Using Float For Currency


#1

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.


#2

Quoting Hunter’s Lists 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


#3

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


#4

unknown wrote:

Quoting Hunter’s Lists 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.


#5

On 12/12/05, Hunter’s Lists 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.


#6

“Wilson B.” 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


#7

On 12/12/05, removed_email_address@domain.invalid 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?


#8

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


#9

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


#10

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


#11

Quoting Malte M. 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


#12

Joe Van D. wrote:

On 12/12/05, removed_email_address@domain.invalid 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 and
the 1.9 release, and can forward you a copy if you like.

mathew


#13

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


#14

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


#15

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
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


#16

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


#17

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. :wink:

James Edward G. II


#18

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!”

:slight_smile:

Cheers.

E


#19

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


#20

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