Modifying an array of tuples in Scala

Question

I am new to Scala and would appreciate any help regarding the following code:

var exp = (-3).to(2, 1) 
var out = exp.map(i => java.lang.Float.floatToIntBits(math.pow(i, 2).toFloat))

Now, I want to write the following code:

for (i <- 0 until exp.length)
{if(exp(i) < 0) 
    {out(i) = out(i) >> exp(i).abs}
}

that is, I want to modify elements of the out vector depending on the elements of the exp vector by having a one-to-one mapping between the two vectors. I can't find a single online source which can help me do this. Please help.

Shadowlands · Accepted Answer · 2015-12-02 09:07:47Z

1

The following should work:

out.zip(exp) map { case (o,e) => if (e < 0) o >> e.abs else o }

answered Dec 2, 2015 at 9:07

Shadowlands

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Comments

Alexandr Dorokhin · Accepted Answer · 2015-12-02 09:19:03Z

1

The vector is immutable collection, you can't modify it, but you can create new one. For this, use for yield comprehension:

val res = for (i <- 0 until exp.length)
  yield if(exp(i) < 0)
          out(i) >> exp(i).abs
        else
          out(i)

or just convert your vector to an array:

val out = exp.map(i => 
  java.lang.Float.floatToIntBits(math.pow(i, 2).toFloat)
).toArray

and modify your array.

answered Dec 2, 2015 at 9:19

Alexandr Dorokhin

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elm · Accepted Answer · 2015-12-02 10:40:20Z

0

The extra iteration can be avoided by adding the shifting to the lambda function either by exp(i) if negative or by 0 (no effect) otherwise, like this,

var out = exp.map { i => 
  val f = java.lang.Float.floatToIntBits(math.pow(i, 2).toFloat) 
  f >> (if (i < 0) -i else 0)
}

Note also

def step(i: Int) = if (i < 0) -i else 0

so that we can simplify the solution above as follows,

var out = exp.map { i => 
  java.lang.Float.floatToIntBits(math.pow(i, 2).toFloat) >> step(i)
}

answered Dec 2, 2015 at 10:40

elm

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Gavin · Accepted Answer · 2015-12-02 09:20:30Z

0

Or make the transformation explicit and arguably more readable by putting it in a function then just having a single map operation.

def fn(i: Int): Int = {
  val o = java.lang.Float.floatToIntBits(math.pow(i, 2).toFloat)
  if (i < 0) o >> i.abs
  else o
}

exp.map(fn)

answered Dec 2, 2015 at 9:20

Gavin

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