>author</key>
<string>George Orwell</string>
<key>title</key>
<string>Animal Farm</string>
</dict>
</array>
</dict>
</plist>
Solution
import Foundation
Now, let’s set our Plist path.
let path = "tests/test.plist"
And read our Plist.
if let data = try? Data(contentsOf: URL(fileURLWithPath: path)) {
var format = PropertyListSerialization.PropertyListFormat.xml
let dict = try? PropertyListSerialization.propertyList(from: data, options: [], format: &format) as! [String : String]
// Finally, we print out the result
print(dict ?? "Error converting Plist")
}
How It Works
In order to read a property list (XML Plist) string as a dictionary, we may use PropertyListSerialization’s propertyList method .
9.18 Read contents of Plist from string into dictionary
Problem
I want to read the contents of a Property List (Plist) string into a dictionary object.
Solution
import Foundation
First, we initialize our Plist string.
let plist = "<?xml version="1.0" encoding="UTF-8"?>" +
"<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">" +
"<plist version="1.0">" +
"<dict>" +
"<key>one</key>" +
"<string>value</string>" +
"<key>two</key>" +
"<string>another value</string>" +
"</dict>" +
"</plist>"
Now, let’s read it.
let data = plist.data(using: .utf8)
var format = PropertyListSerialization.PropertyListFormat.xml
let dict = try? PropertyListSerialization.propertyList(from: data!, options: [], format: &format) as! [String : String]
And print out the result...
print(dict ?? "Error converting plist")
How It Works
In order to read a property list (XML Plist)
string as a dictionary, we may use PropertyListSerialization’s propertyList method
.
["one": "value", "two": "another value"]
9.19 Remove all items from dictionary
Problem
I want to remove all elements from a given dictionary.
Solution
First, we initialize a test dictionary.
Then, we remove all of its contents.
user.removeAll()
Let’s see our new - empty – dictionary...
print("User: (user)")
How It Works
To remove all values from an existing dictionary (basically: empty it), we may use the dictionary’s removeAll method.
User: [:]
9.20 Remove dictionary item by key
Problem
I want to remove a specific item from a given dictionary, by its key.
Solution
First, we initialize a test dictionary.
Then we delete its name or, literally, remove its values with a name key.
user.removeValue(forKey: "name")
Let’s see the resulting dictionary...
print("User: (user)")
How It Works
To remove a particular key-value pair from an existing dictionary, we may use the dictionary’s removeValue(forKey:) method
.
User: ["surname": "Doe", "email": "[email protected]"]
9.21 Set dictionary value by key
Problem
I want to set the value of a specific key in a given dictionary.
Solution
Let’s initialize our test dictionary.
What about changing the user’s name?
user["name"] = "Jane"
And print the result...
print("User: (user)")
How It Works
A dictionary is a collection of key-value pairs. In order to set a specific value, we may use the subscript along with the value’s key and assign the new value.
User: ["name": "Jane", "surname": "Doe", "email": "[email protected]"]
9.22 Summary
In this chapter, we learned how we can make use of one of the most powerful constructs in Swift: the dictionary.
In the next chapter, we’ll be looking into more advanced Swift topics: from working with numbers and dates, to interacting with the file system, performing web requests, or even launching system processes.