improve comments and readme

README.md

@@ -10,3 +10,20 @@ Create a program (or programs) which show how the language you picked (Kotlin) w
 
 ## Report Card Generator
 
+Accepts a file `sample.grades` where each line is an assignment in the format: `<course name>: <grade>`. The program iterates through the file and computes the average and letter grades for all courses listed in `sample.grades`.
+
+Language structures used include:
+
+* Iterable unpacking
+* Implicit type arguments
+* Null safety and propagation (as in `courses[course]!!.add(grade)`)
+* Iteration
+* Actions (as in `file.forEachLine`)
+* Pattern matching (simple example using `when` expression)
+
+Kotlin library features used include:
+
+* File I/O
+* Standard I/O
+* String conversions
+* Mutable maps and lists

src/three_feature.kt

@@ -16,8 +16,10 @@ fun main(args: Array<String>) {
         // This is very brittle, though, if improperly formatted
         val (rawCourse, rawGrade) = it.split(":")
 
+        // 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()  // string conversion is easy with helper methods like `toInt()` and `toIntOrNull()`
+        val grade = rawGrade.trim().toInt()
 
         // collection testing is easy with `in` and `!in` operators
         if (course !in courses)