I have an NSMutableArray with contents I want to replace with NSNull objects.
This is what I do:
NSMutableArray* nulls = [NSMutableArray array];
for (NSInteger i = 0; i < myIndexes.count; i++)
[nulls addObject:[NSNull null]];
[stageMap replaceObjectsAtIndexes:myIndexes withObjects:nulls];
How can I do this more efficiently?
Is there a way to enumerate an NSIndexSet, so I can replace the array content one by one?
Solved
Suggested method turns out to be 2x faster (avg 0.000565s vs 0.001210s):
if (myIndex.count > 0)
{
NSInteger index = [myIndex firstIndex];
for (NSInteger i = 0; i < myIndex.count; i++)
{
[stageMap replaceObjectAtIndex:index withObject:[NSNull null]];
index = [myIndex indexGreaterThanIndex:index];
}
}
You can use a for loop. Start with the first index, use indexGreaterThanIndex: to get the next index, and stop after you hit the last index.
Don't forget to account for an empty index set. Both the first and last index will be NSNotFound in that case. The easiest way is to test the index set's count; if it's zero, don't loop.
Also, what Jason Coco said about profiling. Don't worry too much about efficiency until your program works, and don't go optimizing things until you have run Shark (or Instruments, if that's your thing) and found exactly what is slow.
I realise this is a very old question but I'm posting here in case anyone else finds this question you could use:
[indexes enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
NSLog(#"index: %d", idx);
[objectArray replaceObjectAtIndex:idx
withObject:newObject];
}];
Which is a lot more succinct.
Related
I am having some issues trying to wrap my head around why we can not mutate a collection during enumeration... Apparently, if you are doing any sort of fast enumeration, the system should throw an exception if you try to mutate. Below I have three examples where I am mutating during enumeration. One is a simple C-style loop, and the other two use some form of fast-enumeration. I am only getting enumeration exceptions thrown for case 2. Shouldn't I also be getting exceptions thrown for case 1? Why is case 1 valid? Also, people throughout stack overflow say my case 3 is bad practice, but why? It is simple and seems to work. Inconsistency in how the two different fast-enumeration loops are behaving and the general disgust with the C-style loop is screwing with my understanding here. Instead of vague generic rules of thumb, if someone can really break this down to a science this would really help. From a fundamental level I want to know why the exceptions are not consistent here and why case 3 works for me when it apparently "shouldn't" or is "bad practice."
//Case 1:
NSMutableArray *array = [NSMutableArray arrayWithObjects:#"phuj", #"whub", #"adgh", nil];
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
[array removeObjectAtIndex:idx];
}];
//Case 2:
NSMutableArray *array = [NSMutableArray arrayWithObjects:#"phuj", #"whub", #"adgh", nil];
for (NSString *string in array) {
[array removeObject:string];
}
//Case 3:
NSMutableArray *array = [NSMutableArray arrayWithObjects:#"phuj", #"whub", #"adgh", nil];
for (int i = 0; i < [array count]; i++) {
[array removeObjectAtIndex:i];
}
We love simple and straightforward code.
I won't talk about case 2, because it throws an exception and you can't mutate the array in it.
I want to start from an example, suppose we want to remove the "whub" item from a mutable array and the content of the array is "phuj, whub, whub, adgh", it has two whub as you see.
Let's start to write the code using c style loop:
for (int i = 0; i < [mArray1 count]; i++) {
NSString *str = mArray1[i] ;
if ([str isEqualToString:#"whub"]) {
[mArray1 removeObjectAtIndex:i] ;
}
}
The code has a bug, after it removes the first "whub", all the indexes of item located after it will decrease by 1, so the index of second whub should be 1 and i is 1 at the moment. In the next loop, i is 2, so it skips the second whub. You can change the code to make it correct.
for (int i = 0; i < [mArray1 count]; i++) {
NSString *str = mArray1[i] ;
if ([str isEqualToString:#"whub"]) {
[mArray1 removeObjectAtIndex:i] ;
--i ;
}
}
It works, but it's not straightforward, we modify the index i and it makes the code complex.
