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Ruby Intro

This 3-part homework gives some basic practice in Ruby as well as getting you accustomed to making testing a regular part of your workflow.

It follows a fairly standard Ruby convention for codebases: the code files are stored in lib/ and the test files are stored in spec/. (We use the RSpec unit-testing framework; if we were using Ruby's default framework, known as Test::Unit, the test files would be under test/.)

We've placed "starter code" in lib/ruby_intro.rb; when you're all done, you can submit this single file to the autograder.

However, you can test each of the 3 parts separately. The files spec/part[123]_spec.rb contain RSpec tests for each of the three parts. For example, to test your answers to Part 1, say rspec spec/part1_spec.rb. rspec with no arguments runs the tests in all the files spec/*_spec.rb.

  • The line numbers in the RSpec error report will give you guidance as to which tests failed. (You can check the RSpec documentation to see how the .rspec file can be used to customize the output format.)

  • If you want to be really cool, run autotest. This will run all the tests in spec/, but every time you edit and save your code file, the tests are automatically re-run, so you don't have to run them manually. As we'll see later, this is the first step to TDD or test-driven development: write the tests before you write the code, watch the test fail, fill in the code and save the code file, then watch the test pass!

1. Arrays, Hashes, and Enumerables

Check the Ruby 2.x documentation on Array, Hash and Enumerable as they could help tremendously with these exercises. :-)

  1. Define a method sum(array) that takes an array of integers as an argument and returns the sum of its elements. For an empty array it should return zero.

  2. Define a method max_2_sum(array) which takes an array of integers as an argument and returns the sum of its two largest elements. For an empty array it should return zero. For an array with just one element, it should return that element.

  3. Define a method sum_to_n?(array, n) that takes an array of integers and an additional integer, n, as arguments and returns true if any two elements in the array of integers sum to n. sum_to_n?([], n) should return false for any value of n, by definition.

You can check your progress by running rspec spec/part1_spec.rb, or just running autotest and leaving it running.

2. Strings and Regular Expressions

Check the documentation on String and Regexp as they could help tremendously with these exercises. :-)

  1. Define a method hello(name) that takes a string representing a name and returns the string "Hello, " concatenated with the name.

  2. Define a method starts_with_consonant?(s) that takes a string and returns true if it starts with a consonant and false otherwise. (For our purposes, a consonant is any letter other than A, E, I, O, U.) NOTE: be sure it works for both upper and lower case and for nonletters!

  3. Define a method binary_multiple_of_4?(s) that takes a string and returns true if the string represents a binary number that is a multiple of 4. NOTE: be sure it returns false if the string is not a valid binary number!

3. Object Oriented Basics

Define a class BookInStock which represents a book with an ISBN number, isbn, and price of the book as a floating-point number, price, as attributes.

The constructor should accept the ISBN number (a string, since in real life ISBN numbers can begin with zero and can include hyphens) as the first argument and price as second argument, and should raise ArgumentError (one of Ruby's built-in exception types) if the ISBN number is the empty string or if the price is less than or equal to zero. Include the proper getters and setters for these attributes.

Include a method price_as_string that returns the price of the book formatted with a leading dollar sign and two decimal places, that is, a price of 20 should format as "$20.00" and a price of 33.8 should format as "$33.80".