Dictionary is one of the important collection type. We use dictionaries in most of apps for storing and organizing data.
Dictionary is an unordered collection of key-value pairs. Its also called as hashes or associated arrays in other languages.
Swift’s Dictionary type does not have a defined ordering. To iterate over the keys or values of a dictionary in a specific order, use the sorted() method on its keys or values property.
Dictionary is an unordered collection of key-value pairs. Its also called as hashes or associated arrays in other languages.
Syntax :
[Key: Value] // key must be unique of same type
Example :
var blogDict = [
"name" : "iOS Revisited",
"id" : "111",
"url": "http://iosrevisited.blogspot.com",
"description": "A blog about ios tutorials."
]
Each element is associated with a unique key. We can access elements from dictionary fast using unique keys.
Creating/Initializing Dictionary :
As arrays we can create empty dictionaries using initializers.
Syntax :
var someDict = [someKey: someValue]()
Example :
var responseMessages = [Int: String]() // or var responseMessages: [Int: String] = [:] // responseMessages is an empty dictionaties of type [Int: String]
The keys are of Int type and values are of String type for responseMessages Dictionary.
Storing objects in Dictionary :
For storing values in dictionary we use subscripts as arrays.
Example :
responseMessages[400] = "Bad request" // stored value as string responseMessages[403] = "Access forbidden"
Now responseMessages contains two objects with key 400 and 403.
Accessing and Modifying a Dictionary :
You access and modify a dictionary through its methods and properties, or by using subscript syntax.
As with an array, you find out the number of items in a Dictionary by checking its read-only count property.
print("The responseMessages dictionary contains \(responseMessages.count) items.")
// Prints "The responseMessages dictionary contains 2 items."
For checking whether dictionary is empty or not, we can use count = 0 or use the Boolean isEmpty property as a shortcut.
if responseMessages.isEmpty {
print("The responseMessages dictionary is empty.")
} else {
print("The responseMessages dictionary is not empty.")
}
// Prints "The responseMessages dictionary is not empty."
Here again adding two more element to responseMessages Dict.
responseMessages[500] = "Internal server error" responseMessages[200] = "Success" // the responseMessages dictionary now contains 4 items
For updating or modifying a value we can use subscripts as follows:
responseMessages[500] = "Something went wrong.." // the value for "500" has been changed to "Something went wrong.."
As an alternative to subscripting for updating or modifying a value we have inbuilt functions updateValue(_:forKey:) to set or update the value for a particular key.
The updateValue(_:forKey:) method returns an optional value of the dictionary’s value type. For a dictionary that stores String values, for example, the method returns a value of type String?, or “optional String”. This optional value contains the old value for that key if one existed before the update, or nil if no value existed:
The updateValue(_:forKey:) method returns an optional value of the dictionary’s value type. For a dictionary that stores String values, for example, the method returns a value of type String?, or “optional String”. This optional value contains the old value for that key if one existed before the update, or nil if no value existed:
if let oldValue = responseMessages.updateValue("Access granted", forKey: 403) {
print("The old value for 403 was \(oldValue).")
}
// Prints "The old value for 403 was Access forbidden."
Removing/Deleting object from Dictionary :
For deleting key-value pair from dictionary we can use subscripts by assigning to nil.
// "Access granted" is not the real response for 403, so delete it responseMessages[403] = nil // 403 has now been removed from the dictionary
Alternatively, remove a key-value pair from a dictionary with the removeValue(forKey:) method. This method removes the key-value pair if it exists and returns the removed value, or returns nil if no value existed:
if let removedValue = responseMessages.removeValue(forKey: 500) {
print("The removed responseMessage name is \(removedValue).")
} else {
print("The responseMessages dictionary does not contain a value for 500.")
}
// Prints "The removed responseMessage name is Internal server error"
Iterating Over a Dictionary :
As like arrays, we can iterate over the key-value pairs in a dictionary with a for-in loop.
Each item in the dictionary is returned as a (key, value) tuple, and you can decompose the tuple’s members into temporary constants or variables as part of the iteration:
Each item in the dictionary is returned as a (key, value) tuple, and you can decompose the tuple’s members into temporary constants or variables as part of the iteration:
for (responseCode, message) in responseMessages {
print("\(responseCode): \(message)")
}
// 400: Bad request
// 200: Success
You can also retrieve an iterable collection of a dictionary’s keys or values by accessing its keys and values properties:
for responseCode in responseMessages.keys {
print("response code: \(responseCode)")
}
// response code: 400
// response code: 200
for message in responseMessages.values {
print("message: \(message)")
}
// message: Bad request
// message: Success
Convert to Arrays :
If you need to use a dictionary’s keys or values with an API that takes an Array instance, initialize a new array with the keys or values property:let responseCodes = [Int](responseMessages.keys) // [400, 200] let messages = [String](responseMessages.values) // [Bad request, Success]
Swift’s Dictionary type does not have a defined ordering. To iterate over the keys or values of a dictionary in a specific order, use the sorted() method on its keys or values property.
