在F#中使用float迭代器列出列表 [英] List comprehensions with float iterator in F#

问题描述

考虑以下代码:

let dl = 9.5 / 11.
let min = 21.5 + dl
let max = 40.5 - dl

let a = [ for z in min .. dl .. max -> z ] // should have 21 elements
let b = a.Length

"a"应具有21个元素，但只有20个元素.缺少"max-dl"值.我知道浮点数并不精确，但是我希望F#可以解决这个问题.如果不是，那为什么F#用float迭代器支持List理解?对我来说，这是漏洞的来源.

"a" should have 21 elements but has got only 20 elements. The "max - dl" value is missing. I understand that float numbers are not precise, but I hoped that F# could work with that. If not then why F# supports List comprehensions with float iterator? To me, it is a source of bugs.

在线试用: http://tryfs.net/snippets/snippet-3H

推荐答案

转换为小数并查看数字，似乎第21项将超出"最大值:

Converting to decimals and looking at the numbers, it seems the 21st item would 'overshoot' max:

let dl = 9.5m / 11.m
let min = 21.5m + dl
let max = 40.5m - dl

let a = [ for z in min .. dl .. max -> z ] // should have 21 elements
let b = a.Length

let lastelement = List.nth a 19
let onemore = lastelement + dl
let overshoot = onemore - max

那可能是由于let dl = 9.5m / 11.m中缺乏精度吗?

That is probably due to lack of precision in let dl = 9.5m / 11.m?

要摆脱这种复合错误，您必须使用另一个数字系统，即Rational. F#Powerpack带有一个BigRational类，可以这样使用:

To get rid of this compounding error, you'll have to use another number system, i.e. Rational. F# Powerpack comes with a BigRational class that can be used like so:

let dl = 95N / 110N
let min = 215N / 10N + dl
let max = 405N / 10N - dl

let a = [ for z in min .. dl .. max -> z ] // Has 21 elements
let b = a.Length

这篇关于在F#中使用float迭代器列出列表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！