I have cpp code where the struct objects are put into the CLISTS. I am porting this code into Objective C.
CLIST is similar to a doubly linked list with .RemoveAt , .GetAt , .InsertBefore , .GetNext , .GetHeadPosition functions.
How to implement the same in Objective C.
Do I need to implement doubly linked list in Objective C.Is there any other predefined methods to use it.
A CLIST is presumably circular? Hence the GetHeadPosition?
In any case, NSArray (or, NSMutableArray in this case, since you want to be inserting) is the normal way to keep ordered lists in Objective-C.
For RemoveAt, use removeObjectAtIndex:. For GetAt, use objectAtIndex:. For InsertBefore you're probably going to want to write a little something like:
- (void)insert:(id)objectToInsert before:(id)referenceObject
{
int index = [array indexOfObject:referenceObject];
if(index == NSNotFound) return; // or whatever you'd expect.
// Maybe object is just inserted at the end?
index = index - 1;
if(index < 0) index = [array count];
[array insertObject:objectToInsert atIndex:index];
}
(which would probably go better in an NSArray category, but you get the point)
For GetNext and GetHeadPosition you probably want to keep your array position in a separate variable. So for GetNext:
arrayPosition = (arrayPosition + 1)%[array count];
return [array objectAtIndex:arrayPosition];
And for GetHeadPosition, just:
return arrayPosition;
EDIT: for iterating through an NSArray, the easiest way is actually to ignore anything explicit and just use:
for(ObjectType *object in array)
{
/* do something with object */
}
That generally means you don't really need an analogue of GetNext, but you can't mutate the array while in that loop so it's not always usable.
Related
I have an array in an old objective-C app that I am using to learn more "complicated" coding. It is back from the old days of OS X and was very much broken. I have gotten it to work (mostly)! However, the app has an NSMutableArray of images, 7 in total. I use a random number generator to insert the images on the screen, some code to allow them to fall, and then, using screen bounds, when they reach "0" on the Y axis they are removed from the array.
I initially just had:
if( currentFrame.origin.y+currentFrame.size.height <= 0 )
{
[flakesArray removeObject:myItem];
I have read when removing objects from an array it is best practice to iterate in reverse...so I have this bit of code:
for (NSInteger i = myArray.count - 1; i >= 0; i--)
{ //added for for statement
if( currentFrame.origin.y+currentFrame.size.height <= 0 )
{
[myArray removeObjectAtIndex:i];
}
Sadly both methods result in the same mutated while enumerated error. Am I missing something obvious?
If I add an NSLog statement I can get, I think, the index of the item that needs to be removed:
NSLog (#"Shazam! %ld", (long)i);
2017-01-07 14:39:42.086667 MyApp[45995:7500033] Shazam! 2
I have looked through a lot and tried several different methods including this one, which looks to be the most popular with the same error.
Thank you in advance! I will happily provide any additional information!
Adding more:
Sorry guys I am not explicitly calling NSFastEnumeration but I have this:
- (void) drawRectCocoa:(NSRect)rect
{
NSEnumerator* flakesEnum = [flakesArray objectEnumerator];
then
for( i = 0; i < numberToCreate; i++ )
{
[self newObject:self];
}
while( oneFlake = [flakesEnum nextObject] )
It is here where:
if( currentFrame.origin.y+currentFrame.size.height <= 0 )
{
NSLog (#"Shazam! %i", oneFlake);
[flakesArray removeObject:oneFlake];
}
Thank you all. I am learning a lot from this discussion!
There are two ways to go: (1) collect the objects to remove then remove them with removeObjectsInArray:.
NSMutableArray *removeThese = [NSMutableArray array];
for (id item in myArray) {
if (/* item satisfies some condition for removal */) {
[removeThese addObject:item];
}
}
// the following (and any other method that mutates the array) must be done
// *outside of* the loop that enumerates the array
[myArray removeObjectsInArray:removeThese];
Alternatively, reverseObjectEnumeration is tolerant of removes during iteration...
for (id item in [myArray reverseObjectEnumerator]) {
if (/* item satisfies some condition for removal */) {
[myArray removeObject: item];
}
}
As per the error, you may not mutate any NSMutableArray (or any NSMutable... collection) while it is being enumerated as part of any fast enumeration loop (for (... in ...) { ... }).
