add code for difference between reduce, scan and fold article (#772)

Author: usman-mohyuddin

scala-core-8/src/test/scala/com/baeldung/scala/highorderfunctions/HighOrderFunctionsUnitTest.scala

@@ -0,0 +1,77 @@
+package com.baeldung.scala.highorderfunctions
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class HighOrderFunctionsUnitTest extends AnyFlatSpec with Matchers {
+
+  "reduceLeft" should "calculate the sum correctly" in {
+    val numbers = List(1, 2, 3, 4, 5)
+    val actualSum = numbers.reduceLeft(_ + _)
+    // op: 1 + 2 = 3
+    // op: 3 + 3 = 6
+    // op: 6 + 4 = 10
+    // op: 10 + 5 = 15
+    // res: Int = 15
+    assert(15 == actualSum)
+  }
+
+  "reduceLeft" should "concatenate strings correctly" in {
+    val alphabets = List("A", "B", "C", "D", "E")
+    val actualResult = alphabets.reduceLeft(_ + _)
+    assert("ABCDE" == actualResult)
+  }
+
+  "reduceLeft" should "find the largest number correctly" in {
+    val numbers = List(10, 22, 5, 71, 43)
+    val actualResult = numbers.reduceLeft(_ max _)
+    assert(71 == actualResult)
+  }
+
+  "reduceRight" should "calculate the sum correctly" in {
+    val numbers = List(1, 2, 3, 4, 5)
+    val actualSum = numbers.reduceRight(_ + _)
+    // op: 5 + 4 = 9
+    // op: 9 + 3 = 12
+    // op: 12 + 2 = 14
+    // op: 14 + 1 = 15
+    // res: Int = 15
+    assert(15 == actualSum)
+  }
+
+  "foldLeft" should "calculate the sum correctly" in {
+    val numbers = List(1, 2, 3, 4, 5)
+    val actualSum = numbers.foldLeft(5)(_ + _)
+    // op: 5 + 1 = 6
+    // op: 6 + 2 = 8
+    // op: 8 + 3 = 11
+    // op: 11 + 4 = 15
+    // op: 15 + 5 = 20
+    // res: Int = 20
+    assert(20 == actualSum)
+  }
+
+  "foldRight" should "concatenate the strings correctly" in {
+    val alphabets = List("A", "B", "C", "D", "E")
+    val actualResult = alphabets.foldRight("$")(_ + _)
+    // op: E + $ = E$
+    // op: D + E$ = DE$
+    // op: C + DE$ = CDE$
+    // op: B + CDE$ = BCDE$
+    // op: A + BCDE$ = ABCDE$
+    // res: String = ABCDE$
+    assert("ABCDE$" == actualResult)
+  }
+
+  "scanLeft" should "have correct intermediate states" in {
+    val numbers = List(1, 2, 3, 4, 5)
+    val actualResult = numbers.scanLeft(1)(_ + _)
+    assert(List(1, 2, 4, 7, 11, 16) == actualResult)
+  }
+
+  "scanRight" should "have correct intermediate states" in {
+    val numbers = List(1, 2, 3, 4, 5)
+    val actualResult = numbers.scanRight(1)(_ + _)
+    assert(List(16, 15, 13, 10, 6, 1) == actualResult)
+  }
+}