String formatting in scala - maximum decimal precision

Question

"%.3f".format(1) returns 1.000.
"%.3f".format(4.0/3.0) returns 1.333.

Is there some easy way to have these return 1 and 1.333? I thought the standard printf format specified that precision as the maximum already, but apparently not in Scala.

Answer 1

The default formatter used by printf seems to be a generic one that doesn't have all the same support than [DecimalFormat][1]. You can instantiate a custom formatter along those lines:

scala> import java.text.DecimalFormat 
import java.text.DecimalFormat

scala> val formatter = new DecimalFormat("#.###")
formatter: java.text.DecimalFormat = java.text.DecimalFormat@674dc

scala> formatter.format(1)
res36: java.lang.String = 1

scala> formatter.format(1.34)
res37: java.lang.String = 1.34

scala> formatter.format(4.toFloat / 3)
res38: java.lang.String = 1.333

scala> formatter.format(1.toFloat)
res39: java.lang.String = 1

See: http://docs.oracle.com/javase/tutorial/java/data/numberformat.html for more information.

Answer 2

"%.3f".format(1) will throw an java.util.IllegalFormatConversionException because of the wrong type (Float is expected and you give a Int).

Even if you use "%.3f".format(1.0), you will get 1.000.

You can use a method like the following to obtain the expected result :

def format(x:AnyVal):String = x match {
  case x:Int => "%d".format(x)
  case x:Long => "%d".format(x)
  case x:Float => "%.3f".format(x)
  case x:Double => "%.3f".format(x)
  case _ => ""
}

This method will return the expected format based on argument type.