I have two arrays fetch from web servers, both only contains same numbers of dictionaries. i want to parse every dictionary in two nested for each loops, so used following code:
for (NSDictionary *subscription in feedsArray)
{
for (NSDictionary *unreadCount in unreadCountsArray)
{
[RDInsertObjects insertFeedsWithSubscription:subscription
unreadCount:unreadCount];
}
}
But the results returned isn't me wanted, the inner for loop seemed executed over many times, anyone can tell me how exactly do right things to make this code make sense?
The way you do it, for each single iteration of the first loop the second loop iterates completely.
I think it won't work with a for-in loop. But you said the number of items in the two arrays is the same - so you can easily do:
for (i=0; i< feedsArray.count; i++) {
NSDictionary subscription = feedsArray[i];
NSDictionary unreadCount = unreadCountsArray[i];
[RDInsertObjects insertFeedsWithSubscription: subscription unreadCount: unreadCount];
}
This will only work, if the number of items in the array are always ever the same - if sometime they aren't, you need a modification to prevent an out-of-bounds error or it will crash!
Related
I have an array of core data objects called samples, each sample has a depthFrom and depthToo. I load each sample into a tableView to show the depthFrom and Too. I need to check for gaps between the values and if there is, insert a new sample.
The samples in the table could look like below with depthFrom and depthToo,
The issue is since there is a gap between the numbers from 100 to 210 new samples should be added to the table. using a gap of 50 as much as possible so it would look like this with the auto generated samples.
What im unsure of is how to compare the values, i would rather do it as the view loads before cellForRowAtIndexPath is called so i would not need to reload the table again. I was thinking of looping through each value and comparing them but there all in the same array so im not sure how i would do this. I have all the data displaying correctly in my app its just the gaps i need to account for and if im able to find a way to compare the values in the array then i can manage adding in the new objects i just need pointing in the right direction as this is new to me.
If theres anything about my question that is confusing then just add a comment and i will update it accordingly, thanks for any help.
To fix the gaps, you must keep track of the last depthTo and check if there's a gap between it and the current sample. If there is, insert samples with a spacing of 50*, until we reach our current sample.
Here's a pseudocode solution:
samples = NSMutableArray
int lastDepthTo = 0;
for (i = 0; i < [samples count]; i++) {
s = samples[i]
// add missing samples (upto current s.depthFrom)
while (s.depthFrom > lastDepthTo) {
genDepthTo = MIN(d.depthFrom, lastDepthTo+50)
generated = new sample(depthFrom: lastDepthTo, depthTo: genDepthTo)
[samples insert:generated atIndex:i]
i++ // increment i to skip inserted sample
lastDepthTo = genDepthTo
}
lastDepthTo = s.depthTo
}
Note: this is untested, maybe off by 1 for the indexing of i.
I'm manipulating entries inside a DoubleLinkedQueue via the DoubleLinkedQueueElement.append/prepend methods. This results in the new elements being inserted into the queue, but fails to update the length, and the toList() method results in an error being thrown.
I understand queues are only supposed to have elements added at the start/end, but it looks like the interface should allow for adding in the middle via the entries. I find it hard to believe such a common/well understood data structure would have a bug at this point - so am I using DoubleLinkedQueues incorrectly? Is there another data structure that I should be using? I'm looking to merge values from another iterable into my own sorted iterable - a SplayTreeSet might get me there in n log n time, but a simple merge should get me there in linear time...
Example of code that acts unexpectedly:
main() {
var q = new DoubleLinkedQueue<int>.from([1]);
q.firstEntry().prepend(0);
print('length: ${q.length}');
int i = 0;
for (var qi in q){
print('${i++}: $qi');
}
}
Output:
length: 1
0: 0
1: 1
It looks like the length getter is only pointing to an internal counter. This is done because counting the elements everytime might take very long for long lists.
The internal counter is only updated if you use the methods that directly operate on the list instead of using the prepend method of an element. In your example you should use q.addFirst(0); which results in the length being updates. The .prepend() method just inserts a new element and changes the pointer. Which results in correct traversation of the elements, but the counter is wrong anyway.
