I added sequence in NSMutableDictionary by keys
Here is the key of Dictionary
New,
To Do,
Basic,
Advanced,
In Progress,
Done
But when I print NSMutableDictionary all keys then output was different
Advanced,
In Progress,
To Do,
New,
Done,
Basic
Any solution for this ?
Dictionaries are unordered collections. The sequence of items is no guaranteed, and will not be preserved. Don't make any assumptions about the order objects are fetched from a dictionary. It is not specified and subject to change.
If you need an ordered collection, try using an array of single-element dictionaries, or an array of custom objects with a key and value property.
You can try the following or come up with own data structure if you are worried about the position of elements inside a Dictionary
You can use Array of key-value pairs like this -> [[String : AnyObject]].
Which looks like : [["New" : 1],["In Progress" : 3],...]
You can also use tuples of key and value in an Array like this -> [(key,value)]
Which looks like : [("New" , 1),("In Progress" , 3),...]
Try to get keys in this array
NSArray *orderedArr = [[dict allKeys] sortedArrayUsingSelector:#selector(localizedStandardCompare:)];
i have a json, which i parse as a dictionary. Now most is simply a key value pair like "title":{"this is the title"}. The one thing bugging me is a key value pair that has an array for value.
"hashTags":[{"name":"pizza"},{"name":"salami"},{"name":"diet"}]
i do not want to use SwiftyJSON, since this is the only part i am unable to solve. What i need is to convert this value into an Array, which i can iterate. I had some approaches, they failed because they added new lines or stopped displaying äöü symbols.
Thank you in advance!
hashTags is an array of dictionaries with String keys and values, cast it to [[String:String]]
Iterate thru the array with a for loop and print all values for key name
if let hashTags = json["hashTags"] as? [[String:String]] {
for tag in hashTags {
print(tag["name"])
}
}
It's assumed that json is the parent object which contains the key hashTags.
All you need is using codable.
You can use Codable to parse Json to your desired model easily. you can check following document for more detail:
https://developer.apple.com/documentation/foundation/archives_and_serialization/using_json_with_custom_types
https://developer.apple.com/documentation/swift/codable
Does Hash.slice support strings ?
so for example
a = {"b" => 1, "c" => 2}
a.slice("b")
If not then how does it react to strings ?
I am trying to track down a bug where data is being lost and I think it is because the keys in hashes switch intermittently between strings and symbols. Filtering is done exclusively using Hash.slice(*keys)
In Rails, Hash#slice with a single key value (such as a string) from the hash returns a hash containing the matching key-value pair, or nil if it isn't matched to a key. Hash#slice takes any number of arguments, each representing a key. See the documentation here, and note *keys argument.
I tried to display datas which is in Dictionary format. Below, three attempts are there. First attempt, output order is completely changed. Second attempt, output order is same as input. But, in third attempt, I declared variable as NSDictionary. Exact output I received. Why this changes in Dictionary? Kindly guide me. I searched for Swift's Dictionary tag. But I couldn't found out.
//First Attempt
var dict : Dictionary = ["name1" : "Loy", "name2" : "Roy"]
println(dict)
//output:
[name2: Roy, name1: Loy]
//Second Attempt
var dict : Dictionary = ["name2" : "Loy", "name1" : "Roy"]
println(dict)
//output:
[name2: Loy, name1: Roy]
-----------------------------------------------------------
//Third Attempt With NSDictionary
var dict : NSDictionary = ["name1" : "Loy", "name2" : "Roy"]
println(dict)
//output:
{
name1 = Loy;
name2 = Roy;
}
ANOTHER QUERY: I have used play ground to verify. My screen shot is below:
Here, In NSDictionary, I gave name5 as first, but in right side name2 is displaying, then, in println, it is displaying in ascending order. Why this is happening??
Here, In Dictionary, I gave name5 as first, but in right side name2 is displaying, then, in println, it is displaying, how it is taken on the Dictionary line. Why this is happening??
This is because of the definition of Dictionaries:
Dictionary
A dictionary stores associations between keys of the same type and values of the same type in an collection with no defined ordering.
There is no order, they might come out differently than they were put in. This is comparable to NSSet.
Edit:
NSDictionary
Dictionaries Collect Key-Value Pairs. Rather than simply maintaining an ordered or unordered collection of objects, an NSDictionary stores objects against given keys, which can then be used for retrieval.