#danh's answer works as well, but involves allocating a new array of elements. There are two simpler and more efficient ways to filter an array:
[array filterUsingPredicate:[NSPredicate predicateWithBlock:^(id element, NSDictionary<NSString *,id> *bindings) {
// if element should stay, return YES; if it should be removed, return NO
}];
or
NSMutableIndexSet *indicesToRemove = [NSMutableIndexSet new];
for (NSUInteger i = 0; i < array.count; i += 1) {
if (/* array[i] should be removed */) {
[indicesToRemove addIndex:i];
}
}
[array removeObjectsAtIndexes:indicesToRemove];
filterUsingPredicate: will likely be slightly faster (since it uses fast enumeration itself), but depending on the specific application, removeObjectsAtIndexes: may be more flexible.
No matter what, if you're using your array inside a fast enumeration loop, you will have to perform the modification outside of the loop. You can use filterUsingPredicate: to replace the loop altogether, or you can keep the loop and keep track of the indices of the elements you want to remove for later.
I apologize for this basic question, but I am 2-month new to obj-c.
Problem:
I am not able to find if an object with same data already exists in the NSMutableArray.
What I am doing?
ScanDigInfoForTable* sfile = [[ScanDigInfoForTable alloc]init];
sfile.data = "myData";
int inde = [_DataList indexOfObject:sfile] ;
if(inde == -1)
[_DataList addObject:sfile];
ScanDigInfoForTable* sfile2 = [[ScanDigInfoForTable alloc]init];
sfile2.data = "myData";
inde = [_DataList indexOfObject:sfile2] ;
if(inde == -1)
[_DataList addObject:sfile2];
Issue:
The _DataList get 2 objects instead of 1. Many thanks in advance for your attention.
S.P: I already know that I may traverse the whole array in a loop in order to check the data already exists. Looking for a better solution as the array may have thousands of records.
Well, comparing two custom objects is really not that simple for the simple fact there is no defined way to declare equality. It is individual choice to define the rules for equality for the objects they are creating.
In your case, it would be two step process:
Step 1: Implement isEqual: in your ScanDigInfoForTable class. Assuming ScanDigInfoForTable is a model class and that it has three string properties - code, data & itemID (you can have any type).
- (BOOL)isEqual:(ScanDigInfoForTable *)other {
return [self.code isEqualToString:other.code] && [self.data isEqualToString:other.data] && [self.itemID isEqualToString:other.itemID];
}
Step 2: Call containsObject: method on NSMutableArray. This method would internally call isEqual: to give you the results based on the rules you defined.
// If the object does not exist in the list, we add it
if (![_DataList containsObject:sfile2]) {
[_DataList addObject:sfile2];
}
In Objective-C object equality is determined by the methods -isEqual: and -hash.
When testing object membership in a collection the items of the collection are sent isEqual:. The default implementation only compares the addresses of objects, which is why you are seeing duplicates. Your objects do no provide their own implementation of equality based on the data they contain.
To fix this you can override isEqual: to compare objects based on the data they represent. Using your example in your question, this could just be:
- (BOOL) isEqual:(id)object {
BOOL result = N0;
if (object != self){
if ([object isKindOfClass:[self class]]){
result = [[self data] isEqual:[(ScanDigInfoForTable *)object data]];
}
} else {
result = YES;
}
return result;
}
Mike Ash has a great article about implementing equality. In general, if you are implementing a custom class you should make equality a part of that.
You can user filteredArrayUsingPredicate for example
NSArray * matches = [_DataList filteredArrayUsingPredicate:
[NSPredicate predicateWithFormat:#"data == %# ",sfile2.data]];
if(matches.count == 0) {
[_DataList addObject:sfile2];
}
Something like this?
NSMutableSet* set1 = [NSMutableSet setWithArray:array1];
NSMutableSet* set2 = [NSMutableSet setWithArray:array2];
[set1 intersectSet:set2]; //this will give you only the obejcts that are in both sets
NSArray* result = [set1 allObjects];
This has the benefit of not looking up the objects in the array, while looping through another array, which has N^2 complexity.
and also set2 doesn't have to be mutable, might as well use just
NSSet* set2 = [NSSet setWithArray:array2];
Here is the object, and have following attribute:
NSString attri1;
NSString attri2;
NSString attri3;
NSString attri4;
If I want to list these attri, I can call
NSLog(aObj.attri1);
But can I make the 1 as a variable to call it from a loop? Is this possible to do so in objective-c?
for(int i = 0; i < [array count]; i++)
{
NSLog(aObj.attri1); //is this possible to become one line, dynamic generated variable
}
Thank you. btw, What is this feature called? Thanks.
