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copied!<p><strong>The block approach</strong> avoids running the lookup algorithm for <em>every key</em>:</p> <pre><code>[dict enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL* stop) { NSLog(@"%@ => %@", key, value); }]; </code></pre> <p>Even though <a href="https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSDictionary_Class/Reference/Reference.html"><code>NSDictionary</code></a> is implemented as a <a href="http://en.wikipedia.org/wiki/Hashtable">hashtable</a> (which means that the cost of looking up an element is <a href="http://en.wikipedia.org/wiki/Big_oh"><code>O(1)</code></a>), lookups still slow down your iteration by <em>a constant factor</em>.</p> <p>My measurements show that for a dictionary <code>d</code> of numbers ...</p> <pre><code>NSMutableDictionary* dict = [NSMutableDictionary dictionary]; for (int i = 0; i < 5000000; ++i) { NSNumber* value = @(i); dict[value.stringValue] = value; } </code></pre> <p>... summing up the numbers with the block approach ...</p> <pre><code>__block int sum = 0; [dict enumerateKeysAndObjectsUsingBlock:^(NSString* key, NSNumber* value, BOOL* stop) { sum += value.intValue; }]; </code></pre> <p>... rather than the loop approach ...</p> <pre><code>int sum = 0; for (NSString* key in dict) sum += [dict[key] intValue]; </code></pre> <p>... <strong>is about 40% faster</strong>.</p> <p><strong>EDIT</strong>: The new SDK (6.1+) appears to optimise loop iteration, so <strong>the loop approach is now about 20% faster than the block approach</strong>, at least for the simple case above.</p>

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