I've been trying to figure out a way of checking how many of a certain object are in an NSArray.
I've looked through the docs and I'm pretty sure there is no premade method for this. Also I can't find anything here on SO.
Do anybody know about a good way to do this? Because I seriously can't come up with anything.
In this specific case I have an array with strings (most cases several of each) and I want to count how many strings in the array that matches to whatever I ask for.
If this is a primary use of the data structure and order doesn't matter, consider switching to an NSCountedSet which is specifically for solving this problem efficiently.
If you need an ordered collection, and you don't have a huge set of objects, than the fast enumeration answers are the best approach.
If you want to know where the objects are, then use indexesOfObjectsPassingTest:.
If you have a huge number of object, I would look at indexesOfObjectsWithOptions:passingTest: with the NSEnumerationConcurrent option. This will allow you to search the array on multiple cores. (This is only possibly faster on a multi-core device, and even then is probably only faster if you have a very large collection. You should absolutely test before assuming that concurrent will be faster.) Even if you just need the final count, it may be faster for certain data sets to use this method and then use count on the final index set.
There actually is a method for this: - (NSIndexSet *)indexesOfObjectsPassingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate
NSIndexSet *indexes = [array indexesOfObjectsPassingTest:^(id obj, NSUInteger index, BOOL *stop) {
return [obj isEqualTo:myOtherObject];
}];
Sounds like a case for NSCountedSet, which does what you are after with its initWithArray: initializer:
// Example array of strings
NSArray *array = [NSArray arrayWithObjects:
#"Joe", #"Jane", #"Peter", #"Paul",
#"Joe", #"Peter", #"Paul",
#"Joe",
#"Jane", #"Peter",
nil];
NSCountedSet *countedSet = [[NSCountedSet alloc] initWithArray: array];
// for-in will let you loop over the counted set
for (NSString *str in countedSet) {
NSLog(#"Count of %#: %ld", str, (long)[countedSet countForObject:str]);
}
One approach would be to iterate and check.
- (int)repeatsOf:(NSString *)repeater inArray:(NSArray *)array {
int count = 0;
for (NSString *item in array) {
if ([item isEqualToString:repeater]) {
count++;
}
}
return count;
}
You could try a simple loop. Suppose needle is your reference string and array is your NSArray of strings:
unsigned int n = 0;
for (NSString * str in array)
{
if ([needle isEqualToString:str])
{
++n;
}
}
Now n holds the count of strings in equal to needle.
You could define a function like this:
- (int)countStringsThatMatch:(NSString*)match inArray:(NSArray*)array
{
int matches = 0;
for (id string in array) {
if ([string isEqualToString:match]) {
matches++;
}
}
return matches;
}
And then use it like:
int count = [self countStringsThatMatch:#"someString" inArray:someArray];
- (NSUInteger) objectCountInArray:(NSArray *)array
matchingString:(NSString *)stringToMatch {
NSUInteger count = 0;
for (NSString *string in array) {
count += [string isEqualToString:stringToMatch] ? 1 : 0;
}
return count;
}
You can try to expand this to use a block that gets an object and returns a BOOL. Then you can use it to compare an array of whatever you want.
I want to get some objects from array contains objects where one of objects property satisfy the condition
Like in c#
example :
NSMutablearray * coursesinfo ;
this array contain more than 30 courses
course is object on of its property is finalgrade
i want to get all courses where final grade < 100
can i do this in objective -c like c# ? and how?
the where statement is like using a predicate in cocoa/cocoa touch. here is an example, where I have an array of image file names from a directory and I am looking for the base file name. The indexesOfObjectsWithOptions: method returns a set of Indices that pass the specific test. NSEnumerationConcurrent utilizes a concurrent queue to take advantage of multiple cores, if present.
NSIndexSet *indexSet=[allImageURLs indexesOfObjectsWithOptions:NSEnumerationConcurrent passingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
BOOL match=NO;
NSRange twoXRange=[((NSURL *)obj).absoluteString rangeOfString:#"#2x"];
NSRange iPhoneRange=[((NSURL *)obj).absoluteString rangeOfString:#"~ipad"];
if (twoXRange.location==NSNotFound && iPhoneRange.location==NSNotFound) {
match=YES;
}
return match;
}];
self.imageURLs=[allImageURLs objectsAtIndexes: indexSet];
for your particular case I would do the following:
NSIndexSet *theSet=[coursesinfo indexesOfObjectsWithOptions:NSEnumerationConcurrent passingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
BOOL match=NO;
if( obj.finalGrade<100 ){
match=YES;
}
return match;
}];
NSArray *courses=[coursesinfo objectsAtIndexes: theSet];
Good luck!
t
The closest is use - (NSArray *)filteredArrayUsingPredicate:(NSPredicate *)predicate function of NSArray.
Link to NSPredicate documentation.
I have an NSArray filled with custom objects.
Each object has several variables: pk, amount, date etc..
I want to fetch the object that has the highest number in the pk variable.
I can do this using:
NSUInteger maximumpk = [[bets valueForKeyPath:#"#max.pk"] intValue];
This gives me the actual value from the highest pk. Now I need to get the index for that object. I have seen indexOfObject used when the array has just 1 variable of data, but how do I use it in this instance?
Thanks
Use -indexOfObjectPassingTest:, for example:
NSUInteger idx = [bets indexOfObjectPassingTest:^(id obj, NSUInteger idx, BOOL *stop) {
return [obj pk] == maximumpk;
}];
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];