There is also no order, however there is sorting on print for debugging purposes.
You can't sort a dictionary but you can sort its keys and loop through them as follow:
let myDictionary = ["name1" : "Loy", "name2" : "Roy", "name3" : "Tim", "name4" : "Steve"] // ["name1": "Loy", "name2": "Roy", "name3": "Tim", "name4": "Steve"]
let sorted = myDictionary.sorted {$0.key < $1.key} // or {$0.value < $1.value} to sort using the dictionary values
print(sorted) // "[(key: "name1", value: "Loy"), (key: "name2", value: "Roy"), (key: "name3", value: "Tim"), (key: "name4", value: "Steve")]\n"
for element in sorted {
print("Key = \(element.key) Value = \(element.value)" )
}
A little late for the party but if you want to maintain the order then use KeyValuePairs, the trade-off here is that if you use KeyValuePairs you lose the capability of maintaining unique elements in your list
var user: KeyValuePairs<String, String> {
return ["FirstName": "NSDumb",
"Address": "some address value here",
"Age":"30"]
}
prints
["FirstName": "NSDumb", "Address": "some address value", "Age": "30"]
Dictionaries, by nature, are not designed to be ordered, meaning that they're not supposed to be (although they can be!).
From the Dictionaries (Swift Standard Library documentation):
A dictionary is a type of hash table, providing fast access to the entries it contains. Each entry in the table is identified using its key, which is a hashable type such as a string or number. You use that key to retrieve the corresponding value, which can be any object. In other languages, similar data types are known as hashes or associated arrays.
This requires some basic knowledge of Data Structures, which I'll outline & oversimplify briefly.
Storing associated data without a dictionary
Consider for a minute if there was no Dictionary and you had to use an array of tuples instead, to store some information about different fruits and their colors, as another answer suggested:
let array = [
("Apple", "Red"),
("Banana", "Yellow"),
// ...
]
If you wanted to find the color of a fruit you'd have to loop through each element and check its value for the fruit, then return the color portion.
Dictionaries optimize their storage using hash functions to store their data using a unique hash that represents the key that is being stored. For swift this means turning our key—in this case a String—into an Int. Swift uses Int-based hashes, which we know because we all read the Hashable protocol documentation and we see that Hashable defines a hashValue property that returns an Int.
Storing associated data with a dictionary
The benefits of using a dictionary are that you get fast read access and fast write access to data; it makes "looking up" associated data easy and quick. Typically O(1) time complexity, although the apple docs don't specify, maybe because it depends on the key type's hash function implementation.
let dictionary = [
"Apple": "Red",
"Banana": "Yellow"
// ...
]
The trade off is that the order is typically not guaranteed to be preserved. Not guaranteed means that you might get lucky and it might be the same order, but it's not intended to be, so don't rely on it.
As an arbitrary example, maybe the string "Banana" gets hashed into the number 0, and "Apple" becomes 4. Since we now have an Int we could, under the hood, represent our dictionary as an array of size 5:
// what things *might* look like under, the hood, not our actual code
// comments represent the array index numbers
let privateArrayImplementationOfDictionary = [
"Yellow", // 0
nil, // 1
nil, // 2
nil, // 3
"Red", // 4
] // count = 5
You'll notice, we've converted our keys into array indices, and there are a bunch of blank spaces where we have nothing. Since we are using an array, we can insert data lightning fast, and retrieve it just as quickly.
Those nil spaces are reserved for more values that may come later, but this is also why when we try to get values out of a dictionary, they might be nil. So when we decide to add more values, something like:
dictionary["Lime"] = "Green" // pretend hashValue: 2
dictionary["Dragonfruit"] = "Pink" // pretend hashValue: 1
Our dictionary, under the hood, may look like this:
// what things *might* look like under, the hood, not our actual code
// comments represent the array index numbers
let privateArrayImplementationOfDictionary = [
"Yellow", // 0 ("Banana")
"Pink", // 1 ("Dragonfruit")
"Green", // 2 ("Lime")
nil, // 3 (unused space)
"Red", // 4 ("Apple")
] // count = 5
As you can see, the values are not stored at all in the order we entered them. In fact, the keys aren't even really there. This is because the hash function has change our keys into something else, a set of Int values that give us valid array indices for our actual storage mechanism, an array, which is hidden from the world.
I'm sure that was more information than you wanted and probably riddled with many inaccuracies, but it gives you the gist of how a dictionary works in practice and hopefully sounds better than, "that's just how it works."