If you want to dynamically access a property of an object, that can be done with Key Value Coding.
If the class is KVC-compliant, as most NS classes are, you can use valueForKey: or valueForKeyPath: to access a property with a string:
for(int i = 0; i < [array count]; i++) {
NSLog([[aObj valueForKey:[NSString stringWithFormat:#"attrib%d", i]]);
}
The feature you're looking for is generally called "variable variables." Objective-C does not have this feature. Actually, most languages don't.
The good news is that you don't actually need this feature. Four variables named the same thing with a number at the end is basically equivalent to an array, only with the structure being implicit rather than explicit. Just make attri an array and then you can ask it for a numbered item.
I have seen this over and over, why exactly is it faster to use fast enumeration in loops rather than an NSEnumerator using nextObject:.
NSEnumerator is the old way to enumerate over collections. It involves creating an object to represent the enumeration, then calling a method on it for every single iteration. While this was perfectly serviceable for many years, it's not terribly efficient, as it involves at least one message send for every iteration of the loop. NSFastEnumeration is the more modern approach, which leverages native language support to provide a much more efficient enumeration. The way it works under the hood is it creates a struct that represents the current enumeration state and repeatedly calls -countByEnumeratingWithState:objects:count: on the collection. This method returns a C array of objects in the objects out-param as well as a counter in the count out-param. This allows the caller to then iterate over the C array. In essence, this means one message call per chunk of objects, which, depending on the collection, could be as efficient as a single message call to get all objects.
If you have a bit of code that looks like
for (id obj in myArray) {
[obj doSomething];
}
This gets translated by the compiler into something roughly equivalent to
NSFastEnumerationState __enumState = {0};
id __objects[MAX_STACKBUFF_SIZE];
NSUInteger __count;
while ((__count = [myArray countByEnumeratingWithState:&__enumState objects:__objects count:MAX_STACKBUFF_SIZE]) > 0) {
for (NSUInteger i = 0; i < __count; i++) {
id obj = __objects[i];
[obj doSomething];
}
}
The actual variables used are hidden, and the maximum size of the object buffer is also implementation-dependent, but the basic idea is there. It translates iteration over an obj-c collection into iteration over a C array.
GCC 8.9.4 Fast enumeration
protocol
GNUstep libs/base/trunk/Source/NSEnumerator.m countByEnumeratingWithState:objects:count:
It is not same as Apple's implementation but it is helpful to understand.
- (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*)state
objects: (id*)stackbuf
count: (NSUInteger)len
{
IMP nextObject = [self methodForSelector: #selector(nextObject)];
int i;
state->itemsPtr = stackbuf;
state->mutationsPtr = (unsigned long*)self;
for (i = 0; i < len; i++)
{
id next = nextObject(self, #selector(nextObject));
if (nil == next)
{
return i;
}
*(stackbuf+i) = next;
}
return len;
}
NSArray *array = something;
array = { {1,2}, {2,3}, {3,4} }
that means array is an array of array. so how can you access all the arrays and their values.
we can use for loop like this
for (int i = 0; i < array.count; i++)
{
NSArray x = [array objectAtIndex:i];
}
or a fast enum works like this
for(NSArray array2 in array)
{
// do what ever you want with this new array2.
}
this is a sample example.
PS. I forgot how the array looks in console.
Can I expect it to go from the start of an array to the end in order? Can't find anything in the docs about this.
i.e. is
for (id val in array)
{
NSLog(#"%#", val);
}
always going to print out the same as
for (int i = 0; i < [array count]; ++i)
{
NSLog(#"%#", [array objectAtIndex:i]);
}
From Apples' Objective-C documentation on fast enumeration:
For collections or enumerators that have a well-defined order—such as NSArray or NSEnumerator instance derived from an array—the enumeration proceeds in that order, so simply counting iterations will give you the proper index into the collection if you need it.
Once again I've found the answer right after posting. My old reference didn't mention the order of iteration, but the online one did. The array is indeed iterated in order.