Developers - Best practises for decimal handling

[Seal]

Any decimal number can be exactly represented this way.

Unless you are using javascript which treats decimals as floats 

[1715192594]

1715192594

[1715192594]

1715192594

[Xenland]

Any decimal number can be exactly represented this way.

Unless you are using javascript which treats decimals as floats 

I was waiting for someone else to point that out as well that there was no language being referenced.

[Seal]

Any decimal number can be exactly represented this way.

Unless you are using javascript which treats decimals as floats  

I was waiting for someone else to point that out as well that there was no language being referenced.

Yes, the only way I thought of getting around this is to store multiple values in different formats for different languages. ie. store both integer and decimal or float.

It would be a pain otherwise to have a website which depends on javascript to manage the correct decimal display output. Every single value return or shown would require the use of javascript.

[davout]

Unless you are using javascript which treats decimals as floats 

That just means javascript doesn't support decimals, only floats.
Using floats for display is fine in most cases, using floats for accounting is terribly wrong.

[Zeilap]

Any decimal number can be exactly represented this way.

Unless you are using javascript which treats decimals as floats 

I was waiting for someone else to point that out as well that there was no language being referenced.

No, this is not specific to javascript - no languages (that I can think of) provide built-in type support for arbitrary precision arithmetic, and for good reason - it is expensive in terms of computation.

What people are trying to say is that you should be using a library which provides a Decimal class which does provide this functionality (or a subset), and not use floating point values at all for computation.

[davout]

no languages (that I can think of) provide built-in type support for arbitrary precision arithmetic

• http://docs.oracle.com/javase/1.5.0/docs/api/java/math/BigDecimal.html
• http://www.ruby-doc.org/stdlib-1.9.3/libdoc/bigdecimal/rdoc/BigDecimal.html
• http://docs.python.org/2/library/decimal.html

and for good reason - it is expensive in terms of computation.

If I follow you, languages shouldn't provide standard interfaces to do network I/O, because you know, that's computationally expensive and slow. I guess I won't follow you...

[Zeilap]

no languages (that I can think of) provide built-in type support for arbitrary precision arithmetic

• http://docs.oracle.com/javase/1.5.0/docs/api/java/math/BigDecimal.html
• http://www.ruby-doc.org/stdlib-1.9.3/libdoc/bigdecimal/rdoc/BigDecimal.html
• http://docs.python.org/2/library/decimal.html

and for good reason - it is expensive in terms of computation.

If I follow you, languages shouldn't provide standard interfaces to do network I/O, because you know, that's computationally expensive and slow. I guess I won't follow you...

They're not built-in types, they're types provided by a library. What the original poster seems (to me) to be confused about is that they can write code similar to

Code:

var x = 1.234

in some language and it will magically make x into a 'Decimal' object, and that only javascript will 'convert' this into a float/double. This is not true - they all will, as I say, because no language has a built-in type representing decimals, 1.234 is already a float before it gets assigned to anything. If you want decimals, then you have to specifically create one like

Code:

var x = new Decimal("1.234")

or however your library requires.

[davout]

They're not built-in types, they're types provided by a library.

I call built-in what's part of the standard library of a language. So to me it's built-in the languages I listed but not C++ or PHP for example, because these require an external library.

But yes, I don't know of any language that will natively interpret a numeric string as a decimal type without explicitly telling it to.

[deepceleron]

Your site may need more integer math accuracy than 1 satoshi = 1

example: 1 satoshi - 1.3% fee = .987 satoshis

>>> print int(.987)
0
>>> print int(round(.987))
1

However, even using arbitrary precision, these libraries don't necessary give you the correct answer anywhere near the magnitude of the float precision defined.

>>> getcontext().prec = 6
>>> Decimal (.00000001) * Decimal (.987)
Decimal('9.87000E-9')

>>>getcontext().prec = 2000
>>> Decimal (.00000001) * Decimal (.987)[/b]
Decimal('9.87000000000000009104248083235174268563047313600349515597682205532708579013502123676582211686536 538763903081417083740234375E-9')

It's close enough though - you'd get an extra 9 satoshi if your transaction was 10 billion BTC.

[TheButterZone]

Just going to leave this here, since I'm not a dev: https://github.com/zootreeves/blockchain.info/issues/5

[Xenland]

Your site may need more integer math accuracy than 1 satoshi = 1

example: 1 satoshi - 1.3% fee = .987 satoshis

>>> print int(.987)
0
>>> print int(round(.987))
1

However, even using arbitrary precision, these libraries don't necessary give you the correct answer anywhere near the magnitude of the float precision defined.

>>> getcontext().prec = 6
>>> Decimal (.00000001) * Decimal (.987)
Decimal('9.87000E-9')

>>>getcontext().prec = 2000
>>> Decimal (.00000001) * Decimal (.987)[/b]
Decimal('9.87000000000000009104248083235174268563047313600349515597682205532708579013502123676582211686536 538763903081417083740234375E-9')

It's close enough though - you'd get an extra 9 satoshi if your transaction was 10 billion BTC.

Assuming the total BTC in circulation limit was increased this could ever actually happen yes.