# Using Float For Currency **URL:** \ **Category:** Ruby\ **Created:** [December 13, 2005, 1:05am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102 "2005-12-13T01:05:30Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 1:05am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/1 "2005-12-13T01:05:30Z")
Howdy, I have some methods that manipulate floats that represent a currency amount. I often end up with more precision than I need, i.e.: $9.756. What is the best way to scale that to 9.76? Cheers. ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 1:14am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/2 "2005-12-13T01:14:33Z")
Quoting Hunter’s Lists [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Howdy, > > I have some methods that manipulate floats that represent a > currency amount. > > I often end up with more precision than I need, i.e.: $9.756. > > What is the best way to scale that to 9.76? If you’re doing anything that matters, don’t use floats for currency. There are a lot of really nasty subtle issues that will lose money between the cracks. Usually you want a specialized currency type which uses fixed-precision arithmetic. -mental ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 1:23am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/3 "2005-12-13T01:23:35Z")
Thanks. Fortunately this is just some quick guestimation throw-away stuff. No real need for precision. I will take a look at BigDecimal but in the meantime, any thoughts on the original question? Thx ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 1:15am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/4 "2005-12-13T01:15:57Z")
unknown wrote: > Quoting Hunter’s Lists [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > > > Howdy, > > > > I have some methods that manipulate floats that represent a > > currency amount. > > > > I often end up with more precision than I need, i.e.: $9.756. > > > > What is the best way to scale that to 9.76? > > If you’re doing anything that matters, don’t use floats for > currency. There are a lot of really nasty subtle issues that will > lose money between the cracks. > > Usually you want a specialized currency type which uses > fixed-precision arithmetic. BigDecimal is good. ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 1:23am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/5 "2005-12-13T01:23:36Z")
On 12/12/05, Hunter’s Lists [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Howdy, > > I have some methods that manipulate floats that represent a currency amount. > > I often end up with more precision than I need, i.e.: $9.756. > > What is the best way to scale that to 9.76? Try: val = 9.756 sprintf(“%.2f”, val) That says "print ‘val’ as a floating point value, with two decimal places of precision. ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 5:54am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/6 "2005-12-13T05:54:13Z")
“Wilson B.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > val = 9.756 > sprintf(“%.2f”, val) > That says "print ‘val’ as a floating point value, with two decimal > places of precision. Or just “%.2f” % val Cheers, Dave ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 6:12am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/7 "2005-12-13T06:12:15Z")
On 12/12/05, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > If you’re doing anything that matters, don’t use floats for > currency. There are a lot of really nasty subtle issues that will > lose money between the cracks. > > Usually you want a specialized currency type which uses > fixed-precision arithmetic. What cracks can I lose money through? ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 2:38am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/8 "2005-12-13T02:38:47Z")
Hunter’s Lists wrote: > Howdy, > > I have some methods that manipulate floats that represent a currency amount. > > I often end up with more precision than I need, i.e.: $9.756. > > What is the best way to scale that to 9.76? require ‘bigdecimal’ amount = BigDecimal.new(‘9.756’) rounded = (amount \* 100).round / 100 printf(’%.02f’, rounded) ## Outputs ‘9.76’ Neil S. - removed\_email\_address@domain.invalid ‘A republic, if you can keep it.’ – Benjamin Franklin ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 4:02pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/9 "2005-12-13T16:02:04Z")
On Tue, 13 Dec 2005, Neil S. wrote: > amount = BigDecimal.new(‘9.756’) > rounded = (amount \* 100).round / 100 > printf(’%.02f’, rounded) > > Outputs ‘9.76’ There is at least one point more to make about rounding: this method has a bias when floats ending with a 5 are involved. In the extreme case, when all you three-decimal currency amounts end in a 5 (like $9.755, $9.765) then all are rounded upwards. Instead, half of them should be rounded up, the other half down. A good method to do that is to round to the nearest even digit: $9.755 -\> $9.76 $9.765 -\> $9.76 (not $9.77) The attached method round2 does this (although probably not very efficiently): 8.5.round2 -\> 8 9.765.round2(0.01) -\> 9.76 ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 8:06am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/10 "2005-12-13T08:06:29Z")
Joe Van D. wrote: > What cracks can I lose money through? Floating point numbers represent an extremely wide range of values - much wider than their integer counterparts. This is handled through an exponent and mantissa. For this ability, they trade off precision. Think about the case of adding a large floating point number to a small floating point number: ``` irb(main):001:0> a = 1.0e30 => 1.0e+030 irb(main):002:0> b = 1.0e-30 => 1.0e-030 irb(main):003:0> a + b => 1.0e+030 ``` While this is an extreme example, it does demonstrate the loss of precision. Essentially, in floating point arithmetic we’re trying to squeeze much more out of, say 32 or 48 or 64 bits. Integer arithmetic, on the other hand, is exact. And therefore so is fixed point arithmetic; however, fixed point doesn’t enjoy the wide representation range as floats. The bottom line is you should never use floating point when it comes to money. Eventually you’re going to miss pennies. Instead represent things in the smallest denomination, such as cents, and fix it up in presentation, or use a custom Money column type, or data type. There’s your cracks! Just say no… unless you’re a hot chick. Even then it’s questionable. –Steve ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 11:35pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/11 "2005-12-13T23:35:10Z")
