Incorrect result using float to represent currency values

Question

If I use the following code:

- (NSString*)formatToCurrency:(float)inFloat {
  NSNumberFormatter *currencyFormatter = [[[NSNumberFormatter alloc] init] autorelease];
  [currencyFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
  [currencyFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
  [currencyFormatter setMaximumFractionDigits:2];
  [currencyFormatter setLocale:[NSLocale currentLocale]];

  NSNumber *x = [NSNumber numberWithFloat:inFloat];
  return [NSString stringWithFormat:@"  %@", [currencyFormatter stringFromNumber:x]];
}

then pass in 203500 when calling the code, I receive the output: $203,500.02.

I'd love to hear some thoughts on what is happening.

Thanks,

Matt

EDIT: XCode was reporting 203500 when I viewed it with the debugger, but when I converted the float to an NSString, it would also show 203500.018. Meaning the problem was in the float and not in the NSNumberFormatter.

To give an example of @vikingosegundo .. see http://stackoverflow.com/questions/856019/localize-currency-for-iphone — Peter M, Nov 04 '12 at 21:26
Try it again, only change all occurrences of `float` to `double`. — Hot Licks, Nov 04 '12 at 23:01

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

6

You are creating the NSNumber with a float. Floats always can have a rounding error

What Every Computer Scientist Should Know About Floating-Point Arithmetic

You should familiarize yourself with NSDecimals and NSDecimalNumbers

Cocoa is my GF: Don't be lazy with NSDecimalNumber (like me)

edited Jun 20 '20 at 09:12

Community

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answered Nov 04 '12 at 21:20

vikingosegundo

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+1 for `NSDecimal`, but if fractional amounts of cents are not allowed, a better solution would be to encode the dollar amount in cents instead, and use an integral value type to store it – yfrancis Nov 04 '12 at 21:47
Thanks. I completely blanked that. – Matt Hansen Nov 05 '12 at 04:43

score 0 · Answer 2 · answered Nov 04 '12 at 23:14

Clearly I've lots to learn on this. But at the risk of getting dinged further, I'll report one thing that did help me.

I had this code:

self.myDollarFormat.roundingIncrement = [NSNumber numberWithFloat:roundingInc];
return [self.myDollarFormat stringFromNumber:[NSNumber numberWithFloat: dollars]];

And found 638198.2 with a roundingIncrement of 0.1 was yielding @"$638,198.21".

I changed my code to use double instead of float, and then I got the expected result.

self.myDollarFormat.roundingIncrement = [NSNumber numberWithDouble: roundingInc];
return [self.myDollarFormat stringFromNumber:[NSNumber numberWithDouble: dollars]];

I defer to those who know more, but I thought I'd at least mention this worked for me. Good luck.

Incorrect result using float to represent currency values

2 Answers2