Let's try to write the code using enumerateObjectsUsingBlock method like case 1:
NSMutableArray *array = [NSMutableArray arrayWithObjects:#"phuj", #"whub", #"whub", #"adgh", nil] ;
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
if ([obj isEqualToString:#"whub"]) {
[array removeObjectAtIndex:idx] ;
}
}];
If you build and run it, you will find the result array still contains one "whub" and this time, we can't modify the index to fix it.
Conclude: You talk much about that you can mutate the array in the loop, the fact you can mutate the array doesn't mean you will get the correct answer. In fact, with c style loop you can do what ever you want, it won't crash, but we say it is bad because we have a better way to go. In practice, we'd like to use simple code to achieve the target, so others can understand easily, ourselves can also benefit from it.
I'd like to use this way, I think it is simple and straightforward.
NSMutableArray *arrayRemove = [NSMutableArray array] ;
for (NSString *str in mArray1) {
if ([str isEqualToString:#"whub"]) {
[arrayRemove addObject:str] ;
}
}
[mArray1 removeObjectsInArray:arrayRemove] ;
I try to delete items from NSMutableArray in loop. I have an array :(2,3,4,5,6).
int j = [array count];
while (array != NULL) {
NSUInteger g = 0;
for (int q = 0; q < j; q++) {
[array removeObjectAtIndex:g];
}
When i set a breakpoint (after first iteration) i got the following:
[0]=(id)0x00000000
[1]=(id)0x071421a0(int)3
[2]=(id)0x071421b0(int)4
[3]=(id)0x071421e0(int)5
I don't understand how to delete in each iteration a first object. I mean that following it will be first. And why the last one is disappeared every time also?
Thanks.
Another way to delete a collection of objects from an array is to add the objects you want to delete to a separate array, then use that to delete your objects from the primary array in one fell swoop. There are benefits to this, least of which would be no risk of out-of-bounds, and also the possibility for rolling back since you are essentially removing in a single batch process:
int j = [array count];
NSMutableArray *theseObjects = [NSMutableArray array];
for (int q = 0; q < j; q++)
{
id thisObject = [array objectAtIndex:q];
BOOL shouldRemoveThisObject = ...//<--determine if you want to remove this object
if (shouldRemoveThisObject)
[theseObjects addObject:thisObject];
}
[array removeObjectsInArray:theseObjects];
It looks like you want to remove all entries. Then you could also call [myArray removeAllObjects];
You cant do as you are trying to do.
j is set to 5 (array count) but the array's size is reducing on every iteration. So at one point of time it will try to access array out-of-bound, hence exception and your app will crash.
Instead you need to do as below:
while (array.count > 0) {
NSLog(#"Arr before removal is : %#",array);
[array removeObjectAtIndex:0];
NSLog(#"Arr after removal is : %#",array);
}
If for some other reason you want to remove only first then the above will work.
If you want to remove all the objects at one go: you can use:
[array removeAllObjects];
You CAN'T remove objects from NSMutableArray inside a loop. This is because, in that case, array size is changing over each iteration (worst case) and you could try to access to an out of bounds index.
The best option is Jeremy's solution. Keep desired objects on separated array and then remove these objects from the main array, outside of the loop.
I'm writing a for in loop to read a list of names from an NSArray, here is my code.
NSArray *names = [NSArray arrayWithObjects:#"Luke",#"James",#"Fred",#"Harry", nil];
for (NSString *name in names) {
NSLog(#"%#",name);
}
What I am trying to determine is wether there is an easier way to get the current loop number without adding a variable outside the loop like so..
int number = 0;
for (NSString *name in names) {
number++;
NSLog(#"%i - %#",number,name);
}
Is there a built in 'loop number' property that can be accessed during a for loop? - Am I barking up the wrong tree? Should I just use the variable and get over it?
What you've done is reasonable (although I'd put number++ at the end of the loop). Usually when I need a loop index, I avoid fast enumeration and use a more traditional for loop:
for (NSUInteger i=0; i<[names count]; i++) {
NSString *name = names[i];
NSLog(#"%i - %#", i, name);
}
If you need the index, use a standard 3-part for loop, or the number counter with a while loop. No way to magically get the index in the for...in style loop.
