I want to be able to use NSString, or something similar to store an array of strings, but access them by a key (I think this is what it's called), instead of a numeric index.
For example, instead of doing this:
return names[3];
Do something like this:
return names["Bob"];
How do you store and retrieve values like this?
Please excuse my lack of knowledge of this. I've been working with Objective C for about a year now, but never had to do anything like this.
In Objective-C, you do this with the NSDictionary class and its mutable subclass NSMutableDictionary. You can get the value for a key using
NSString *value = [myDictionary objectForKey:#"Bob"];
NSDictionary
Those kind of arrays are taken from C (of which Objective-C is a superset) which only allow integers as their indices.
You may wish to use NSDictionary and it's objectWithKey style messages.
Related
How do you fill a NSMutableArray with a set capacity for later use?
Basically I want to set up a NSMutableArray to act as a map for my game objects, so I have this line...
gameObjects = [[NSMutableArray alloc] initWithCapacity:mapWidth*mapHeight];
Which I had hoped would create and fill my MutableArray so I can get then access it with this kind of index...
int ii = (cellY*mapWidth)+cellX;
NSDictionary *currentObject = [gameObjects objectAtIndex:ii];
But I just learned initWithCapacity doesn't fill the array, so should I create blank objects to fill it with, or is there a Null that I can fill it with? Also would I do that with 2 for loops or is there an instruction something like "initWith:myObject" ?
I want to be able to check at a certain index within the array to see if there's an object there or not, so I need to be able to acces that index point, and I can only do that if there's something there or I get an out of bounds error.
I'll be using this NSMutableArray pretty much as a grid of objects, it's a 1 dimensional array organised as a 2 dimensional array, so I need to be able to fill it with mapWidth*mapHeight of something, and then calculate the index and do a check on that index within the array.
I've looked on here and googled but couldn't find anything like what I'm asking.
Thanks for any advice.
I think what you are looking for is [NSNull null]. It is exactly what you want- a placeholder value.
You can find more information on the topic in this question.
initWithCapacity is just a performance optimization -- it has no effect on the array behavior, it just keeps the code "under the covers" from having to repeatedly enlarge the internal array as you add more entries.
So if you want a "pre-allocated" array, you'd need to fill it with NSNull objects or some such. You can then use isKindOfClass to tell if the object is the right type, or simply == compare the entry to [NSNull null]. (Since there's only ever one NSNull object it always has the same address).
(Or you could use a C-style array of pointers with nil values for empty slots.)
But you might be better off using an NSMutableDictionary instead -- no need to pre-fill, and if the element isn't there you get a nil pointer back. For keys use a NSNumber object that corresponds to what would have been your array index.
initWithCapacity only hints to NSMutableArray that it should support this many objects. It won't actually have any objects in it until you add them. Besides, every entry in the array is a pointer to an object, not a struct like you'd normally have in a standard c array.
You need to change how you're thinking about the problem. If you don't add an object to the array, it's not in there. So either you pre-fill the array with "empty" objects as you've said, which is weird. Or you can add the objects as you need them.
I have an NSArray consisting of NSNumbers and I want to convert this to an NSArray of NSStrings, by getting the stringValue of each NSNumber in the first array.
The method that comes to my mind is iterating each value in the first one, getting its string value and adding it into another array. But there should be a more elegant solution for this. Do you know one?
NSArray implements the Key-Value Coding method valueForKey: in such a way that it returns a new array. The new array contains the results of asking each object in the original array for the specified value. In this case, NSNumber has its stringValue, so all you have to do is:
NSArray * b = [a valueForKey:#"stringValue"];
Plain old fast enumeration (or enumerateObjectsUsingBlock:) wouldn't be a terrible solution, though. NSArray's implementation of valueForKey: most likely uses a for loop internally, and that would be pretty readily understood by anyone who reads it later.
OK this is probably a really stupid question but I can't seem to find the answer.
I know how to sort arrays using sort descriptors but what about a simple array containing only a series of numbers?
