Solution to a simple Hackerrank problem: Compare the Triplets

Question

Given two equally sized arrays of integers A and B, compute the number of times A[i] > B[i] and the number of times A[i] < B[i] for each index i.

New to scala, I put together the following solution:

import scala.io.Source
object Solution {
 def main(args: Array[String]) {
 val lines = Source.stdin.getLines
 val alice = lines.next.split(" ").map(_.toInt)
 val bob = lines.next.split(" ").map(_.toInt)
 val competitions = alice zip bob
 val scoreA = competitions.count(c => c._1 > c._2)
 val scoreB = competitions.count(c => c._1 < c._2)
 printf("%d %d", scoreA, scoreB)
 }
}

I normally solve these problems using python, so I simply searched online for my usual programming constructs. I have a few questions.

• Is there a more general way to print a list of space separated integers? Currently my use of printf is limited to exactly two.
• Are there any pitfalls I'm unaware of?
• Ideas of more concise solutions without sacrificing readability?

Any other insights would be greatly appreciated!

Answer 1

You don't have to traverse the competitions array twice. The results can be collected in a tuple result after a single traversal.

val results = competitions.foldLeft((0,0)){
 case ((aw,bw),(a,b)) => 
 if (a>b) (aw+1,bw) //a won
 else if (b>a) (aw,bw+1) //b won
 else (aw,bw) //tie
 }

The tuple's elements can be accessed via indexing, results._1 and results._2, but it can be more convenient to unpack it via pattern matching.

val (scoreA, scoreB) = results

So, putting it all together, and removing the competitions step (just because you can).

val (aWins
 ,bWins) = alice.zip(bob)
 .foldLeft((0,0)){
 case ((aw,bw),(a,b)) =>
 if (a>b) (aw+1,bw)
 else if (b>a) (aw,bw+1)
 else (aw,bw)
 }

Sending the results to STDOUT is usually done via println(), either with string catenation...

println(aWins + " " + bWins)

...or with string interpolation.

println(s"$aWins $bWins")

• Martin R: I am not a Scala expert, but it seems to me that your code handles the case A[i] == B[i]differently from the original code in the question.
• jwvh: @MartinR; Good catch. While trying to demonstrate my major point on code efficiency I sort of glossed over the rules for a tie. A simple if else addition fixes that.