When searching for the actual performance of Swift dictionaries, Is Swift dictionary ... indexed for performance? ... StackOverflow had some extra possible relevant details to offer.
If you're still interested to know more details about this, you can try implementing your own dictionary as an academic exercise. I'd also suggest picking up a book on Data Structures and Algorithms, there are many to choose from, unfortunately I don't have any suggestions for you.
The deeper you get into this topic the more you'll understand why you'll want to use one particular data structure over another.
Hope that helps!
✅ It is possible!
Although the Dictionary is not ordered, you can make it preserve the initial order by using the official OrderedDictionary from the original Swift Repo
The ordered collections currently contain:
Ordered Dictionary (That you are looking for)
Ordered Set
They said it is going to be merged in the Swift's source code soon (reference WWDC21)
Neither NSDictionary nor Swift::Dictionary orders its storage. The difference is that some NSDictionary objects sort their output when printing and Swift::Dictionary does not.
From the documentation of -[NSDictionary description]:
If each key in the dictionary is an NSString object, the entries are
listed in ascending order by key, otherwise the order in which the
entries are listed is undefined. This property is intended to produce
readable output for debugging purposes, not for serializing data.
From The Swift Programming Language:
A dictionary stores associations between keys of the same type and values of the same type in an collection with no defined ordering.
Basically, order of items as seen in output is arbitrary, dependant on internal implementation of data structure, and should not be relied on.
This is indeed an issue with dictionaries. However, there's a library available to make sure the order stays the way you initialised it.
OrderedDictionary is a lightweight implementation of an ordered dictionary data structure in Swift.
The OrderedDictionary structure is an immutable generic collection which combines the features of Dictionary and Array from the Swift standard library. Like Dictionary it stores key-value pairs and maps each key to a value. Like Array it stores those pairs sorted and accessible by a zero-based integer index.
Check it out here:
https://github.com/lukaskubanek/OrderedDictionary
What's the difference between arrays and hashes in Ruby?
From Ruby-Doc:
Arrays are ordered, integer-indexed collections of any object. Array indexing starts at 0, as in C or Java. A negative index is assumed to be relative to the end of the array—that is, an index of -1 indicates the last element of the array, -2 is the next to last element in the array, and so on. Look here for more.
A Hash is a collection of key-value pairs. It is similar to an Array, except that indexing is done via arbitrary keys of any object type, not an integer index. Hashes enumerate their values in the order that the corresponding keys were inserted.
Hashes have a default value that is returned when accessing keys that do not exist in the hash. By default, that value is nil. Look here for more.
Arrays:
Arrays are used to store collections of data. Each object in an array has an unique key assigned to it. We can access any object in the array using this unique key. The positions in an array starts from " 0 ". The first element is located at " 0 ", the second at 1st position etc.
Example:
Try the following in - irb.
bikes = Array.new
bikes = %w[Bajaj-Pulsar, Honda-Unicorn, TVS-Apache, Yamaha, Suzuki]
You have added 4 elements in the array.
puts bikes[3]
Yamaha,
Add a new element to position 5.
bikes[5] = "Hardly Davidson"
Hashes:
Like arrays, Hashes are also used to store data. Hashes points an object to another object. Consider of assigning a certain "meaning" to a string. Each time you refer that string, it refers its "meaning".
Example:
bikes = Hash.new
bikes = {
'Bajaj' => 'Pulsar 220, Pulsar 200, Pulsar 180 and Pulsar 150',
'Honda' => 'Unicorn, Shine and Splendor',
'TVS' => 'Apache, Star City, and Victor'
}
Try this now:
bikes['Bajaj']
You get => "Pulsar 220, Pulsar 200, Pulsar 180 and Pulsar 150"
An array is an ordered list of things: a, b, c, d
A hash is a collection of key/value pairs: john has a peugeot, bob has a renault, adam has a ford.
The two terms get "hashed" together these days. I think this is how it goes:
A "hash" will have key -> value pairs:
(top -> tshirt, bottom -> shorts, feet -> shoes)
And an "array" will typically have an index:
([0]tshirt, [1]shorts, [2]shoes)
But, right or wrong, you'll see things with key -> value pairs called "arrays", too.
I think the difference depends mainly on when and how you want to use them. You won't get into much trouble calling an array a hash, or vice versa, but you should know the difference.