Quoting Malte M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Joe Van D.: > > > What cracks can I lose money through? > > irb\> 0.2 - 0.05 - 0.15 > =\> 2.77555756156289e-17 > > You’re actually gaining money here. Indeed. The critical take-home lesson: Floating point arithmetic only approximates arithmetic with real numbers. Never write code which assumes that math with floating-point code will observe the normal laws of arithmetic (most common mistake: assuming == is useful for testing the equality of two floating-point results). If you’re unsure, it might be better not to use floating-point at all. -mental ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 16, 2005, 11:15pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/12 "2005-12-16T23:15:41Z")
Joe Van D. wrote: > On 12/12/05, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > If you’re doing anything that matters, don’t use floats for > > currency. There are a lot of really nasty subtle issues that will > > lose money between the cracks. > > […] > > What cracks can I lose money through? It’s not just that; your program logic can also behave unexpectedly. e.g. irb\> 1.20 - 1.00 == 0.20 =\> false Use BigDecimal for currency. I’ve just finished some new Rdoc documentation for it which will hopefully be added to [ruby-doc.org](http://ruby-doc.org) and the 1.9 release, and can forward you a copy if you like. mathew ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 16, 2005, 11:42pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/13 "2005-12-16T23:42:54Z")
mathew wrote: > Use BigDecimal for currency. I’ve just finished some new Rdoc > documentation for it which will hopefully be added to [ruby-doc.org](http://ruby-doc.org) and > the 1.9 release, and can forward you a copy if you like. Hi mathew, Excuse my ignorance, but I’m wondering what BigDecimal does different than the built-in integer types? Thanks, Steve ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 13, 2005, 9:34pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/14 "2005-12-13T21:34:32Z")
Joe Van D.: > What cracks can I lose money through? irb\> 0.2 - 0.05 - 0.15 =\> 2.77555756156289e-17 You’re actually gaining money here. Malte ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 16, 2005, 11:45pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/15 "2005-12-16T23:45:55Z")
On Dec 16, 2005, at 4:39 PM, Stephen W. wrote: > mathew wrote: > > > Use BigDecimal for currency. I’ve just finished some new Rdoc > > documentation for it which will hopefully be added to [ruby-doc.org](http://ruby-doc.org) > > and the 1.9 release, and can forward you a copy if you like. > > Hi mathew, > > Excuse my ignorance, but I’m wondering what BigDecimal does > different than the built-in integer types? It doesn’t use floating point arithmetic. That means it is accurate, but slower. James Edward G. II ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 17, 2005, 12:01am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/16 "2005-12-17T00:01:00Z")
James Edward G. II wrote: > On Dec 16, 2005, at 4:39 PM, Stephen W. wrote: > > > Excuse my ignorance, but I’m wondering what BigDecimal does different > > than the built-in integer types? > > It doesn’t use floating point arithmetic. That means it is accurate, > but slower. Hi James, I think you misread my question. I realize the difference between Integer and FP math, I even explained it earlier in this thread. I’m asking why one would use BigDecimal, specifically, as opposed to the built-in Integer types? Thanks, Steve ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 17, 2005, 12:04am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/17 "2005-12-17T00:04:00Z")
On Dec 16, 2005, at 4:58 PM, Stephen W. wrote: > I think you misread my question. Sorry about that. > I’m asking why one would use BigDecimal, specifically, as opposed > to the built-in Integer types? Well, BigDecimal lets you work with decimals, but will be slower. Integers will be faster, but you’ll need to handle the conversions, as needed. Hope I got it right that time. 😉 James Edward G. II ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 16, 2005, 11:51pm UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/18 "2005-12-16T23:51:26Z")
Hunter’s Lists wrote: > Howdy, > > I have some methods that manipulate floats that represent a currency > amount. > > I often end up with more precision than I need, i.e.: $9.756. > > What is the best way to scale that to 9.76? I think this has already been covered, but: “Money does not float!” 🙂 > Cheers. E ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 17, 2005, 12:13am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/19 "2005-12-17T00:13:08Z")
Stephen W. wrote: > I’m asking why one would use BigDecimal, specifically, as opposed to the > built-in Integer types? One would use BigDecimal instead of Fixnum or Bignum when one is going to do some operation that might produce a fraction. If you’re certain your operations will always result in whole numbers, though, I don’t see a point in dragging in BigDecimal. – Neil S. - removed\_email\_address@domain.invalid ‘A republic, if you can keep it.’ – Benjamin Franklin ---
**Author:** ![HunterSs\_Lists](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/6bbea6/32.png) [@HunterSs\_Lists](https://www.ruby-forum.com/u/HunterSs_Lists)\ **Post date:** [December 17, 2005, 12:28am UTC](https://www.ruby-forum.com/t/using-float-for-currency/50102/20 "2005-12-17T00:28:44Z")
James Edward G. II wrote: > Well, BigDecimal lets you work with decimals, but will be slower. > Integers will be faster, but you’ll need to handle the conversions, as > needed. Ahh, I see what BigDecimal is now. Think I’d rather just use cents and Integers. –Steve [Next page](https://www.ruby-forum.com/t/using-float-for-currency/50102.md?page=2)