Rounding issue when using NSString and %f

Question

I was under the impression that using a format specifier like %0.1f would round float values following a standard 4/5 split (i.e. 3.24 would round to 3.2 and 3.25 would round to 3.3). Yet I'm not getting results I expect. Here is my test code:

NSLog(@"%0.1f", 3.24); //Output: 3.2
NSLog(@"%0.1f", 3.25); //Output: 3.2  Expected Output: 3.3
NSLog(@"%0.1f", 3.26); //Output: 3.3

Even using NSNumberFormatter generates the same result:

    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    [formatter setNumberStyle:NSNumberFormatterDecimalStyle];
    [formatter setRoundingMode: NSNumberFormatterRoundHalfUp]; //Tried with and without setting this.  Same results either way.
    [formatter setMaximumFractionDigits:1];

    NSString *numberString1 = [formatter stringFromNumber:[NSNumber numberWithFloat:3.24]];
    NSString *numberString2 = [formatter stringFromNumber:[NSNumber numberWithFloat:3.25]];
    NSString *numberString3 = [formatter stringFromNumber:[NSNumber numberWithFloat:3.26]];

    NSLog(@"Result...%@",numberString1); //Output: 3.2
    NSLog(@"Result...%@",numberString2); //Output: 3.2  Expected: 3.3
    NSLog(@"Result...%@",numberString3); //Output: 3.3

Seems like basic stuff. What am I doing wrong?

"What am I doing wrong?" You're using floating point without understanding it. Likely your "3.25" value actually has a closest approximation of something like 3.2499987, so that is the input to the rounding operation. — Hot Licks, Jul 15 '13 at 17:07
@HotLicks - Ok, so is there a better way to handle rounding? I'm not looking for extreme precision. Just for 1 or 2 decimal places. — user2584382, Jul 15 '13 at 17:33
Rounding is being handled correctly. Your perception of it is incorrect. — Hot Licks, Jul 15 '13 at 17:36
Use NSDecimalNumber instead of float if you need a precise representation of decimal numbers. — Martin R, Jul 15 '13 at 17:46

Rounding issue when using NSString and %f

0 Answers0