How to apply filter on list of _2 of tuples in Scala?

Question

I want all the lists(the first element of below tuple) if second element is the powers of 2. I am new to Scala and stuck with syntax. Could anyone help me here I want to apply filter on below code to get below output.

Output:

List(1,3)
List(1,7)
List(3,5)
List(7,9)

input:

scala> a.toList.combinations(2).map(x => (x, x.sum)).foreach(println)
(List(1, 3),4)
(List(1, 5),6)
(List(1, 7),8)
(List(1, 9),10)
(List(3, 5),8)
(List(3, 7),10)
(List(3, 9),12)
(List(5, 7),12)
(List(5, 9),14)
(List(7, 9),16)

Answer 1

val tups = List((List(1, 3),4), (List(1, 5),6), (List(1, 7),8) , 
                (List(1, 9),10), (List(3, 5),8), (List(3, 7),10), 
                (List(3, 9),12), (List(5, 7),12),
                (List(5, 9),14), (List(7, 9),16))

Using fold :

tups.foldLeft(List[List[Int]]())((a, b) => if((b._2 & (b._2 - 1)) == 0) a :+ b._1 else a)

Using filter and map :

tups.filter(e => (e._2 & (e._2 - 1)) == 0).map(_._1)

x & (x - 1) == 0 is the bitwise operation that checks if a number is a power of 2

For a more detailed explanation of the syntax, documentation for fold and other List operations can be found here

Answer 2

For

val xs = List(1,3,5,7,9)

another approach that incorporates the generation of input and corresponding filtering includes

import scala.math.log

// testPower true if log is integral value
// log_base n = log_e n / log_e base

def testPower(n: Int, base: Int = 2): Boolean = {
  val lg = log(n) / log(base)
  lg.toInt == lg 
}

xs.combinations(2).filter(ys => testPower(ys.sum)).toList

The test provided in @sinanspd answer is more succinct as well as more efficient; the test here generalises to any power in addition to 2. This solution works for any combination size as it sums up the entire combination (sub)list.

Answer 3

You can use the fact that the binary representation of an element that is the power of 2, has only 1 bit which is 1, and all. others are 0's. Then use toBinaryString method:

val xs = List(1, 3, 5, 7, 9).combinations(2).map(x => (x, x.sum)).toList
val result = xs.filter(_._2.toBinaryString.count(_ == '1') == 1).map(_._1)

Code run at Scastie .