complete grade calcuator

src/three_feature.kt

@@ -1,11 +1,48 @@
-import java.util.*
+import java.io.File
+
 
 fun main(args: Array<String>) {
-    val in1 = readLine() ?: ""
+    // File I/O is easily done with `File()` constructor
+    val file = File("res/sample.grades")
 
-    val reader = Scanner(System.`in`)
-    val in2 = reader.nextInt()
+    // Maps and Lists are, by default, immutable.
+    // Specify our data structure to be mutable at both levels
+    val courses = mutableMapOf<String, MutableList<Int>>()
 
-    println(in1)
-    print(in2)
+    // File supports line iteration with forEachLine, which accepts a code block.
+    // `it` is the iteration variable in the code block
+    file.forEachLine {
+        // It is possible to unpack iterables into multiple variables
+        // This is very brittle, though, if improperly formatted
+        val (rawCourse, rawGrade) = it.split(":")
+
+        val course = rawCourse.trim()
+        val grade = rawGrade.trim().toInt()  // string conversion is easy with helper methods like `toInt()` and `toIntOrNull()`
+
+        // collection testing is easy with `in` and `!in` operators
+        if (course !in courses)
+            courses[course] = mutableListOf()  // implicit type arguments are passed: `mutableListOf<Int>()`
+
+        // use `!!` to assert the hash lookup will not be null. We just added it to the map.
+        courses[course]!!.add(grade)
+    }
+
+
+    // Iteration over maps is easy when combined with iterable unpacking
+    for ((course, grades) in courses) {
+        val average = grades.average()  // List<Int> has helper methods such as `sum()` and `average()`
+
+        // use a `when` block as an alternative to case. Supports rich pattern matching,
+        // but we just use it here to check which grade range we're in
+        val letter = when {
+            average > 90 -> "A"
+            average > 80 -> "B"
+            average > 70 -> "C"
+            average > 70 -> "D"
+            else -> "F"
+        }
+
+        // input and output from console are as easy as `println()` and `readline()`
+        println("$course:\n\t$letter ($average)\n")
+    }
 }