[KTLN-476] Converting a List of Maps to a Map Grouped by Key in Kotlin (#797)

* Added unit test

* fixed code

* fixed code

* Formatting tweaks

---------

Co-authored-by: Brandon Ward <[email protected]>

Author: albertache1998

core-kotlin-modules/core-kotlin-collections-map/src/test/kotlin/com/baeldung/listOfMapsToMaps/ListOfMapsToMaps.kt

@@ -0,0 +1,56 @@
+package com.baeldung.listOfMapsToMaps
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+class ListOfMapsToMaps {
+
+    val listOfMaps = listOf(
+        mapOf("name" to "Albert", "age" to "18"),
+        mapOf("name" to "Naomi", "age" to "26"),
+        mapOf("name" to "Dru", "age" to "18"),
+        mapOf("name" to "Steve", "age" to "30")
+    )
+    val expectedMap = mapOf(
+        "name" to listOf("Albert", "Naomi", "Dru", "Steve"),
+        "age" to listOf("18", "26", "18", "30")
+    )
+
+    @Test
+    fun `converts list of maps to maps grouped by key using for loop`() {
+        assertEquals(expectedMap, groupByUsingForLoop(listOfMaps))
+    }
+
+    @Test
+    fun `converts list of maps to maps grouped by key using groupBy method`() {
+        val result = listOfMaps
+            .flatMap { map -> map.entries }
+            .groupBy({ it.key }, { it.value })
+
+        assertEquals(expectedMap, result)
+    }
+
+    @Test
+    fun `converts list of maps to maps grouped by key using fold method`() {
+        assertEquals(expectedMap, groupByUsingFoldMethod(listOfMaps))
+    }
+}
+
+fun groupByUsingForLoop(input: List<Map<String, String>>): Map<String, List<String>> {
+    val result = mutableMapOf<String, MutableList<String>>()
+    for (map in input) {
+        for ((key, value) in map) {
+            result.getOrPut(key) { mutableListOf() }.add(value)
+        }
+    }
+    return result
+}
+fun groupByUsingFoldMethod(input: List<Map<String, String>>): Map<String, List<String>> {
+    return input.fold(emptyMap()) { acc, map ->
+        acc.keys.union(map.keys).associateWith { key ->
+            acc.getOrDefault(key, emptyList()) + map.getOrDefault(key, "")
+        }
+    }
+}
+
+