How can I get the first element in an NSDictionary?

Question

I have an array of NSDictionaries. How can I pull out the first element in the dictionary?

   NSArray *messages = [[results objectForKey:@"messages"] valueForKey:@"message"];         
    for (NSDictionary *message in messages)
    {
        STObject *mySTObject = [[STObject alloc] init];
        mySTObject.stID = [message valueForKey:@"id"];      
        stID = mySTObject.stID;
    }

Solution 1

There is no "first" element in an NSDictionary; its members have no guaranteed order. If you just want one object from a dictionary, but don't care which key it's associated with, you can do:

id val = nil;
NSArray *values = [yourDict allValues];

if ([values count] != 0)
    val = [values objectAtIndex:0];

Solution 2

NSDictionaries are unordered, meaning that there are not first or last element. In fact, the order of the keys are never guaranteed to be the same, even in the lifetime of a specific dictionary.

If you want any object, you can get one of the keys:

id key = [[message allKeys] objectAtIndex:0]; // Assumes 'message' is not empty
id object = [message objectForKey:key];

Solution 3

NSArray has a selector named firstObject that simplifies the code and makes it more readable:

id val = [[yourDict allValues] firstObject];

If yourDict is empty val will be nil, so is not necessary to check the dictionary/array size.

Solution 4

Simplest:

[[dict objectEnumerator] nextObject];

Solution 5

According to Apple, calls to allKeys or allValues incur the cost of creating new arrays:

A new array containing the dictionary’s values, or an empty array if the dictionary has no entries (read-only)

So, an alternative option that does not incur such cost could look like this:

NSString* key = nil;
for(key in yourDict)
{ // this loop will not execute if the dictionary is empty
   break; // exit loop as soon as we enter it (key will be set to some key)
}
id object = yourDict[key]; // get object associated with key. nil if key doesn't exist.

Note: If the dictionary is empty, the key will remain nil, and the object returned will also be nil, we therefore don't need special handling of the case where the dictionary is actually empty.

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Comments

[Chris Hillery]

Yes, so you can still message val without crashing if [values count] is zero.

[Jeremy Fox]

Please be aware that the allValues method returns an unordered array. Per the documentation "The order of the values in the array isn’t defined.". So you may not get the object you are expecting to get.

[cbh2000]

You can use subscripts too. val = values[0]; is legal. More on subscripts here: clang.llvm.org/docs/ObjectiveCLiterals.html

[TheRealRonDez]

Thanks - this makes sense.

[mikekavouras]

allValues doesn't guarantee order.

[trojanfoe]

While that's true, and saves a line or two of code, the question is about NSDictionary, not NSArray.

[ASLLOP]

Yes, and is the "first" value of the dictionary what you get with this code. Is just that it uses an array conversion, like almost all other answers.

[trojanfoe]

There is no "first" with a dictionary as it has no order. You will potentially get a different "first" each time you add an object to the dictionary.

[ASLLOP]

Of course, a dictionary is not an array, there is no implicit order on it. But the user doesn't asked for a specific order on the data.