Once and for all: Do not use double for money

What do you mean with “Use a distribute algorithm to distribute money to severel slots without loosing cents.”? Could you please explain this further? Anyway thanks for the links, good discussions there.

@digital.alterego: Fowler calls it allocate I guess. You have 3 people and need to distribute (allocate) 10 EUR to them. 10/3 = 3.33 in cents, which means 1 cent is lost. Distributing 10 EUR correctly will result in 3 buckets [3.33, 3.33, 3.34].

I always distribute as [3.33, 3.33, 3.33] and then deposit 0.01 into my own secret account.

The comic hero, I´m really honoured.

A lot of people tried this, most of them got caught when getting greedy :-)

Quote:
————————–
I always distribute as [3.33, 3.33, 3.33] and then deposit 0.01 into my own secret account.
————————–

Just make sure you don’t mess up on some mundane detail like putting a decimal point in the wrong place.

But seriously, it’s been my opinion for a while that using float or double should cause a compiler warning optionally suppressed by a compiler flag. I believe the only place a typical programmer would run into floats being used correctly would be 3D graphics programming.

[...] Stephan advised to use the "money" data type to store currency amounts instead of doubles. For good reasons. [...]

@Greg: I have the same feeling. Developers using double or float do use the wrong type in most cases except for 3D graphics or statistics.

I don’t know about USA, but in Europe you would use ROUND_HALF_UP for money rounding!?

ROUND_HALF_EVEN is called fair rounding because it distributes the error. It’s also called bankers rounding.

http://en.wikipedia.org/wiki/Rounding#Round-to-even_method

“Rounding mode to round towards the “nearest neighbor” unless both neighbors are equidistant, in which case, round towards the even neighbor. Behaves as for ROUND_HALF_UP if the digit to the left of the discarded fraction is odd; behaves as for ROUND_HALF_DOWN if it’s even. Note that this is the rounding mode that minimizes cumulative error when applied repeatedly over a sequence of calculations.”

http://java.sun.com/j2se/1.5.0/docs/api/java/math/BigDecimal.html#ROUND_HALF_EVEN

[...] No signal, no noise. « Once and for all: Do not use double for money [...]

http://en.wikipedia.org/wiki/Rounding#Round-to-even_method:

“The origin of the term bankers’ rounding is more obscure. If this rounding method was ever a standard in banking, the evidence has proved extremely difficult to find. To the contrary, section 2 of the European Commission report The Introduction of the Euro and the Rounding of Currency Amounts [2] suggests that there had previously been no standard approach to rounding in banking.”

funny enough, but Fowler did use double for money amount in his examples :-)

Alex: Sure?

Looking at page 491 of Patterns of Enterprise Application Architecture he explicitly discourages the use of floats and suggests using long (See google book search).

Today I came back to this posting. I’m maintaining an app made by other people no longer in the company, and there is an issue with money rounding – they used Double. I’m so pissed off I have to deal with this!!!
So yeah folks, I guess this posting is great advice.