This seems quite a bit of a hack. You could either use a normal for loop instead of fast enumeration, or enumerate the objects using a block, or you can also use the indexOfObject: method inside the for loop (but this is really discouraged since it works only if you have only unique objects in the array, and anyways the repeated lookup makes it slower). All in all, try this:
int i;
for (i = 0; i < names.count; i++) {
NSString *name = [names objectAtIndex:i];
// and "i" already stores the index
}
or this:
[names enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
// use obj and idx
}];
and this is also possible, but don't do it:
for (NSString *name in names) {
int idx = [names indexOfObject:name];
// etc.
}
I want to get the index of the current object when using fast enumeration, i.e.
for (MyClass *entry in savedArray) {
// What is the index of |entry| in |savedArray|?
}
Look at the API for NSArray and you will see the method
- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block
So give that one a try
[savedArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
//... Do your usual stuff here
obj // This is the current object
idx // This is the index of the current object
stop // Set this to true if you want to stop
}];
I suppose the most blunt solution to this would be to simply increment an index manually.
NSUInteger indexInSavedArray = 0;
for (MyClass *entry in savedArray) {
indexInSavedArray++;
}
Alternatively, you could just not use fast enumeration.
for (NSUInteger indexInSavedArray = 0; indexInSavedArray < savedArray.count; indexInSavedArray++) {
[savedArray objectAtIndex:indexInSavedArray];
}
This question has already been answered, but I thought I would add that counting iterations is actually the technique mentioned in the iOS Developer Library documentation:
NSArray *array = <#Get an array#>;
NSUInteger index = 0;
for (id element in array) {
NSLog(#"Element at index %u is: %#", index, element);
index++;
}
I was sure there would be a fancy trick, but I guess not. :)
If you want to access the index or return outside block here is a piece of code that can be useful. (considering the array is an array of NSString).
- (NSInteger) findElemenent:(NSString *)key inArray:(NSArray *)array
{
__block NSInteger index = -1;
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
if ([obj isEqualToString:key]) {
*stop = YES;
index = idx;
}
}];
return index;
}
A simple observation: If you initialize the index to -1 and then put the ++index as the first line in the for loop, doesn't that cover all bases (provided someone doesn't slip code in front of the increment)?
I just had a pretty bad bug because I was doing this the way everyone else in here has suggested. That is, "create an index variable and increment it at the end of your loop".
I propose that this should be avoided and instead the following pattern should be followed:
int index = -1;
for (a in b) {
index++;
//Do stuff with `a`
}
The reason I recommend this odd pattern, is because if you use the continue; feature of fast enumeration, it will skip the final index++ line of code at the end of your loop, and your index count will be off! For this reason I recommend starting at -1 and incrementing before doing anything else.
As for people who said just use indexOfObject: this won't work with duplicate entries.
In Cocoa, if I want to loop through an NSMutableArray and remove multiple objects that fit a certain criteria, what's the best way to do this without restarting the loop each time I remove an object?
Thanks,
Edit: Just to clarify - I was looking for the best way, e.g. something more elegant than manually updating the index I'm at. For example in C++ I can do;
iterator it = someList.begin();
while (it != someList.end())
{
if (shouldRemove(it))
it = someList.erase(it);
}
For clarity I like to make an initial loop where I collect the items to delete. Then I delete them. Here's a sample using Objective-C 2.0 syntax:
NSMutableArray *discardedItems = [NSMutableArray array];
for (SomeObjectClass *item in originalArrayOfItems) {
if ([item shouldBeDiscarded])
[discardedItems addObject:item];
}
[originalArrayOfItems removeObjectsInArray:discardedItems];
Then there is no question about whether indices are being updated correctly, or other little bookkeeping details.
Edited to add:
It's been noted in other answers that the inverse formulation should be faster. i.e. If you iterate through the array and compose a new array of objects to keep, instead of objects to discard. That may be true (although what about the memory and processing cost of allocating a new array, and discarding the old one?) but even if it's faster it may not be as big a deal as it would be for a naive implementation, because NSArrays do not behave like "normal" arrays. They talk the talk but they walk a different walk. See a good analysis here:
The inverse formulation may be faster, but I've never needed to care whether it is, because the above formulation has always been fast enough for my needs.
