Question

我必须自动比较C程序的文本输出和转换为Scala。不幸的是，我找不到以同样的方式格式化double值的方法。在C中我有例如：

double diff = -1.0;  // just an example
fprintf(stdout, "diff %g\n", diff);

这写成

diff -1

（无小数点）。

在Scala我有

val diff = -1.0;  // just an example
println(f"diff $diff%g\n")

这写成

diff -1.00000

换句话说，%g格式化程序不会这样做。我正在寻找at this document，但无法获得100％兼容的fprintf格式。

修改：Without using JNI or writing page long special methods.

Answer 1

Scala使用记录的here Java Formatter类。得到你想要的：

scala> "%.0g".format(-1.0)
res0: String = -1

Answer 2

好的，@ {john-douthat this is an identical question great answer，需要进行一些小调整：

def ¬(x: Double) = {
  val obj = x.toFloat.asInstanceOf[AnyRef]
  String.format("%.6g", obj).replaceFirst("\\.?0+(e|$)", "$1")
}

val diff = -1.0
println(s"diff ${¬(diff)}")
println(s"Pi ${¬(math.Pi)}")

输出：

diff -1
Pi 3.14159

但仍有一些问题。例如，-3.59847e-10输出为-3.59847e-1。这是replaceFirst的正则表达式中的错误。如果有人能比这更好地解决它，我会非常高兴（正则表达式与我有精神错配）：

def ¬(x: Double) = {
  val obj = x.toFloat.asInstanceOf[AnyRef]
  val s0  = String.format("%.6g", obj)
  val i0  = s0.lastIndexOf('e')
  val i   = if (i0 < 0) s0.length else i0
  s0.substring(0, i).replaceFirst("\\.?0+($)", "$1") + s0.substring(i)
}

Answer 3

检查

  val diff = -1.0               //> diff  : Double = -1.0
  "diff %g\n".format(diff)      //> res6: String = "diff -1.00000
                                //| "

C fprintf％g Scala / Java中的对应关系

3 个答案: