I have a to-many relationship, where a List can have many Items.
I then have a List variable, whith the list Entity that I need. No problems there.
The list is a tableview and the user can add rows, so when they add a row it adds it as an Item relationship to the List.
I can then use List.valueForKey("item") to get the Items. But the return type is AnyObject. Is there a better way than casting it to an NSSet and then having to go through and grt the values and putting them into an array?
In other words, how can I get the value of all the Item entities and put them into an array ?
Pd: the Relationship is Ordered.
You cannot successfully cast something to something it is not. Casting is merely a way of revealing the truth to the compiler. The implication is that you know more than the compiler does. But you must tell the compiler the truth! If you lie to the compiler, you will crash at runtime.
So, if casting to an array causes a crash at runtime, that's because this is not an array. You can only cast to what it really is.
Related
In which of NSArray, NSDictionary, or NSSet can I store a nil value?
In none.
You can insert instances (it is a shared one) of NSNull. But in many cases the need to insert "no value" into a collection is a code smell. The selection of a specific collection should not depend on such facilities, but on how you logically want to access them.
There are collections that can deal with nil values, i. e. NSPointerArray. But as mentioned: Usually you do something wrong.
What is the suggested approach for updating an objects value in an array, bearing in mind the array may have been reordered?
I'm wondering how dangerous using index based paths is, when an array could have possibly changed via a deletion, or reorder.
Would it be better to use objects instead, I wonder.
If you are using a mutable list, it is inherently unsafe to update an object by its position in a list. The right thing to do is to use deref. Assuming you have a list of references (the most common case) you can dereference a Model at its position in the list. This will ensure it points to the object's identity path rather than the index in the list. Then you can update the object directly without worrying about whether it has moved around in the list.
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.
Is the best way to sort an array in Delphi is "alphanumeric".
I found this comment in an old code of my application
" The elements of this array must be in ascending, alphanumeric
sort order."
If so ,what copuld be the reason?
-Vas
There's no "best" way as to how to sort the elements of an array (or any collection for that fact). Sort is a humanized characteristic (things are not usually sorted) so I'm guessing the comment has more to do with what your program is expecting.
More concretely, there's probably other section of code elsewhere that expect the array elements to be sorted alphanumerically. It can be something so simple as displaying it into a TreeView already ordered so that the calling code doesn't have to sort the array first.
Arrays are represented as a contiguous memory assignment so that access is fast. Internally the compiler just does a call to GetMem asking for SizeOf(Type) * array size. There's nothing in the way the elements are sorted that affects the performance or memory size of the arrays in general. It MUST be in the program logic.
Most often an array is sorted to provide faster search times. Given a list of length L, I can compare with the midpoint (L DIV 2) and quickly determine if I need to look at the greater half, or the lesser half, and recursively continue using this pattern until I either have nothing to divide by or have found my match. This is what is called a Binary search. If the list is NOT sorted, then this type of operation is not available and instead I must inspect every item in the list until I reach the end.
No, there is no "best way" of sorting. And that's one of the reasons why you have multiple sorting techniques out there.
With QuickSort, you even provide the comparison function where you determine what order you ultimately want.
Sorting an array in some way is useful when you're trying to do a binary search on the array. A binary search can be extremely fast, compared to other methods. But if the sort error is wrong, the search will be unable to find the record.
Other reasons to keep arrays sorted are almost always for cosmetic reasons, to decide how the array is sent to some output.
The best way to re-order an array depends of the length of the array and the type of data it contains. A QuickSort algorithm would give a fast result in most cases. Delphi uses it internally when you're working with string-lists and some other lists. Question is, do you really need to sort it? Does it really need to stay an array even?
But the best way to keep an array sorted is by keeping it sorted from the first element that you add to it! In general, I write a wrapper around my array types, which will take care of keeping the array ordered. The 'Add' method will search for the biggest value in the array that's less or equal to the value that I want to add. I then insert the new item right after that position. To me, that would be the best solution. (With big arrays you could use the binary search method again to find the location where you need to insert the new record. It's slower than appending records to the end but you never have to wonder if it's sorted or not, since it is...