kCFNumberFormatter round differences

Question

I found an interesting question what's not described on the internet, even is misleading in the apple documentation. What are the differences between:

kCFNumberFormatterRoundCeiling

kCFNumberFormatterRoundFloor

kCFNumberFormatterRoundDown

kCFNumberFormatterRoundUp

kCFNumberFormatterRoundHalfEven

kCFNumberFormatterRoundHalfDown

kCFNumberFormatterRoundHalfUp

?

kCFNumberFormatterRoundCeiling explanation is equal with kCFNumberFormatterRoundUp but they are not working the same, the same case in kCFNumberFormatterRoundFloor and kCFNumberFormatterRoundDown

kCFNumberFormatterRoundCeiling explanation is equal with kCFNumberFormatterRoundUp but they are not working the same, the same case in kCFNumberFormatterRoundFloor and kCFNumberFormatterRoundDown — flatronka, Jun 28 '13 at 10:01
Perhaps you should add that to the question to make clear what you are asking. (Seems a good question to me.) — Martin R, Jun 28 '13 at 10:03

Martin R · Accepted Answer · 2013-06-28T11:28:22.287

The documentation is indeed ambigous. Both kCFNumberFormatterRoundFloor and kCFNumberFormatterRoundDown are documented as

Round down to next larger number with the proper number of fraction digits.

A small test shows that these rounding modes give different results for negative numbers.

kCFNumberFormatterRoundFloor behaves similar to the floor() function. It rounds always to a smaller (or equal) value. On the other hand kCFNumberFormatterRoundDown rounds "towards zero", i.e. it returns gives a number whose absolute value is smaller or equal to the absolute value of the input.

Example: (using NSNumberFormatter)

NSNumberFormatter *f1 = [[NSNumberFormatter alloc] init];
[f1 setMaximumFractionDigits:0];
[f1 setRoundingMode:NSNumberFormatterRoundFloor];

NSNumberFormatter *f2 = [[NSNumberFormatter alloc] init];
[f2 setMaximumFractionDigits:0];
[f2 setRoundingMode:NSNumberFormatterRoundDown];

double x = -1.5;
NSLog(@"%@, %@", [f1 stringFromNumber:@(x)], [f2 stringFromNumber:@(x)]);
// Output: -2, -1

x = + 3.5;
NSLog(@"%@, %@", [f1 stringFromNumber:@(x)], [f2 stringFromNumber:@(x)]);
// Output: 3, 3

Similarly, kCFNumberFormatterRoundCeiling returns a value that is larger or equal to the input (like ceil()), and kCFNumberFormatterRoundUp rounds "away from zero".

kCFNumberFormatter round differences

1 Answers1