For me the take-home message is to use whatever formulation is clearest to you. Optimize only if necessary. I personally find the above formulation clearest, which is why I use it. But if the inverse formulation is clearer to you, go for it.
One more variation. So you get readability and good performace:
NSMutableIndexSet *discardedItems = [NSMutableIndexSet indexSet];
SomeObjectClass *item;
NSUInteger index = 0;
for (item in originalArrayOfItems) {
if ([item shouldBeDiscarded])
[discardedItems addIndex:index];
index++;
}
[originalArrayOfItems removeObjectsAtIndexes:discardedItems];
This is a very simple problem. You just iterate backwards:
for (NSInteger i = array.count - 1; i >= 0; i--) {
ElementType* element = array[i];
if ([element shouldBeRemoved]) {
[array removeObjectAtIndex:i];
}
}
This is a very common pattern.
Some of the other answers would have poor performance on very large arrays, because methods like removeObject: and removeObjectsInArray: involve doing a linear search of the receiver, which is a waste because you already know where the object is. Also, any call to removeObjectAtIndex: will have to copy values from the index to the end of the array up by one slot at a time.
More efficient would be the following:
NSMutableArray *array = ...
NSMutableArray *itemsToKeep = [NSMutableArray arrayWithCapacity:[array count]];
for (id object in array) {
if (! shouldRemove(object)) {
[itemsToKeep addObject:object];
}
}
[array setArray:itemsToKeep];
Because we set the capacity of itemsToKeep, we don't waste any time copying values during a resize. We don't modify the array in place, so we are free to use Fast Enumeration. Using setArray: to replace the contents of array with itemsToKeep will be efficient. Depending on your code, you could even replace the last line with:
[array release];
array = [itemsToKeep retain];
So there isn't even a need to copy values, only swap a pointer.
You can use NSpredicate to remove items from your mutable array. This requires no for loops.
For example if you have an NSMutableArray of names, you can create a predicate like this one:
NSPredicate *caseInsensitiveBNames =
[NSPredicate predicateWithFormat:#"SELF beginswith[c] 'b'"];
The following line will leave you with an array that contains only names starting with b.
[namesArray filterUsingPredicate:caseInsensitiveBNames];
If you have trouble creating the predicates you need, use this apple developer link.
I did a performance test using 4 different methods. Each test iterated through all elements in a 100,000 element array, and removed every 5th item. The results did not vary much with/ without optimization. These were done on an iPad 4:
(1) removeObjectAtIndex: -- 271 ms
(2) removeObjectsAtIndexes: -- 1010 ms (because building the index set takes ~700 ms; otherwise this is basically the same as calling removeObjectAtIndex: for each item)
(3) removeObjects: -- 326 ms
(4) make a new array with objects passing the test -- 17 ms
So, creating a new array is by far the fastest. The other methods are all comparable, except that using removeObjectsAtIndexes: will be worse with more items to remove, because of the time needed to build the index set.
Either use loop counting down over indices:
for (NSInteger i = array.count - 1; i >= 0; --i) {
or make a copy with the objects you want to keep.
In particular, do not use a for (id object in array) loop or NSEnumerator.
For iOS 4+ or OS X 10.6+, Apple added passingTest series of APIs in NSMutableArray, like – indexesOfObjectsPassingTest:. A solution with such API would be:
NSIndexSet *indexesToBeRemoved = [someList indexesOfObjectsPassingTest:
^BOOL(id obj, NSUInteger idx, BOOL *stop) {
return [self shouldRemove:obj];
}];
[someList removeObjectsAtIndexes:indexesToBeRemoved];
Nowadays you can use reversed block-based enumeration. A simple example code:
NSMutableArray *array = [#[#{#"name": #"a", #"shouldDelete": #(YES)},
#{#"name": #"b", #"shouldDelete": #(NO)},
#{#"name": #"c", #"shouldDelete": #(YES)},
#{#"name": #"d", #"shouldDelete": #(NO)}] mutableCopy];
[array enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
if([obj[#"shouldDelete"] boolValue])
[array removeObjectAtIndex:idx];
}];
Result:
(
{
name = b;
shouldDelete = 0;
},
{
name = d;
shouldDelete = 0;
}
)
another option with just one line of code:
[array filterUsingPredicate:[NSPredicate predicateWithFormat:#"shouldDelete == NO"]];
In a more declarative way, depending on the criteria matching the items to remove you could use:
[theArray filterUsingPredicate:aPredicate]
#Nathan should be very efficient
Here's the easy and clean way. I like to duplicate my array right in the fast enumeration call:
for (LineItem *item in [NSArray arrayWithArray:self.lineItems])
{
if ([item.toBeRemoved boolValue] == YES)
{
[self.lineItems removeObject:item];
}
}
This way you enumerate through a copy of the array being deleted from, both holding the same objects. An NSArray holds object pointers only so this is totally fine memory/performance wise.
