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.
Quoting Hunter's Lists <lists@lastonepicked.com>:
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
Try:
val = 9.756
sprintf("%.2f", val)
That says "print 'val' as a floating point value, with two decimal
places of precision.
···
On 12/12/05, Hunter's Lists <lists@lastonepicked.com> 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?
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 Stevens - neil@hakubi.us
'A republic, if you can keep it.' -- Benjamin Franklin
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
···
--
Posted via http://www.ruby-forum.com/\.
unknown wrote:
Quoting Hunter's Lists <lists@lastonepicked.com>:
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.
···
--
Posted via http://www.ruby-forum.com/\.
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
···
From: <mental@rydia.net>
Reply-To: <ruby-talk@ruby-lang.org>
Date: Tue, 13 Dec 2005 09:12:56 +0900
To: ruby-talk ML <ruby-talk@ruby-lang.org>
Subject: Re: Using Float For CurrencyQuoting Hunter's Lists <lists@lastonepicked.com>:
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
Or just
"%.2f" % val
Cheers,
Dave
···
"Wilson Bilkovich" <wilsonb@gmail.com> wrote:
val = 9.756
sprintf("%.2f", val)
That says "print 'val' as a floating point value, with two decimal
places of precision.
What cracks can I lose money through?
···
On 12/12/05, mental@rydia.net <mental@rydia.net> wrote:
Quoting Hunter's Lists <lists@lastonepicked.com>:
> 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.
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
round2.rb (2.14 KB)
···
On Tue, 13 Dec 2005, Neil Stevens wrote:
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'
--
Wybo
Joe Van Dyk 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
Joe Van Dyk:
What cracks can I lose money through?
0.2 - 0.05 - 0.15
=> 2.77555756156289e-17
You're actually gaining money here.
Malte
Joe Van Dyk 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.
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 and the 1.9 release, and can forward you a copy if you like.
mathew
···
On 12/12/05, mental@rydia.net <mental@rydia.net> wrote:
--
<URL:http://www.pobox.com/~meta/>
My parents went to the lost kingdom of Hyrule
and all I got was this lousy triforce.
Quoting Malte Milatz <malte__@gmx-topmail.de>:
Joe Van Dyk:
> What cracks can I lose money through?> 0.2 - 0.05 - 0.15
=> 2.77555756156289e-17You'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
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 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
It doesn't use floating point arithmetic. That means it is accurate, but slower.
James Edward Gray II
···
On Dec 16, 2005, at 4:39 PM, Stephen Waits 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 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?
James Edward Gray II 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
···
On Dec 16, 2005, at 4:39 PM, Stephen Waits wrote:
And built-in integer types represent integers, not decimals.
That does however, bring up the point of using an integer
representation instead of floating point, eg. manipulating integer
pennies instead of decimal dollars. If you are certain that integer
arithmetic -- with it's corresponding truncation; eg. (5 / 3) * 3
returns 3, not 5 -- is sufficient for your application, it can be
faster than BigDecimal. AFAIK, BigDecimal is doing this same thing
(integer representation), behind the scenes, but is slower due to the
requirements of being flexible in how many decimal places it
represents.
Another class to look into along these lines -- slower but more
encapsulated than a "roll your own" integer representation, and more
domain specific than BigDecimal -- is the great Money library from
Tobias Lütke:
http://dist.leetsoft.com/api/money/
Jacob Fugal
···
On 12/16/05, James Edward Gray II <james@grayproductions.net> wrote:
On Dec 16, 2005, at 4:39 PM, Stephen Waits 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.
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 Gray II
···
On Dec 16, 2005, at 4:58 PM, Stephen Waits wrote:
Stephen Waits 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 Stevens - neil@hakubi.us
'A republic, if you can keep it.' -- Benjamin Franklin