Objective-C - Wrong output on simple math - ios

Question

I am learning Objective-C and iOS development by making a simple tip calculator. However, the issue I am having is when I try to calculate the tip. This is simple math (tip percent / total bill) * 100. This is exactly what I am doing, but I am really confused as to why my output is wrong.
This is the method in my ViewController.m file that is giving me issues
- (IBAction)doCalculate:(id)sender {
NSInteger totalBillAmount = self.inputTotalBill.text.intValue;
NSLog(#"input total bill: %i", totalBillAmount);
NSInteger tipPercent = self.inputTip.text.intValue;
NSLog(#"input tip percent: %i", tipPercent);
NSInteger tipAmount = (tipPercent / totalBillAmount) * 100;
NSLog(#"tip amount: %i", tipAmount);
NSInteger billAmount = totalBillAmount + tipAmount;
NSLog(#"total bill: %i", billAmount);
// Set labels accordingly
self.labelTipAmount.text = [NSString stringWithFormat:#"%i", tipAmount];
self.labelBillAmount.text = [NSString stringWithFormat:#"%i", billAmount];
}
And this is my output:
2016-02-28 01:39:36.283 conversion[1533:58347] input total bill: 100
2016-02-28 01:39:36.285 conversion[1533:58347] input tip percent: 15
2016-02-28 01:39:36.285 conversion[1533:58347] tip amount: 0
2016-02-28 01:39:36.285 conversion[1533:58347] total bill: 100
I am really confused so any help is appreciated, thanks!

Answer 1

Computer numbers does not behave like numbers you learned at school. Integers only use integer arithmetic, and then integer division (look on the web).
4/100 (as an integer division) gives 0 (remember Euclidian division?) If you want to make more natural computations, use floats or doubles (but they will surprise you even more later!).

Answer 2

When you divide two NSIntegers, the result is one NSInteger. If that fraction is <1 and >0 then the output is 0. Using NSInteger might not be the best option here if you want it to be simple.
NSInteger tipAmount = (tipPercent * totalBillAmount) / 100;
NSLog(#"tip amount: %i", tipAmount); // tip amount: 0
If you used a float, it would be a whole lot cleaner:
float tipAmount = (tipPercent * totalBillAmount) / 100;
NSLog(#"tip amount: $%.02f", tipAmount); // tip amount: $15.00
However, using floats for currency can be very bad. So, it would be a more sage decision to use NSInteger to keep track of the smallest unit of currency instead. For the USD this is $0.001, or one tenth of a cent.
This means, when someone enters the bill total, let's say $100.00, you would record that value as 100000.
Then, to calculate 15%, you would multiply the bill by 15 and then divide by 100.
NSInteger tipAmount = (tipPercent * totalBillAmount) / 100;
NSLog(#"tip amount: %i", tipAmount); // tip amount: 15000
To show the user again, I would use a method like the following to convert from tenth cent units to a formatted string for dollars:
- (NSString *)tenthCentToDollarString:(NSInteger)tenthCents {
if (tenthCents >= 0) {
NSInteger roundedCents = (tenthCents + 5) / 10;
if (roundedCents < 10) {
return [NSString stringWithFormat:#"$0.0%zd", roundedCents];
}
if (roundedCents < 100) {
return [NSString stringWithFormat:#"$0.%zd", roundedCents];
}
NSInteger cents = roundedCents % 100;
NSInteger dollars = roundedCents / 100;
if (cents < 10) {
return [NSString stringWithFormat:#"$%zd.0%zd", dollars, cents];
}
return [NSString stringWithFormat:#"$%zd.%zd", dollars, cents];
}
// Dollar amount is negative
NSInteger positiveTenthCents = ABS(tenthCents);
NSString *dollarString = [self tenthCentToDollarString:positiveTenthCents];
return [NSString stringWithFormat:#"-%#", dollarString];
}

Answer 3

A couple issues: You should be doing tipAmount = (tipPercent / 100) * totalBillAmount and cast to doubles because NSInts can't do fractions.