Pow doesn't work for decimal exponents

[alexandrinaw]

The current implementation of Pow only looks at the IntPart of the exponent, so something like 4^2.5 returns 16 :(

I'm not sure why the exponent argument is supposed to be a decimal if it actually only works for ints.

[l0k18]

I also discovered this bug today and it's a real downer because I wanted to use it for compound interest calculations.

[victorquinn]

Sorry all, would love a PR to help resolve this :)

I'll try to get to it but perhaps the community can get to it sooner as I'm currently swamped

[martskins]

Any pointers on how would you go about implementing this? Also, there's probably people using this believing that the result is correct, so I think some sort of measures should be taken. How do you feel about either panic-ing (as wrong as that sounds) when a non-int decimal is given as the exponent or returning an inexact value (maybe just use math.Pow?) while we wait for a new implementation?

[mediocregopher]

This is still a problem, and it bit me today. Reproduction is very easy:

	two := decimal.RequireFromString("2")
	onePointOne := decimal.RequireFromString("1.1")
	fmt.Printf("%v^%v = %v\n", two, onePointOne, two.Pow(onePointOne))

should output something like 2.14355..., but just outputs 2.

Is the underlying problem that decimal exponents can't always be represented to a non-infinite number of decimal places? If so, would it make sense to do something like what is done for division in this package, where it's possible to specify how much precision you want both on the package and method level (DivisionPrecision and DivRound)?

[mediocregopher]

Somewhat related: I implemented the ability to take the nth root of a number: #153.

For performing decimal exponentiation, if you know that d2 is a fraction then you can combine Pow and Root. For example:

three, four, threeQuarters := decimal.New(3, 0), decimal.New(4, 0), decimal.RequireFromString("0.75")

// broken
three.Pow(threeQuarters)

// works
three.Pow(three).Root(four)

[mwoss]

Thanks for creating this issue @alexandrinaw. This "bug" should be at least documented in code, because as @martskins said many people use this believing that the result is correct.
I'm adding this issue to the backlog and try o fix it before v1.2.0 release

[bartmika]

Issue still exists, I ran:

go get -u github.com/shopspring/decimal

I have v1.2.0 installed and this error still exists.

[juby-gif]

+1

[zhengjiejiang]

+1

[mwoss]

Yes, we know that it's still not a resolved issue. We weren't able to fix it with 1.2.0 release. Recently, I had a lot of o things on my mind and I wasn't able to focus on decimal development. Apologize for that. I will try to work on the issue this weekend. Please be patient

[bartmika]

@mwoss Hello how are you? I was wondering how the progress is coming along with this issue? Thank you!

[mwoss]

Hi @bartmika. This week I had my last uni assignments and other Master's degree related stuff and now I'll be free to invest some time into decimal library development.
I have a draft implementation on my local branch. I need to test it correctly and refactor code overall before I create a pull request with it.
Thank you all for your patient!

[bartmika]

Dziękuję bardzo!