To use uisearchdisplaycontroller to specify a search on specific array fields, I found that the syntax caters only to key-value (dictionary?) data. Now, when I need to sort an array, I see again that the methods cater to key references, not mere array indexing. (I've built complex data with keyless arrays, not dictionaries.) Do I understand correctly that I should default to using dictionaries everywhere? Is there any situation to use plain arrays?
I see now the Key-Value Coding Guide (document 1812 of 3894)... should get to it any minute now.
It is NOT mandatory but it will be much comfortable when using NSDictionary.
Also, with the plain array, you can sort it as well without using "%K" at predicate format or using block comparator
Related
I've run into the same problem as found in this question. However, I have a follow-up question. I seem to be in the same situation as the original asker: I have a plist with a hierarchy of dictionaries that define a configuration screen. These are not mutable and will stay the same throughout the application. Since the original discussion seems to focus on problems arising from mutating the dictionary, I must ask for comfirmation: is the order of a dictionary guaranteed the same as they are in the plist, i.e. as it is read (with initWithContentsOfFile)? Can I use allKeys on it in this case to get a correct-order array of keys if the dictionary never changes?
No, the keys are not likely to appear in the same order as your plist file. I can't look under the hood, but I would guess that the keys are ordered in whatever way that provides an efficient look-up. The documentation for allKeys says that the order of the elements in the array is not defined.
If you need an NSDictionary that maintains order, take a look at CHDataStructure's CHOrderedDictionary, which does exactly that. I use it all the time.
I would not assume that the keys will be in any kind of order because allKeys does not guarantee order.
Internally, NSDictionary uses a hash table.
From allKeys description:
The order of the elements in the array
is not defined.
If you want to display values in some order then get the keys, sort the keys array and then get the value for each key.
Nothing at all is guaranteed about order. It’s not even guaranteed that iterating over a dictionary twice will give you the keys in the same order.
I'm making a plist to hold genre synonyms. The list of synonyms for a given genre doesn't have any inherent order to it.
Should I use an array (which implies an order that doesn't exist) or a dictionary (which implies there's a corresponding value for each key, which doesn't exist).
Simply put--to store an unordered set in a plist, how should I represent it and why?
(To clarify: If there were a Set data structure in the plist editor, I would use that, but I only have Array and Dictionary to choose from.)
More details: I'm going to be looking up by the primary representation of the genre, thus the outer data structure in the plist has to be a dictionary.
But then for the synonyms, the only operation necessary is to enumerate them using a block.
So either an array or a dictionary will do. However, I'm concerned that using an array will imply an order that doesn't have any semantic meaning. On the other hand, is it a common occurrence to have dictionaries in plists that don't have a corresponding value?
Editing again to respond to Josh's comments:
I like your idea of converting into an NSSet after reading in the plist. However, I could still do that with a dictionary, right? So not sure why an array is the obvious choice.
If someone else edits the plist, they might think there's a meaning to the order, when in reality, the ordering is arbitrary.
Surprised no-one has defended using a dictionary instead of an array. Is there a reason a dictionary shouldn't be used in a plist for this purpose?
If you don't care about order, then the arbitrary order you get from building an array is equivalent to the arbitrary order you'd get by using a set. You can also very easily convert an array in a plist to an NSSet after reading it back: +[NSSet setWithArray:]
So use an array.
I would just use an array, since you say there's no corresponding key for a dictionary entry.
At the same time, if you're typing in a large number of entries into plist files (www), your fingers may get tired from dealing with the raw XML or plist editor stuff. You might want to consider a different way to save your synonyms?
Use an NSArray if lookup by item is not needed. If lookup is needed use an NSDictionary.
Assuming I have a hash value of some NSObject during runtime.
Is there a way to find a pointer to that object using just hash value?
I don't want to store pointers to objects and their hashes as keys. I imagine that iOS already doas that.
There is no way, not even an unreliable way, to do this.
Many objects have hashes in ways that makes it impossible to reference it. You will have duplicates because of this. One example, as #Martin said, is NSArrays. NSArrays' hashes are just the number of elements in the array.
This question already has answers here:
What's the difference between a dictionary and an array?
(6 answers)
Closed 10 years ago.
I'm in a dilemma in terms of which of the two I should use. I will be retrieving a group of data via a restful API (returns json) and I'm not sure how I should store them before I display it on my UI View Table.
eg.
{"Events":[{"Id":5,"Name":"Event 1 2013"},{"Id":6,"Name":"Event 2 2013"}]}
I've been reading tutorials and some would use NSMutableArrays while some would use NSMutableDictionary.
How should I go about it?
BTW: I'm displaying the data on UI View table that will redirect the user to another page when tapped and if they decide to go back will have to show the previous view without reloading (uses UinavigationController)
Thanks!
EDIT:
Also, just to give you an idea on what I'm trying to do. I'm trying to follow this tutorial on splitting the data I get into section headers. On this tutorial it's using NSDictionary.
http://www.icodeblog.com/2010/12/10/implementing-uitableview-sections-from-an-nsarray-of-nsdictionary-objects/
If I use NSArray, would that affect the performance?
In NSArray - every item in the collection has an integer index, so there is an explicit order to the items. When you're retrieving/replacing/removing the stored object from the NSARRY,you need to specify the corresponding object index of that stored object.
