Question

I am looking for a simple but efficient Objective-C algorithm which, starting from a given quarter of a year (3 months), adds or subtracts a specific number of quarters.

For example, adding 10 quarters to Q1 of 2012, I get the year: 2012 + Floor(10 / 4) = 2012 + 2 = 2014, then the quarter: 1 + Mod (10, 4) = 1 + 2 = Q3. Subtracting 2 quarters from Q3 of 2011, I want to end up with Q1 of 2011.

What would be the most elegant code to accomplish this?

Answer 1

- (void)addQuarters:(NSInteger)quarters toYear:(NSInteger *)currentYear andQuarter:(NSInteger *)currentQuarter {
    if (*currentYear < 0 ||
        *currentQuarter < 1 ||
        *currentQuarter > 4) {
        return;
    }

    *currentYear += quarters / 4;
    *currentQuarter += quarters % 4;

    if (*currentQuarter  > 4) {
        *currentQuarter -= 4;
        *currentYear += 1;
    } else if (*currentQuarter < 1) {
        *currentQuarter += 4;
        *currentYear -= 1;
    }
}

// Example of use:
NSInteger year = 2012;
NSInteger quarter = 2;

[self addQuarters:-7 toYear:&year andQuarter:&quarter];
NSLog(@"Quarter %i of %i", quarter, year);
// Output: Quarter 3 of 2010