iPhone: counting unique items in an array [duplicate] - iphone

Question

I need to perform what I feel is a basic function but I can't find any documentation on how to do it. Please help!
I need to count how many times a certain object occurs in an array. See example:
array = NSArray arrayWithObjects:#"Apple", #"Banana", #"Cantaloupe", #"Apple", #"DragonFruit", #"Eggplant", #"Apple", #"Apple", #"Guava",nil]retain];
How can I iterate through the array and count the number of times it finds the string #"Apple"?
Any help is appreciated!

Answer 1

One more solution, using blocks (working example):
NSInteger occurrences = [[array indexesOfObjectsPassingTest:^(id obj, NSUInteger idx, BOOL *stop) {return [obj isEqual:#"Apple"];}] count];
NSLog(#"%d",occurrences);

Answer 2

As #bbum said, use an NSCounted set. There is an initializer thet will convert an array directly into a counted set:
NSArray *array = [[NSArray alloc] initWithObjects:#"A", #"B", #"X", #"B", #"C", #"D", #"B", #"E", #"M", #"X", nil];
NSCountedSet *countedSet = [[NSCountedSet alloc] initWithArray:array];
NSLog(#"%#", countedSet);
NSLog output:
(D [1], M [1], E [1], A [1], B [3], X [2], C [1])
Just access items:
count = [countedSet countForObject: anObj]; ...

Answer 3

A Simple and specific answer:
int occurrences = 0;
for(NSString *string in array){
occurrences += ([string isEqualToString:#"Apple"]?1:0); //certain object is #"Apple"
}
NSLog(#"number of occurences %d", occurrences);
PS: Martin Babacaev's answer is quite good too. Iteration is faster with blocks but in this specific case with so few elements I guess there is no apparent gain. I would use that though :)

Answer 4

Use an NSCountedSet; it'll be faster than a dictionary and is designed to solve exactly that problem.
NSCountedSet *cs = [NSCountedSet new];
for(id anObj in someArray)
[cs addObject: anObj];
// then, you can access counts like this:
.... count = [cs countForObject: anObj]; ...
[cs release];

Answer 5

Just came across this pretty old question. I'd recommend using a NSCountedSet:
NSCountedSet *countedSet = [[NSCountedSet alloc] initWithArray:array];
NSLog(#"Occurrences of Apple: %u", [countedSet countForObject:#"Apple"]);

Answer 6

I would encourage you to put them into a Dictionary (Objective C's version of a map). The key to the dictionary is the object and the value should be the count. It should be a MutableDictionary of course. If the item is not found, add it and set the count to 1.

Answer 7

- (int) numberOfOccurrencesForString:(NSString*)needle inArray:(NSArray*)haystack {
int count = 0;
for(NSString *str in haystack) {
if([str isEqualToString:needle]) {
count++;
}
}
return count;
}

Answer 8

I up-voted Rob's answer, but I wanted to add some code that I hope will be of some assistance.
NSArray *array = [[NSArray alloc] initWithObjects:#"A", #"B", #"B", #"B", #"C", #"D", #"E", #"M", #"X", #"X", nil];
NSMutableDictionary *dictionary = [[NSMutableDictionary alloc]init];
for(int i=0; i < [array count]; i++) {
NSString *s = [array objectAtIndex:i];
if (![dictionary objectForKey:s]) {
[dictionary setObject:[NSNumber numberWithInt:1] forKey:s];
} else {
[dictionary setObject:[NSNumber numberWithInt:[dictionary objectForKey:s] intValue]+1 forKey:s];
}
}
for(NSString *k in [dictionary keyEnumerator]) {
NSNumber *number = [dictionary objectForKey:k];
NSLog(#"Value of %#:%d", k, [number intValue]);
}

Answer 9

If the array is sorted as in the problem statement then you don't need to use a dictionary.
You can find the number of unique elements more efficiently by just doing 1 linear sweep and incrementing a counter when you see 2 consecutive elements being the same.
The dictionary solution is O(nlog(n)), while the linear solution is O(n).
Here's some pseudo-code for the linear solution:
array = A,B,B,B,B,C,C,D,E,M,X,X #original array
array = array + -1 # array with a dummy sentinel value to avoid testing corner cases.
# Start with the first element. You want to add some error checking here if array is empty.
last = array[0]
count = 1 # you have seen 1 element 'last' so far in the array.
for e in array[1..]: # go through all the elements starting from the 2nd one onwards
if e != last: # if you see a new element then reset the count
print "There are " + count + " " + last elements
count = 1 # unique element count
else:
count += 1
last = e

Answer 10

the complete code with reference to #bbum and #Zaph
NSArray *myArray = [[NSArray alloc] initWithObjects:#"A", #"B", #"X", #"B", #"C", #"D", #"B", #"E", #"M", #"X", nil];
NSCountedSet *countedSet = [[NSCountedSet alloc] initWithArray:myArray];
for (NSString *item in countedSet) {
int count = [countedSet countForObject: item];
NSLog(#"the String ' %# ' appears %d times in the array",item,count);
}
Thank you.

Answer 11

If you want it more generic, or you want to count equals/different objects in array, try this:
Sign "!" count DIFFERENT values. If you want SAME values, remove "!"
int count = 0;
NSString *wordToCheck = [NSString string];
for (NSString *str in myArray) {
if( ![str isEqualToString:wordToCheck] ) {
wordToCheck = str;
count++;
}
}
hope this helps the community!
I've used it to add correct number of sections in uitableview!

Answer 12

You can do this way,
NSArray *array = [[NSArray alloc] initWithObjects:#"A", #"B", #"X", #"B", #"C", #"D", #"B", #"E", #"M", #"X", nil];
NSOrderedSet *orderedSet = [NSOrderedSet orderedSetWithArray:array];
NSArray *uniqueStates = [[orderedSet set] allObjects];
NSCountedSet *countedSet = [[NSCountedSet alloc] initWithArray:array];
for(int i=0;i<[uniqueStates count];i++){
NSLog(#"%# %d",[uniqueStates objectAtIndex:i], [countedSet countForObject: [uniqueStates objectAtIndex:i]]);
}
The result is like : A 1