calculation not working as expected.. any ideas?? (iphone / int)

Question

I have a calculation which isn't working and I cannot work out why!

int numHoursInWeek;
int numDays;
int averageSalary;

int total_seconds_in_year = ((numHoursInWeek * 60 * 60) * numDays);

NSLog(@"average sal in pence=%i", (averageSalary * 100));
NSLog(@"num seconds in year=%i", total_seconds_in_year);
NSLog(@"cost per second=%i", ((averageSalary * 100) / total_seconds_in_year));

int cost_per_person_per_second = ((averageSalary*100) / total_seconds_in_year);

costPerSecond = (cost_per_person_per_second * numPeople);
lblCostPerPerson.text = [NSString stringWithFormat:@"%.2f",cost_per_person_per_second];

the above returns the following in NSLog

average sal in pence=3400000
num seconds in year=31968000
cost per second=-1.991753

I know everything else is being set correctly (numDays, averageSalary for example). When I do the calc manually, I get 0.1063. So that should show on my label?? (cost per person per second).

any ideas? should I be using floats instead of ints for the variables?

Answer 1

When you do integer division, numbers are truncated, so:

6 / 4
>>> 1

Change your data type to float or double, and write all of your numbers as "100.0" for example - otherwise it will be treated as an int.

Answer 2

Looking at

total_seconds_in_year = ((numHoursInWeek * 60 * 60) * numDays);

Shouldn't that be

total_seconds_in_year = ((numHoursInWeek * 60 * 60) * numWeeks);

or

 total_seconds_in_year = ((numHoursInDay * 60 * 60) * numDays);

Answer 3

When you use "%f", make sure that the argument is a float or double, not an integer. GCC will warn you about this for printf, but for some reason it can't do this for stringWithFormat:.