========================
Working With Collections
========================


Working With Lists
==================

.. highlight:: none

#. In a Kotlin or Kotlinscript file, write some code that creates a small
   list contains between 5 and 10 integer values.

#. Add code that will

   * Print out the contents of the list (without using a loop)
   * Print a count of how many elements are in the list (without using a loop)
   * Display the first three and last three values
     (e.g., using the ``take`` and ``takeLast`` methods)
   * Print a reversed version of the list, leaving the original unchanged
   * Print a sorted version of the list, leaving the original unchanged
   * Filter the list, displaying only the odd numbers

  Here's an example of what program output should look like::

     List is [1, 2, 7, 4, 11, -1, 8]
     List has 7 values
     First three values are [1, 2, 7]
     Last three values are [11, -1, 8]
     Reversed list is [8, -1, 11, 4, 7, 2, 1]
     Sorted list is [-1, 1, 2, 4, 7, 8, 11]
     Odd values are [1, 7, 11, -1]


Reading From a File Into a List
===============================

#. In a new directory (or a new IntelliJ project), create a Kotlin file
   named :file:`stats.kt`.  In this file, add the following code:

   .. code-block:: kotlin

      import java.io.File
      import java.util.Scanner

      fun main(args: Array<String>) {
        if (args.size > 0) {
          val input = Scanner(File(args[0])
        }
      }

   When it runs, this program expects a filename to be supplied as a
   command line argument.  It uses this filename to construct a ``File``
   object representing the named file, then creates a ``Scanner`` object
   attached to this file.  The ``Scanner`` object can be used to parse
   the contents of the file.

   .. note::

      The important point here is that ``File`` and ``Scanner`` are part
      of Java's API.  Java classes can be used seamlessly within Kotlin
      code.

   Check that the program compiles and runs before proceeding further.

#. Add the following code to :file:`stats.kt`:

   .. code-block:: kotlin

      val data = mutableListOf<Double>()
      while (input.hasNextDouble()) {
        data.add(input.nextDouble())
      }

   This creates an empty list, then uses the ``Scanner`` to read
   ``Double`` values from the file and add them to this list.

#. Add code that displays the number of values that have been added to
   the list.

#. Add code that computes and displays the mean and `standard deviation`_
   of values in the list.  Note that computing the mean can be done with a
   single method call!

   Program output should look something like this::

      6 values read from file
      Mean value = 3.5917
      Std deviation = 1.1082


Working With Maps
=================

#. In a Kotlin or Kotlinscript file, write some code that creates a small
   map of ``String`` to ``Int``, containing a few key-value pairs.
   Your program should

   * Print out the contents of the map (without using a loop)
   * Print a count of how many key-value pairs it contains
     (without using a loop)
   * Use a for loop to sum up the values, then print the sum
   * Remove one of the key-value pairs, then print map contents again

   Here's an example of what program output should look like::

      Map is {oranges=1, bananas=7, plums=5, apples=2}
      4 key-value pairs, sum of values = 15
      Removing oranges...
      Map is now {bananas=7, plums=5, apples=2}

#. Create a new directory (or new IntelliJ project) and put
   :download:`input.kt </fundamentals/input.kt>` in it.  Add a new file,
   :file:`phonebook.kt`.  In this new file, write a program that creates and
   maintains a phonebook, using a map defined like so:

   .. code-block:: kotlin

      val phonebook = mutableMapOf<String,String>()

   Your program should use a loop to repeatedly prompt the user to enter
   a name.  If the string is empty (i.e., the user has simply pressed
   the Enter key) then the program should break out of the loop; otherwise,
   it should use the entered name to look up the corresponding phone number
   in ``phonebook``.

   If a number is found for the entered name, it should be displayed;
   otherwise, the program should print "Not found", the user should be
   prompted to supply a phone number and then the name and number should be
   inserted into the map.

   Here's an example of what interaction with the program should look like::

      Enter a name: nick
      Not found
      Enter number for nick: 0123 456789
      Enter a name: nick
      Number for nick is 0123 456789


.. _standard deviation:
   https://en.wikipedia.org/wiki/Standard_deviation#Discrete_random_variable