Is there a way to iterate over a dictionary?

I know `NSDictionaries` as something where you need a `key` in order to get a `value`. But how can I iterate over all `keys` and `values` in a `NSDictionary`, so that I know what keys there are, and what values there are? I know there is something called a *for-in-loop* in `JavaScript`. Is there something similar in `Objective-C`?

Yes, `NSDictionary` supports fast enumeration. With Objective-C 2.0, you can do this:

// To print out all key-value pairs in the NSDictionary myDict
for(id key in myDict)
NSLog(@"key=%@ value=%@", key, [myDict objectForKey:key]);

The alternate method (which you have to use if you're targeting Mac OS X pre-10.5, but you can still use on 10.5 and iPhone) is to use an `NSEnumerator`:

NSEnumerator *enumerator = [myDict keyEnumerator];
id key;
// extra parens to suppress warning about using = instead of ==
while((key = [enumerator nextObject]))
NSLog(@"key=%@ value=%@", key, [myDict objectForKey:key]);

**The block approach** avoids running the lookup algorithm for *every key*:

[dict enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL* stop) {
NSLog(@"%@ => %@", key, value);
}];

Even though [`NSDictionary`][1] is implemented as a [hashtable][2] (which means that the cost of looking up an element is [`O(1)`][3]), lookups still slow down your iteration by *a constant factor*.

My measurements show that for a dictionary `d` of numbers ...

NSMutableDictionary* dict = [NSMutableDictionary dictionary];
for (int i = 0; i < 5000000; ++i) {
NSNumber* value = @(i);
dict[value.stringValue] = value;
}

... summing up the numbers with the block approach ...

__block int sum = 0;
[dict enumerateKeysAndObjectsUsingBlock:^(NSString* key, NSNumber* value, BOOL* stop) {
sum += value.intValue;
}];

... rather than the loop approach ...

int sum = 0;
for (NSString* key in dict)
sum += [dict[key] intValue];

... **is about 40% faster**.

**EDIT**: The new SDK (6.1+) appears to optimise loop iteration, so **the loop approach is now about 20% faster than the block approach**, at least for the simple case above.

[1]:

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This is iteration using block approach:

NSDictionary *dict = @{@"key1":@1, @"key2":@2, @"key3":@3};

[dict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
NSLog(@"%@->%@",key,obj);
// Set stop to YES when you wanted to break the iteration.
}];

With autocompletion is very fast to set, and you do not have to worry about writing iteration envelope.