cocoa: why for-each is faster than for loop? - cocoa

Question

My application read all people in contact in two ways:
for-loop:
CFAbsoluteTime startTime = CFAbsoluteTimeGetCurrent ();
long count = macContact.addressBook.people.count;
for(int i=0;i<count;++i){
ABPerson *person = [macContact.addressBook.people objectAtIndex:i];
NSLog(#"%#",person);
}
NSLog(#"%f",CFAbsoluteTimeGetCurrent() - startTime);
for-each
CFAbsoluteTime startTime = CFAbsoluteTimeGetCurrent ();
for(ABPerson *person in macContact.addressBook.people){
NSLog(#"%#",person);
}
NSLog(#"%f",CFAbsoluteTimeGetCurrent() - startTime);
for-each only took 4 seconds to enumerate 5000 people in addressBook, while for-loop took 10 minutes to do the same job.
I want to know why there is a huge difference in performance?

Answer 1

The difference in performance almost certainly has to do with the macContact.addressBook.people part. You're calling that every single time through the for loop but only once with the for-each loop. I'm guessing either the addressBook or the people properties are not returning cached data but rather new data every time.
Try using
NSArray *people = macContact.addressBook.people;
for (int i = 0; i < [people count]; i++) {
NSLog(#"%#", [people objectAtIndex:i];
}
You'll probably find the performance is very similar again.
That said, for-each is faster than for in general. The reason is because a for loop invokes a method send on every single pass through the loop (-objectAtIndex:), whereas for-each can fetch the objects much more efficiently by grabbing them in large batches.
In more recent versions of the OS you can go a step further and use a block-based enumeration method. This looks like
[macContact.addressBook.people enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL stop){
NSLog(#"%#", obj);
}];
For NSArrays this should have very similar performance to a for-each loop. For other data structures such as dictionaries this style can be faster because it can fetch the value along with the key (whereas a for-each only gives you the key and requires you to use a message send to get the value).