Add the objects you want to remove to a second array and, after the loop, use -removeObjectsInArray:.
this should do it:
NSMutableArray* myArray = ....;
int i;
for(i=0; i<[myArray count]; i++) {
id element = [myArray objectAtIndex:i];
if(element == ...) {
[myArray removeObjectAtIndex:i];
i--;
}
}
hope this helps...
Why don't you add the objects to be removed to another NSMutableArray. When you are finished iterating, you can remove the objects that you have collected.
How about swapping the elements you want to delete with the 'n'th element, 'n-1'th element and so on?
When you're done you resize the array to 'previous size - number of swaps'
If all objects in your array are unique or you want to remove all occurrences of an object when found, you could fast enumerate on an array copy and use [NSMutableArray removeObject:] to remove the object from the original.
NSMutableArray *myArray;
NSArray *myArrayCopy = [NSArray arrayWithArray:myArray];
for (NSObject *anObject in myArrayCopy) {
if (shouldRemove(anObject)) {
[myArray removeObject:anObject];
}
}
benzado's anwser above is what you should do for preformace. In one of my applications removeObjectsInArray took a running time of 1 minute, just adding to a new array took .023 seconds.
I define a category that lets me filter using a block, like this:
#implementation NSMutableArray (Filtering)
- (void)filterUsingTest:(BOOL (^)(id obj, NSUInteger idx))predicate {
NSMutableIndexSet *indexesFailingTest = [[NSMutableIndexSet alloc] init];
NSUInteger index = 0;
for (id object in self) {
if (!predicate(object, index)) {
[indexesFailingTest addIndex:index];
}
++index;
}
[self removeObjectsAtIndexes:indexesFailingTest];
[indexesFailingTest release];
}
#end
which can then be used like this:
[myMutableArray filterUsingTest:^BOOL(id obj, NSUInteger idx) {
return [self doIWantToKeepThisObject:obj atIndex:idx];
}];
A nicer implementation could be to use the category method below on NSMutableArray.
#implementation NSMutableArray(BMCommons)
- (void)removeObjectsWithPredicate:(BOOL (^)(id obj))predicate {
if (predicate != nil) {
NSMutableArray *newArray = [[NSMutableArray alloc] initWithCapacity:self.count];
for (id obj in self) {
BOOL shouldRemove = predicate(obj);
if (!shouldRemove) {
[newArray addObject:obj];
}
}
[self setArray:newArray];
}
}
#end
The predicate block can be implemented to do processing on each object in the array. If the predicate returns true the object is removed.
An example for a date array to remove all dates that lie in the past:
NSMutableArray *dates = ...;
[dates removeObjectsWithPredicate:^BOOL(id obj) {
NSDate *date = (NSDate *)obj;
return [date timeIntervalSinceNow] < 0;
}];
Iterating backwards-ly was my favourite for years , but for a long time I never encountered the case where the 'deepest' ( highest count) object was removed first. Momentarily before the pointer moves on to the next index there ain't anything and it crashes.
Benzado's way is the closest to what i do now but I never realised there would be the stack reshuffle after every remove.
under Xcode 6 this works
NSMutableArray *itemsToKeep = [NSMutableArray arrayWithCapacity:[array count]];
for (id object in array)
{
if ( [object isNotEqualTo:#"whatever"]) {
[itemsToKeep addObject:object ];
}
}
array = nil;
array = [[NSMutableArray alloc]initWithArray:itemsToKeep];