Question

I’d like to increment over days in a loop, so that it counts 2012-11-10, 2012-11-11, 2012-11-12, …

What’s the most performant way to achieve this?

NSDate *iterationDate = [NSDate date];

for (int i = 0; i < 100; i++) {
    NSDateComponents *comps = [[NSDateComponents alloc] init];
    [comps setYear:0];
    [comps setMonth:0];
    [comps setWeek:0];
    [comps setDay:1];
    [comps setHour:0];
    [comps setMinute:0];
    [comps setSecond:0];
    iterationDate = [currentCalendar dateByAddingComponents:comps toDate:iterationDate options:0];
}

Answer 1

If you need all the intermediate NSDates, just pull comps out of the loop:

NSDate *iterationDate = [NSDate date];
NSDateComponents *comps = [[NSDateComponents alloc] init];
[comps setYear:0];
[comps setMonth:0];
[comps setWeek:0];
[comps setDay:1];
[comps setHour:0];
[comps setMinute:0];
[comps setSecond:0];

for (int i = 0; i < 100; i++) {
    iterationDate = [currentCalendar dateByAddingComponents:comps toDate:iterationDate options:0];
}

---

You can achieve something similar using CoreFoundation APIs:

CFCalendarRef calendar = CFCalendarCreateWithIdentifier(0, kCFGregorianCalendar);
CFAbsoluteTime at = CFAbsoluteTimeGetCurrent();
const CFOptionFlags options = 0;
for (int i = 0; i < NIter; ++i) {
    if (0 == CFCalendarAddComponents(calendar, &at, options, "d", 1)) {
        assert(0 && "uh-oh");
    }
    CFDateRef date = CFDateCreate(0, at);
    // store result
    CFRelease(date);
}
CFRelease(calendar);

That measured out to be 33% faster than Foundation. It’s even faster if you don’t need to create the CFDates and can simply store the CFAbsoluteTime values.