[英]List[Int] => Int without String Conversion?
I came up with the following to convert a List[Int] => Try[BigDecimal]
: 我想出了以下内容来转换List[Int] => Try[BigDecimal]
:
import scala.util.Try
def f(xs: List[Int]): Try[BigDecimal] =
Try { xs.mkString.toInt }.map ( BigDecimal(_) )
Example: 例:
scala> f(List(1,2,3,4))
res4: scala.util.Try[BigDecimal] = Success(1234)
scala> f(List(1,2,3,55555))
res5: scala.util.Try[BigDecimal] = Success(12355555)
Is there a way to write this function without resorting to a String conversion step? 有没有办法编写这个函数而不需要求助于字符串转换步骤?
Not very pretty, and I'm not convinced it's much more efficient. 不是很漂亮,我不相信它更有效率。 Here's the basic outline. 这是基本的大纲。
val pwrs:Stream[BigInt] = 10 #:: pwrs.map(_ * 10)
List(1,2,3,55555).foldLeft(0:BigInt)((p,i) => pwrs.find(_ > i).get * p + i)
Here it is a little more fleshed out with error handling. 这里有一个更加充实的错误处理。
import scala.util.Try
def f(xs: List[Int]): Try[BigDecimal] = Try {
lazy val pwrs: Stream[BigDecimal] = 10 #:: pwrs.map(_ * 10)
xs.foldLeft(0: BigDecimal) {
case (acc, i) if i >= 0 => pwrs.find(_ > i).get * acc + i
case _ => throw new Error("bad")
}
}
UPDATE UPDATE
Just for giggles, I thought I'd plug some code into Rex Kerr's handy benchmarking/profiling tool, Thyme . 只是为了咯咯笑,我想我会把一些代码插入到Rex Kerr的方便的基准测试/分析工具Thyme中 。
the code 编码
import scala.util.Try
def fString(xs: List[Int]): Try[BigInt] = Try { BigInt(xs.mkString) }
def fStream(xs: List[Int]): Try[BigInt] = Try {
lazy val pwrs: Stream[BigInt] = 10 #:: pwrs.map(_ * 10)
xs.foldLeft(0: BigInt) {
case (acc, i) if i >= 0 => pwrs.find(_ > i).get * acc + i
case _ => throw new Error("bad")
}
}
def fLog10(xs: List[Int]): Try[BigInt] = Try {
xs.foldLeft(0: BigInt) {
case (acc, i) if i >= 0 =>
math.pow(10, math.ceil(math.log10(i))).toInt * acc + i
case _ => throw new Error("bad")
}
}
fString()
is a slight simplification of Kevin's original question. fString()
是凯文原始问题的略微简化。 fStream()
is my proposed non-string implementation. fStream()
是我提出的非字符串实现。 fLog10
is the same but with Alexey's suggested enhancement. fLog10
与Alexey建议的增强功能相同。
You'll note that I'm using BigInt
instead of BigDecimal
. 你会注意到我使用的是BigInt
而不是BigDecimal
。 I found that both non-string methods encountered a bug somewhere around the 37th digit of the result. 我发现两个非字符串方法在结果的第37位左右遇到了一个错误。 Some kind of rounding error or something, but there was no problem with BigInt
so that's what I used. 某种舍入错误或其他什么,但BigInt
没有问题,所以这就是我使用的。
test setup 测试设置
// create a List of 40 Ints and check its contents
val lst = List.fill(40)(util.Random.nextInt(20000))
lst.min // 5
lst.max // 19858
lst.mkString.length // 170
val th = ichi.bench.Thyme.warmed(verbose = print)
th.pbenchWarm(th.Warm(fString(lst)), title="fString")
th.pbenchWarm(th.Warm(fStream(lst)), title="fStream")
th.pbenchWarm(th.Warm(fLog10(lst)), title="fLog10")
results 结果
Benchmark for fString (20 calls in 345.6 ms) Time: 4.015 us 95% CI 3.957 us - 4.073 us (n=19) Garbage: 109.9 ns (n=2 sweeps measured) fString的基准测试(在345.6 ms内调用20次)时间:4.015 us 95%CI 3.957 us - 4.073 us(n = 19)垃圾:109.9 ns(n = 2次测量扫描)
Benchmark for fStream (20 calls in 305.6 ms) Time: 7.118 us 95% CI 7.024 us - 7.213 us (n=19) Garbage: 293.0 ns (n=3 sweeps measured) fStream的基准测试(在305.6 ms内进行20次调用)时间:7.118 us 95%CI 7.024 us - 7.213 us(n = 19)垃圾:293.0 ns(n = 3次测量扫描)
Benchmark for fLog10 (20 calls in 382.8 ms) Time: 9.205 us 95% CI 9.187 us - 9.222 us (n=17) Garbage: 73.24 ns (n=2 sweeps measured) fLog10的基准测试(20次调用382.8 ms)时间:9.205 us 95%CI 9.187 us - 9.222 us(n = 17)垃圾:73.24 ns(测量n = 2次扫描)
So I was right about the efficiency of the non-string algorithm. 所以我对非字符串算法的效率是正确的。 Oddly, using math._
to avoid Stream
creation doesn't appear to be better. 奇怪的是,使用math._
来避免Stream
创建似乎并不是更好。 I didn't expect that. 我没想到。
takeaway 带走
Number-to-string and string-to-number transitions are reasonably efficient. 数字到字符串和字符串到数字的转换是相当有效的。
import scala.util.{Try, Success}
import scala.annotation.tailrec
def findOrder(i: Int): Long = {
@tailrec
def _findOrder(i: Int, order: Long): Long = {
if (i < order) order
else _findOrder(i, order * 10)
}
_findOrder(i, 1)
}
def f(xs: List[Int]): Try[BigDecimal] = Try(
xs.foldLeft(BigDecimal(0))((acc, i) => acc * findOrder(i) + i)
)
To find the correct power of 10 more efficiently (replace pwrs.find(_ > i).get
with nextPowerOf10(i)
in @jwvh's answer): 要更有效地找到10的正确功率nextPowerOf10(i)
在@jwvh的答案中用nextPowerOf10(i)
替换pwrs.find(_ > i).get
):
def nextPowerOf10(x: Int) = {
val n = math.ceil(math.log10(x))
BigDecimal(math.pow(10, n))
}
Since you start with an Int
, there should be no rounding issues. 因为你从Int
开始,所以不应该有舍入问题。
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