I have a NSMutableArray that is loaded with values.
Later in the application, I need to change the values of one of the elements in the array.
How do I accomplish this?
thanks
tony
Take a look at the class reference instance methods:
http://developer.apple.com/library/ios/#documentation/cocoa/reference/foundation/Classes/NSMutableArray_Class/Reference/Reference.html
You can use:
replaceObjectAtIndex:withObject: given you know the index of the object.
replaceObjectsAtIndexes:withObjects: to replace multiple objects at once.
Call -[NSMutableArray replaceObjectAtIndex:withObject:]. Or, if the object in the array is mutable, just grab it with -objectAtIndex: and modify its properties directly.
You would probably want to use 'replaceObjectAtIndex:withObject:' for replacing the object itself.
refer following link. It may help you
http://developer.apple.com/library/ios/#documentation/cocoa/reference/foundation/Classes/NSMutableArray_Class/Reference/Reference.html#//apple_ref/occ/instm/NSMutableArray/replaceObjectAtIndex:withObject:
From that you can use replaceObjectAtIndex:withObject: or
replaceObjectsAtIndexes:withObjects: for replacing objects.
just you have to know the index of objects.
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.
The Apple documentation says that the - (void)removeObject:(id)anObject method removes all occurrences of the given object from an NSMutableArray.
Is there a way to remove only one occurrence of the object from the array?
If you have a particular instance that you want removed, which has a unique memory address but would otherwise compare equal to other instances, you would use removeObjectIdenticalTo:.
If you want to remove the first object in the array that fits the bill, use indexOfObject:, which finds the lowest index, followed by removeObjectAtIndex: You can also use indexesOfObjectsPassingTest: to get the list of all indexes that contain equal objects, as an NSIndexSet, and then pick one out from there -- perhaps lastIndex, e.g.
It is really simple:
[yourArray removeObjectAtIndex:[yourArray indexOfObject:yourObject]]
Yes, you want to find the index of the specific object you want to remove and call:
- (void)removeObjectAtIndex:(NSUInteger)index
See the Apple documentation for NSMutableArray here.
I have an NSMutableDictionary with keys and values set up initially.
Later I need to update the values for certain keys, and I thought I would just need to do
[mutableDict setValue:val forKey:key], but this throws an exception saying:
[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object
What am I doing wrong?
(I have another mutable dictionary inside each key of the mutable dictionary, but I don't think that's why)
I'll copy an paste my comment just incase you want to keep your 100% accept
Paul.s
For some reason you are actually working with an NSDictionary not an NSMutableDictionary. Hard to say why without more code, maybe you are using a copy obtained by calling copy or you have loaded the dictionary from a plist/userDefaults.. Can we see some more code?
Dennis
#Paul.s Yes, I did copy an NSMutableDictionary. Would that be why? If it is, then what else should I use to copy?
Paul.s
Use mutableCopy instead of copy to get a mutable dictionary
Dennis
#Paul.s oh.. didn't know about that. I'm new to Objective-C and I tried to search for what I am doing wrong but I missed it :( Thank you!!
I have used sortUsingSelector to sort an NSMutableArray of custom objects.
Now I'm trying to sort an NSMutableArray containing NSMutableArrays of custom objects.
Can you use sortUsingSelector on an NSMutableArray, or does it only work for custom classes?
If you can use blocks, the most straightforward way using sortUsingComparator:. Otherwise, you'll need to use sortUsingFunction:.
In either case, you are going to need to write a custom block or function that takes two arrays as arguments and returns a sort order based on their contents (I'm not sure what logic you are using to determine if array A or array B is "before" or "after" the other).
You'd do something like:
static NSInteger MySorterFunc(id leftArray, id rightArray, void *context) {
... return ascending/descending/same based on leftArray vs. rightArray ...
}
Then:
[myArrayOfArrays sortUsingFunction: MySorterFunc context: NULL];
It sends the selector to the objects, so you'll need to use one of the other sorters. Probably sortUsingFunction:context:.
Of course you can also use sortUsingSelector:, it really doesn’t matter whats the object in your array as long as it responds to the selector you want to use. But NSMutableArray and NSArray don’t have any comparison methods themselves, so you’d have to extend them using a category to implement your compare method.
So you probably want to use the other sorting methods pointed out in the other answers here. It’s not impossible to use sortUsingSelector: but it is rather inconvenient and most people (including me) would argue that it’s bad style to write a category for that.
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;