For example an array containing the following numbers:
21,3,11,58,32,76,19,45,7,92
I just need the numbers in ascending order.
I'm assuming you mean an NSArray of NSNumbers.
Fairly simple:
NSArray *unorderedNumbers; // assume exists
NSArray *sortedArray = [unorderedNumbers
sortedArrayUsingSelector:#selector(compare:)];
(See sortedArrayUsingSelector:).
This causes the objects (which happen to be numbers) to be compared using compare:.
The C standard library includes qsort
See: http://www.cplusplus.com/reference/clibrary/cstdlib/qsort/
so I'm having the most difficult of time pulling values out of an NSDictionary. Right now I just have a dictionary that is populated from a JSON call and it only contains a key named 'Success' with a value of 0 or 1.
How do I do a conditional on that value to check if its 0 or 1? I've tried a bunch of things, but I'm not getting anywhere. Here's my current code:
[[jsonDictionary objectForKey:#"Success"] isEqualToNumber:1]
I'm getting passing argument 1 of 'isEqualToNumber:' makes pointer from integer without a cast' as a warning, and the app crashes when it hits that line anyway.
And a subquestion, what's the difference between objectForKey and valueForKey? Which one should I use by default?
Anyway, this noob in Objective-C would truly appreciate some help on this. Thanks in advance!
Since dictionaries contain Objective-C objects, an entry containing a number is an NSNumber instance. NSNumber provides a convenience method, -intValue, for extracting its underlying int value:
if ([[jsonDictionary objectForKey:#"Success"] intValue] == 1) { … }
Note that NSNumber has other convenience methods for extracting its underlying value as other C data types.
In most cases, you should use -objectForKey: instead of -valueForKey:. The former is the canonical method to obtain an entry in the dictionary and is declared in NSDictionary. The latter is declared in NSObject and is used in Key-Value Coding contexts, where the key must be a valid KVC key, and there’s additional processing — for instance, if you’re using -valueForKey: in a dictionary with a key that starts with #, that character is stripped from the key and [super valueForKey:key] is called.
The number 1 is not an object pointer. Use an NSNumber instance instead if you want to use a number in an NSDictionary.
[[jsonDictionary objectForKey:#"Success"]
isEqualToNumber:[NSNumber numberWithInteger:1]]
[[jsonDictionary objectForKey:#"Success"] isEqualToNumber: [NSNumber numberWithInt:1]]
Number and Value Programming Topics: Using Numbers
NSNumber: What is the point ?
You can get the value of dictionary in different ways like checking
the value first.
Solution 1: Using simple if statement.
int value = 0;
if ([[jsonDictionary objectForKey:#"Success"]intValue]==1){
value = [[jsonDictionary objectForKey:#"Success"]intValue];
}
Solution 2: Using ternary operator
value = ([[jsonDictionary objectForKey:#"Success"]intValue]==1) ? 1:0;
I want to use the object's reference value as a key into a dictionary, as opposed to a copy of value of the object. So, I essentially want to store an object associated with a particular instance of another object in a dictionary and retrieve that value later.
Is this possible? Is it completely against the idea of NSDictionary? I can tell that I am probably approaching this the wrong way because the dictionary wants me to implement NSCopying on the object itself, which doesn't really make sense in terms of what I'm doing. I can see that what I should really be doing is wrapping the pointer value, but that seems a little mad.
Advice would be appreciated.
I think you can use [NSValue valueWithPointer:object].
NSMutableDictionary has been designed to only deal with Objective-C object instances. For example, when you call setObject:forKey: method calls copyWithZone: on the key and retain on the value.
If you want to have a dictionary structure and to be able to deal with arbitrary key and value, then you can go with CFMutableDictionary. You can describe precisely what is done with key and values; it is flexible enough to deal with arbitrary pointer or event char * strings.
This did the trick for me
aDictionary[#((intptr_t)object)] = ...;
You can use the address in memory of myObejct as a key in myDictionary
NSString *myObject_addressInMemory = [NSString stringWithFormat:#"%p", myObject];
myDictionary[myObject_addressInMemory] = someValue;