Find out frequency of certain integers appearing in an array in Scala - arrays

Question

Say I have two arrays. One array A of a set of integers - all distinct. Another array B of a list of integers, all appearing in array A, but not necessarily distinct. For example:
A could be Array(123, 456, 789)
B could be Array(123, 123, 456, 123, 789, 456)
I want to create an array C, which tells us the frequency of each element (from array A) appearing in array B. In this case, C would be Array(3, 2, 1) because 123 appears 3 times, 456 appears 2 times, and 789 appears 1 time.
What is an efficient way to do this in Scala?
My attempt is
val C: Array[Int] = Array.fill(3)(0)
var idx = 0
for(i <- A){for(j <- B){if(j == i){C(idx) += 1}}
idx += 1}
for(i <- C){println(i)}
But I understand that this is probably inefficient, and would take a long time if I am dealing with a much larger array A and array B. But I am restricted to for loops and if statements since I am only a beginner with Scala. Is there a more efficient way to do this?

Answer 1

Lets say that n is length of Array A and m is length of array B.
As of now your solution is O(n * m)
You can improve this to O(n + m) by using a mutable HashMap and O(n) extra space.
import scala.collection.mutable
val a = Array(123, 456, 789)
val b = Array(123, 123, 456, 123, 789, 456)
val countMap = mutable.HashMap.empty[Int, Int]
// add all integers in `a` with count 0
for (i <- a) {
countMap.put(i, 0)
}
// iterate on b
// and update the count in countMap (if exists)
for (i <- b) {
countMap.get(i).foreach(c => countMap.put(i, c + 1))
}
// fill your array `c`
val c = Array.ofDim[Int](a.length)
for ((i, index) <- a.zipWithIndex) {
c(index) = countMap.getOrElse(i, 0)
}
println(c.mkString(", "))
// 3, 2, 1
Keep in mind that for's for Scala collections have their own costs, you can improve it further by using while loops.
import scala.collection.mutable
val a = Array(123, 456, 789)
val b = Array(123, 123, 456, 123, 789, 456)
val countMap = mutable.HashMap.empty[Int, Int]
// to use with our while loops
var i = 0
// add all integers in `a` with count 0
i = 0
while (i < a.length) {
countMap.put(a(i), 0)
i = i + 1
}
// iterate on b
// and update the count in countMap (if exists)
i = 0
while (i < b.length) {
if (countMap.contains(b(i))) {
countMap.put(b(i), countMap(b(i)) + 1)
}
i = i + 1
}
// fill your array `c`
val c = Array.ofDim[Int](a.length)
i = 0
while (i < a.length) {
c(i) = countMap.getOrElse(a(i), 0)
i = i + 1
}
println(c.mkString(", "))
// 3, 2, 1