improved control structure usage

.idea/misc.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_10" default="true" project-jdk-name="10" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_11" default="true" project-jdk-name="11" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>

res/sample.grades

@@ -24,7 +24,7 @@ programming languages: 89
 differential equations: 59
 differential equations: 97
 software architecture: 56
-parallel and distributed computing: 89
+parallel and distributed computing:
 parallel and distributed computing: 66
 differential equations: 58
 software architecture: 98

src/three_feature.kt

@@ -1,51 +1,64 @@
 import java.io.File
 
 
+class ReportCard {
+    // Maps and Lists are, by default, immutable.
+    // Specify our data structure to be mutable at both levels
+    private val courses = mutableMapOf<String, MutableList<Int>>()
+
+    // use getOrPut to get a list of course grades, and add the new grade to that list
+    fun addAssignment(course: String, grade: Int) = courses.getOrPut(course) { mutableListOf() }.add(grade)
+
+    fun grades(func: (String, String, Double) -> Unit) {
+        // 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 > 60 -> "D"
+                else -> "F"
+            }
+
+            // call back to the block given
+            func(course, letter, average)
+        }
+    }
+}
+
+// Create an extension method on `Double` that allows us to specify precision
+fun Double.format(digits: Int) = java.lang.String.format("%.${digits}f", this)
+
 fun main(args: Array<String>) {
     // File I/O is easily done with `File()` constructor
     val file = File("res/sample.grades")
 
-    // Maps and Lists are, by default, immutable.
+    val report = ReportCard()
-    // Specify our data structure to be mutable at both levels
-    val courses = mutableMapOf<String, MutableList<Int>>()
 
-    // File supports line iteration with forEachLine, which accepts a code block.
+    // `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 (rawCourse, rawGrade) = it.split(":").let {
+            Pair(it[0], it.getOrElse(1) { _ -> "" })
+        }
 
         // use `trim()` to strip whitespace from either side of the strings
         // string conversion is easy with helper methods like `toInt()` and `toIntOrNull()`
         val course = rawCourse.trim()
-        val grade = rawGrade.trim().toInt()
+        val grade = rawGrade.trim().toIntOrNull()
-
+        grade?.let { report.addAssignment(course, it) }
-        // 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)
     }
 
-
+    // input and output from console are as easy as `println()` and `readline()`
-    // Iteration over maps is easy when combined with iterable unpacking
+    // uses string interpolation for easier formatting
-    for ((course, grades) in courses) {
+    // grade precision is specified with the `format` extension method defined above
-        val average = grades.average()  // List<Int> has helper methods such as `sum()` and `average()`
+    report.grades { course: String, letter: String, average: Double ->
-
+        println("$course:\n\t$letter (${average.format(2)})\n")
-        // use a `when` block as an alternative to case. Supports rich pattern matching,
-        // but we just use it here to check which grade range we're in
-        val letter = when {
-            average > 90 -> "A"
-            average > 80 -> "B"
-            average > 70 -> "C"
-            average > 70 -> "D"
-            else -> "F"
-        }
-
-        // input and output from console are as easy as `println()` and `readline()`
-        // uses string interpolation for easier formatting
-        println("$course:\n\t$letter ($average)\n")
     }
 }