Why system can not find the method BigInteger.ToDouble?

Question

I am using F# Interactive, and I have added the reference of FSharp.PowerPack.dll.

When I try to convert BigNum to double as the following code,

let n = 2N
let d = double n

error comes out that

"System.MissingMethodException: Method not found: 'Double System.Numerics.BigInteger.ToDouble(System.Numerics.BigInteger)'. at Microsoft.FSharp.Math.BigNum.ToDouble(BigNum n)"

What can I do if I want to make such conversions like "BigNum to int" and "BigNum to double"? Thanks very much.

Answer 1

What you've written will work in the F# standalone CTP.

This error occurs in VS2010, because the BigInteger type has been moved from the F# library to the .Net4.0 core library. I'm not sure whether this issue has something to do with having both the F# CTP and the VS2010 beta installed.

Until a better solution comes along, you could roll your own conversion like this:

let numToDouble (n:bignum) = double n.Numerator / double n.Denominator

To convert a bignum to an integer you could then think of something like this:

let numToInt (n:bignum) = int n.Numerator / int n.Denominator

But this is rather dangerous: it'll overflow quite easily. A better version of numToInt would be to convert to a double first and then convert to int:

let numToInt = int << numToDouble

Still, both conversions aren't ideal for numerators/denominators of more than 308 digits, which will still overflow a double, while the fraction itself could be small.

ex: 11^300 / 13^280 ~= 3.26337, but

> numToDouble (pown 11N 300 / pown 13N 280);;
val it : float = nan