Question

我往往想在发言时产生范围很广的变量。有些计算仅与某一具体数据有关,如果说的话——用临时变数 bad坏对外部范围进行污染。

What I would like to do:

val data = (whatever)

if (val x = data*2+5.4345/2.45; val y = data/128.4; x*y < 10) 
  x * y
else
  x * 2

println(x) //ERROR!

一种替代办法是:

val data = (whatever)

if (data*2+5.4345/2.45*data/128.4 < 10) 
  data*2+5.4345/2.45*data/128.4
else
  data*2+5.4345/2.45 * 2

The obvious alternative I m trying to avoid:

val data = (whatever)
val x = data*2+5.4345/2.45
val y = data/128.4

if (x*y < 10) 
  x*y
else
  x * 2

println(x) //OK

Is something like this possible in Scala? Is there a decent workaround? If not, what other languages support an idea like this?

Answer 1

由于if in Scala is an expression, i.e. itsam a Value, 通常由您对if/code>的表述确定某种价值。因此,你的第三种选择只是罚款:把它放在法典的栏目中,即:



val data = (whatever)
val myValue = {
  val x = data*2+5.4345/2.45
  val y = data/128.4

  if (x*y < 10) 
    x*y
  else
    x * 2
}


None of the vals declared within the block are available outside it.

Answer 2


You can use a pattern match:

val data = 123

val (result, x) = (data*2+5.4345/2.45, data/128.4) match {
  case (x, y) if x * y < 10 => (x * y, x)
  case (x, _)               => (x * 2, x)
}

println(x)

包括<代码>x* y或x* 2,视其计算结果而定;<代码>x 载有按要求分列的<编码>*2+5.4345/2.45数值。

Answer 3

你们能够为它创造空间。

{
  val x = data*2+5.4345/2.45
  val y = data/128.4;
  if ( x*y < 10) 
    x * y
  else
    x * 2
}

或更清楚,

locally {
  val x = data*2+5.4345/2.45
  val y = data/128.4;
  if ( x*y < 10) 
    x * y
  else
    x * 2
}

友情链接