Unfortunately it looks like you cannot insert elements in the middle of the list, nor can you make the list recount the elements. You should consider creating a bug over at www.dartbug.com.
// Update:
toList() throws an error because there are more elements than length.
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Solution:
I have marked #BlackRider's answer as correct as it is the most versatile especially for complex comparisons however there are other very good answers and comments. I would encourage anyone with the same or similar question to review them and evaluate the best course of action for your specific situation.
In my situation, I am actually not using BlackRider's solution in my implementation. I have elected to use my own solution (see Edit #2 below) with help from #JoshCaswell's comments as well as #voromax's suggestion of indexesOfObjectsWithOptions:passingTest: due to the fact that my comparisons are very simple in this situation.
Thanks to everyone who answered and provided insight.
I am looking for an efficient way to retrieve an object from an NSArray based on a property of that object (a unique identifier, in this case). In C#.NET using Linq I would do something like
MyObject obj = myList.Single(o => o.uuid == myUUID);
I am also wondering if there is an efficient way to get an array of objects matching a non-unique property. Again, with Linq it would look like
List<MyObject> objs = myList.Where(o => o.flag == true).ToList();
Of course I can write loops to do this but they would not be reusable and I'm suspicious of their performance.
Finding an object with a unique ID:
-(MyObject*)findObjectWithUUID:(NSString*)searchUUID{
for (MyObject* obj in _myArray){
if([obj.uuid isEqualToString: searchUUID])
return obj;
}
}
Finding an array of objects:
-(NSArray*)findObjectsWithFlag:(BOOL)f{
NSMutableArray* arr = [NSMutableArray array];
for (MyObject* obj in _myArray){
if(obj.flag == f)
[arr addObject:obj];
}
return arr;
}
-- EDIT --
Luckily in the first situation the object I am looking for has a unique identifier and I know there will only be one. I came up with a solution to implement isEqual on my object which will be invoked by indexOfObject:
- (BOOL)isEqual:(id)object{
return [self.uuid isEqualToString: ((MyObject*)object).uuid];
}
And then create a "fake" lookup object and use that to find the real one
MyObject *lookupObject = [[MyObject alloc] init];
lookupObject.uuid = searchUUID;
MyObject *actualObject =
[_myArray objectAtIndex:[_myArray indexOfObject:lookupObject]];
This is essentially the same as the for-in loop I posted above, but might be more readable & be more reusable. Of course, this only works for finding one unique object and does not address the second half of my question.
-- EDIT 2 --
Checking Class and implementing hash as recommended in comments.
- (BOOL)isEqual:(id)object{
return [object isKindOfClass:[MyObject class]] &&
[self.uuid isEqualToString: ((MyObject*)object).uuid];
}
- (NSUInteger)hash{
return [self.uuid hash];
}
You can use [NSPredicate], which gives you a query-like syntax for search. Check out this page for the predicate syntax description. Here's a simple example:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"propertyName == %#", #"value"];
NSArray *filteredArray = [myArray filteredArrayUsingPredicate:predicate];
As to performance, I think your solution is OK since any search in an array needs to iterate through all the elements anyway, and then, for each object, compare the value of a field against the value you search for. You can optimize repeat searches within the same data, e.g. by creating and populating a dictionary that maps values of some field to the matching objects (or collections of objects, if the mapping is one to many).
You may also look at modern block syntax: indexOfObjectWithOptions:passingTest: or indexesOfObjectsWithOptions:passingTest: which support concurrency and search order.
I was intrigued by rmaddys comment so I've checked the difference between looping and predicate.
Let's assume a simple object with NSString property. I've inserted it into array 10 000 times , every time with different property value.
In the worst case scenario when desired object was on the last position of the array, loop approach was 3.5x faster than NSPredicate (0.39s vs 0.11s, arraySize = 10000, 10 iterations, iPad Mini)
Code I used for reference: pastebin
I know its related with NSArray but if we do it using Swift and using the swift Array which is a struct, then that will be lot easier.
Swift 2.2 / Swift 3.0 / Swift 4.x Working fine on all versions
Lets assume we have a custom model class
class User {
var userId = 0
var userName = ""
}
And lets assume we have an array named as usersArray which has custom objects of User class.
And we want to fetch an object from this array with userId = 100 for example:-
let filteredArray = usersArray.filter({$0.userId == 100})
This filtered array will contain all the custom objects which have userId as 100
print(filteredArray[0].userName) //will print the name of the user with userId = 100
just for those who are interested, I've found the fastest way to search through NSArray is by using a for loop on a background thread. using the [self performSelectorInBackground...] method.
In an NSArray of 10000 custom objects I searched through the whole thing thoroughly in around 1 second. On the main thread it took around 10 seconds or more.
Here is my situation:
I manipulate 6 NSMutableArrays. One of them has NSDates objects in it, the other ones have NSNumbers. When I populate them, I use addObject: for each of them, so index 0 of each array contains all the values I want for my date at index 0 in the dates array.
I want to make sure that the arrays are all sorted according to the dates array (order by date, ascending), meaning that during the sorting, if row 5 of the dates array is moved to row 1, it has to be applied to all the other arrays as well.
I was previously using CoreData, but I must not use it anymore (please don't ask why, this is off-topic ;) ). In CoreData, I could use an NSSortDescriptor, but I have no idea on how to do it with multiple NSArrays...
As always, hints/answers/solutions are always appreciated :)
This is a common problem. Use the following approach.
Encapsulate your six arrays into an object - every instance will have six properties.
Implement compare: method on this object, using [NSDate compare:] (this step can be skipped but it's cleaner this way).
Now you have only one array - sort it using the method from step 2.
I think the better solution for you to have NSArray of NSDictionary objects.
NSArray *allValues = [[NSArray alloc] init];
NSDictionary *dict = #{"Date" : [NSDate date], #"Key1" : #"Value1", #"Key2" : #"Value2"};
Then you can sort this array with sortDescriptor without any problems.
And then you can also use Comparator or Sort Desriptor as you wish.
Wrap all your items that you are storing in an array into a single object. Each one of your previous 6 arrays will be a property.
Inside that object you can implement
- (NSComparisonResult)compare:(YourClass *)otherObject {
return [self.date compare:otherObject.date];
}
You can now sort the array and they will sort by date.
I have two NSMutableArrays. The content of the first is numerically, which is paired to the content of the second one:
First Array Second Array
45 Test45
3 Test3
1 Test1
10 Test10
20 Test20
That's the look of both arrays. Now how could I order them so numerically so they end up like:
First Array Second Array
1 Test1
3 Test3
10 Test10
20 Test20
45 Test45
Thanks!
I would put the two arrays into a dictionary as keys and values. Then you can sort the first array (acting as keys in the dictionary) and quickly access the dictionary's values in the same order. Note that this will only work if the objects in the first array support NSCopying because that's how NSDictionary works.
The following code should do it. It's actually quite short because NSDictionary offers some nice convenience methods.
// Put the two arrays into a dictionary as keys and values
NSDictionary *dictionary = [NSDictionary dictionaryWithObjects:secondArray forKeys:firstArray];
// Sort the first array
NSArray *sortedFirstArray = [[dictionary allKeys] sortedArrayUsingSelector:#selector(compare:)];
// Sort the second array based on the sorted first array
NSArray *sortedSecondArray = [dictionary objectsForKeys:sortedFirstArray notFoundMarker:[NSNull null]];
Rather than keep two parallel arrays, I'd keep a single array of model objects. Each number from the first array would be the value of one property, and each string from the second array would be the value of the other property. You could then sort on either or both properties using sort descriptors.
Generally, in Cocoa and Cocoa Touch, parallel arrays make work while model objects save work. Prefer the latter over the former wherever you can.