NSDictionary - derived from the word called entry. Each entry consists of one object that represents the key and a second object that is that key’s value. Within a dictionary, the keys are unique. That is, no two keys in a single dictionary are equal (as determined by isEqual:).When you're retrieving the object from the dictionary you need to specify the key value for the objectForKey
Whenever if you're parsing the plist then NSDictionary would be ideal.You can refer apple's document here for more explanation about NSDictionary.Happy coding :)
The lookup times on NSDictionaries are faster than on NSArrays. That's one of the main advantages. Here's a link to the Apple documentation.
Generally, if you need to access data in an indexed fashion (like you need to for rows in a table) then you should use an array because you can access any specific index using indexOfObject:
Now, if you have a lot of information for each row then you should have an array of either custom objects or an array of dictionaries.
Dictionary are always faster than Arrays. Dictionary maps keys to objects, just like a hash table. It's an associative array.
For searching some value you need to iterate for arrays, in dictionary you retrieve it by key.
If you want the collection to be in some sorted order or arrival order then Array is the proper type for you.
Dictionary lacks when you end up getting two same keys.
And I feel good to use arrays for tableViews as I can directly associate row to index.
I'm simply curious as lately I have been seeing the use of Hashmaps in Java and wonder if Delphi's Sorted String list is similar at all.
Does the TStringList object generate a Hash to use as an index for each item? And how does the search string get checked against the list of strings via the Find function?
I make use of Sorted TStringLists very often and I would just like to understand what is going on a little bit more.
Please assume I don't know how a hash map works, because I don't :)
Thanks
I'm interpreting this question, quite generally, as a request for an overview of lists and dictionaries.
A list, as almost everyone knows, is a container that is indexed by contiguous integers.
A hash map, dictionary or associative array is a container whose index can be of any type. Very commonly, a dictionary is indexed with strings.
For sake of argument let us call our lists L and our dictionaries D.
Lists have true random access. An item can be looked-up in constant time if you know its index. This is not the case for dictionaries and they usually resort to hash-based algorithms to achieve efficient random access.
A sorted list can perform binary search when you attempt to find a value. Finding a value, V, is the act of obtaining the index, I, such that L[I]=V. Binary search is very efficient. If the list is not sorted then it must perform linear search which is much less efficient. A sorted list can use insertion sort to maintain the order of the list – when a new item is added, it is inserted at the correct location.
You can think of a dictionary as a list of <Key,Value> pairs. You can iterate over all pairs, but more commonly you use index notation to look-up a value for a given key: D[Key]. Note that this is not the same operation as finding a value in a list – it is the analogue of reading L[I] when you know the index I.
In older versions of Delphi it was common to coax dictionary behaviour out of string lists. The performance was terrible. There was little flexibility in the contents.
With modern Delphi, there is TDictionary, a generic class that can hold anything. The implementation uses a hash and although I have not personally tested its performance I understand it to be respectable.
There are commonly used algorithms that optimally use all of these containers: unsorted lists, sorted lists, dictionaries. You just need to use the right one for the problem at hand.
TStringList holds the strings in an array.
If you call Sort on an otherwise unsorted (Sorted property = false) string list then a QuickSort is performed to sort the items.
The same happens if you set Sorted to true.
If you call Find (or IndexOf which calls find) on an unsorted string list (Sorted property = false, even if you explicitly called Sort the list is considered unsorted if the Sorted property isn't true) then a linear search is performed comparing all strings from the start till a match is found.
If you call Find on a sorted string list (Sorted property = true) then a binary search is performed (see http://en.wikipedia.org/wiki/Binary_search for details).
If you add a string to a sorted string list, a binary search is performed to determine the correct insertion position, all following elements in the array are shifted by one and the new string is inserted.
Because of this insertion performance gets a lot worse the larger the string list is. If you want to insert a large number of entries into a sorted string list, it's usually better to turn sorting off, insert the strings, then set Sorted back to true which performs a quick sort.
The disadvantage of that approach is that you will not be able to prevent the insertion of duplicates.
EDIT: If you want a hash map, use TDictionary from unit Generics.Collections
You could look at the source code, since that comes with Delphi. Ctrl-Click on the "sort" call in your code.
It's a simple alphabetical sort in non-Unicode Delphi, and a slightly more complex Unicode one in later versions. You can supply your own comparison for custom sort orders. Unfortunately I don't have a recent version of Delphi so can't confirm, but I expect that under the hood there's a proper Unicode-aware and locale-aware string comparison routine. Unicode sorting/string comparison is not trivial and a little web searching will point out some of the pitfalls.
Supplying your own comparison routine is often done when you have delimited text in the strings or objects attached to them (the Objects property). In those cases you often wat to sort by a property of the object or something other than the first field in the string. Or it might be as simple as wanting a numerical sort on the strings (so "2" comes after "1" rather than after "19")
There is also a THashedStringList, which could be an option (especially in older Delphi versions).
BTW, the Unicode sort routines for TStringList are quite slow. If you override the TStringList.CompareStrings method then if the strings only contain Ansi characters (which if you use English exclusively they will), you can use customised Ansi string comparisons. I use my own customised TStringList class that does this and it is 4 times faster than the TStringList class for a sorted list for both reading and writing strings from/to the list.
Delphi's dictionary type (in generics-enabled versions of Delphi) is the closest thing to a hashmap, that ships with Delphi. THashedStringList makes lookups faster than they would be in a sorted string list. you can do lookups using a binary search in a sorted stringlist, so it's faster than brute force searches, but not as fast as a hash.
The general theory of a hash is that it is unordered, but very fast on lookup and insertion. A sorted list is reasonably fast on insertion until the size of the list gets large, although it's not as efficient as a dictionary for insertion.
The big benefit of a list is that it is ordered but a hash-lookup dictionary is not.