Check key exists in NSDictionary

Question

how can I check if this exists?:

[[dataArray objectAtIndex:indexPathSet.row] valueForKey:@"SetEntries"]

I want to know whether this key exists or not. How can I do that?

Thank you very much :)

EDIT: dataArray has Objects in it. And these objects are NSDictionaries.

Solution 1

I presume that [dataArray objectAtIndex:indexPathSet.row] is returning an NSDictionary, in which case you can simply check the result of valueForKey against nil.

For example:

if ([[dataArray objectAtIndex:indexPathSet.row] valueForKey:@"SetEntries"] != nil) {
    // The key existed...

}
else {
    // No joy...

}

Solution 2

So I know you already selected an answer, but I found this to be rather useful as a category on NSDictionary. You start getting into efficiency at this point with all these different answers. Meh...6 of 1...

- (BOOL)containsKey: (NSString *)key {
     BOOL retVal = 0;
     NSArray *allKeys = [self allKeys];
     retVal = [allKeys containsObject:key];
     return retVal;
}

Solution 3

Check if it's nil:

if ([[dataArray objectAtIndex:indexPathSet.row] valueForKey:@"SetEntries"] != nil) {
    // SetEntries exists in this dict
} else {
    // No SetEntries in this dict
}

Solution 4

this also works using Objective-C literals using the following syntax:

NSDictionary *dict = @{ @"key1" : @"value1", @"key2" : @"value2" };
if (dict[@"key2"])
NSLog(@"Exists");
else
NSLog(@"Does not exist");

Solution 5

Try this:

if ([dict objectForKey:@"bla"]) {
   // use obj
} else {
   // Do something else like create the object
}

View more solutions

Comments

[Admin]

Any reason why you’re using -valueForKey: instead of -objectForKey:? The former is used in key-value coding and is slightly slower than the latter.

[kaala]

I also wrote the same method in my category, but I have a weird crash happened when I use UIImageWriteToSavedPhotosAlbum(). after narrow down, I change the method name from containsKey => hasKey and the crash disappeared

[Houman]

This is much better than the accepted answer. I want to search by a key and not if the value of the key is nil or not. You could have a key in a dictionary that points to a nil value. Yet the key exists. The accepted answer doesn't cover this scenario. But this solution here does.

[Jarno Lamberg]

@Hooman: Not true about a key having a value of nil: the second paragraph of the NSDictionary documentation (developer.apple.com/library/mac/documentation/Cocoa/Referen‌​ce/…) states that "Neither a key nor a value can be nil; if you need to represent a null value in a dictionary, you should use NSNull." Both this and the accepted answer work for your scenario, i.e. with a dictionary where a value for some key is null (of type NSNull).

[Palpatim]

This may be a technically correct explanation, but you'll be serving the community better (and likely getting more upvotes) if you provide more detailed discussion about your reasoning in the body of your answer. Why is it appropriate to use objectForKey: instead of another method? Why don't you explicitly have to check for a value? At the very least, using the code example provided in the question would make it easier to tie your example to the original question.

[SmileBot]

Or: return [[self allKeys] containsObject:key] ? YES : NO;

[NiñoScript]

what's the cast from BOOL to NSNull* for?

[orion elenzil]

good callout, @Jamo. I'd steer way clear of this answer. my guess is that under the covers, both the call to allKeys and the call to containsObject are each at least O(n), while valueForKey is likely much less.

[Narasimha Nallamsetty]

For swift version below code works. let headers:Dictionary = httpResponse .allHeaderFields if (headers["errorcode"] != nil) { print("error key exists ") }

[Andrei A.]

the value of that key can also be nil therefore this approach will not fully test if a key exists or not in the dictionary

[Andrei A.]

the value of that key can also be nil therefore this approach will not fully test if a key exists or not in the dictionary

[John Parker]

@AndreiA.You can't add nil to an NSDictionary.

[Aaron Ash]

@AndreiA. Neither a key nor a value can be nil in an NSDictionary. To represent nil, you can use NSNull.

[fishinear]

This does not answer the question

[fishinear]

Casting the pointer to an object to unsigned long serves no purpose.