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).