How to find if a Scala String is parseable as a Double or not?
ScalaScala Problem Overview
Suppose that I have a string in scala and I want to try to parse a double out of it.
I know that, I can just call toDouble
and then catch the java num format exception if this fails, but is there a cleaner way to do this? For example if there was a parseDouble
function that returned Option[Double]
this would qualify.
I don't want to put this in my own code if it already exists in the standard library and I am just looking for it in the wrong place.
Thanks for any help you can provide.
Scala Solutions
Solution 1 - Scala
For Scala 2.13+ see Xavier's answer below. Apparently there's a toDoubleOption
method now.
For older versions:
def parseDouble(s: String) = try { Some(s.toDouble) } catch { case _ => None }
Fancy version (edit: don't do this except for amusement value; I was a callow youth years ago when I used to write such monstrosities):
case class ParseOp[T](op: String => T)
implicit val popDouble = ParseOp[Double](_.toDouble)
implicit val popInt = ParseOp[Int](_.toInt)
// etc.
def parse[T: ParseOp](s: String) = try { Some(implicitly[ParseOp[T]].op(s)) }
catch {case _ => None}
scala> parse[Double]("1.23")
res13: Option[Double] = Some(1.23)
scala> parse[Int]("1.23")
res14: Option[Int] = None
scala> parse[Int]("1")
res15: Option[Int] = Some(1)
Solution 2 - Scala
Scalaz provides an extension method parseDouble
on String
s, which gives a value of type Validation[NumberFormatException, Double]
.
scala> "34.5".parseDouble
res34: scalaz.Validation[NumberFormatException,Double] = Success(34.5)
scala> "34.bad".parseDouble
res35: scalaz.Validation[NumberFormatException,Double] = Failure(java.lang.NumberFormatException: For input string: "34.bad")
You can convert it to Option
if so required.
scala> "34.bad".parseDouble.toOption
res36: Option[Double] = None
Solution 3 - Scala
scala> import scala.util.Try
import scala.util.Try
scala> def parseDouble(s: String): Option[Double] = Try { s.toDouble }.toOption
parseDouble: (s: String)Option[Double]
scala> parseDouble("3.14")
res0: Option[Double] = Some(3.14)
scala> parseDouble("hello")
res1: Option[Double] = None
Solution 4 - Scala
You could try using util.control.Exception.catching
which returns an Either
type.
So using the following returns a Left wrapping a NumberFormatException
or a Right wrapping a Double
import util.control.Exception._
catching(classOf[NumberFormatException]) either "12.W3".toDouble
Solution 5 - Scala
Scala 2.13
introduced String::toDoubleOption
:
"5.7".toDoubleOption // Option[Double] = Some(5.7)
"abc".toDoubleOption // Option[Double] = None
"abc".toDoubleOption.getOrElse(-1d) // Double = -1.0
Solution 6 - Scala
Unfortunately, this isn't in the standard library. Here's what I use:
class SafeParsePrimitive(s: String) {
private def nfe[T](t: => T) = {
try { Some(t) }
catch { case nfe: NumberFormatException => None }
}
def booleanOption = s.toLowerCase match {
case "yes" | "true" => Some(true)
case "no" | "false" => Some(false)
case _ => None
}
def byteOption = nfe(s.toByte)
def doubleOption = nfe(s.toDouble)
def floatOption = nfe(s.toFloat)
def hexOption = nfe(java.lang.Integer.valueOf(s,16))
def hexLongOption = nfe(java.lang.Long.valueOf(s,16))
def intOption = nfe(s.toInt)
def longOption = nfe(s.toLong)
def shortOption = nfe(s.toShort)
}
implicit def string_parses_safely(s: String) = new SafeParsePrimitive(s)
Solution 7 - Scala
There's nothing like this not only in Scala, but even in basic Java.
Here's a piece code that does it without exceptions, though:
def parseDouble(s: String)(implicit nf: NumberFormat) = {
val pp = new ParsePosition(0)
val d = nf.parse(s, pp)
if (pp.getErrorIndex == -1) Some(d.doubleValue) else None
}
Usage:
implicit val formatter = NumberFormat.getInstance(Locale.ENGLISH)
Console println parseDouble("184.33")
Console println parseDouble("hello, world")
Solution 8 - Scala
I'd usually go with an "in place" Try:
def strTimesTen (s: String) = for (d <- Try(s.toDouble)) yield d * 10
strTimesTen("0.1") match {
Success(d) => println( s"It is $d" )
Failure(ex) => println( "I've asked for a number!" )
}
Note, that you can do further calculation in the for and any exception would project into a Failure(ex). AFAIK this is the idiomatic way of handling a sequence of unreliable operations.