Test-Driven Development

Do all of the following exercises using Test-Driven Development (TDD). That means writing specs first!

When approaching each problem, make sure to

1. Read the entire problem statement.
2. Talk with your partner to identify test cases and key functionality to expect from your code.
3. Write RSpec tests.
4. Write the method you now have tests for. Follow the red-green-refactor approach until all specs pass.

Expected time: 2 hrs

Learning goals

By the end of this practice, you should

• Be able to explain what functionality your testing will cover
• Know the hierarchy / syntax of RSpec methods (describe, before, let, it, expect, etc.)
• Be comfortable writing RSpec tests
• Know when to use let and subject

uniq

Array has a uniq method that removes duplicates from an array. It returns the unique elements in the order in which they first appeared:

[1, 2, 1, 3, 3].uniq # => [1, 2, 3]

Write your own version of this method called my_uniq; it should take in an array and return a new array.

two_sum

Write a new Array#two_sum method that finds all pairs of positions where the elements at those positions sum to zero.

N.B.: Ordering matters. You want each of the pairs to be sorted with the smaller index before the bigger index. You then want the array of pairs to be sorted "dictionary-wise":

[-1, 0, 2, -2, 1].two_sum # => [[0, 4], [2, 3]]

• [0, 2] before [2, 1] (smaller first elements come first)
• [0, 1] before [0, 2] (then smaller second elements come first)

my_transpose

To represent a matrix, or two-dimensional grid of numbers, you can write an array containing arrays which represent rows:

rows = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8]
]

row1 = rows[0]
row2 = rows[1]
row3 = rows[2]

You could equivalently have stored the matrix as an array of columns:

cols = [
  [0, 3, 6],
  [1, 4, 7],
  [2, 5, 8]
]

Write a method, my_transpose, which will convert between the row-oriented and column-oriented representations. You may assume square matrices for simplicity's sake. Usage will look like the following:

my_transpose([
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8]
])
 # => [[0, 3, 6],
 #     [1, 4, 7],
 #     [2, 5, 8]]

Stock Picker

Write a method that takes an array of stock prices (prices on days 0, 1, ...), and outputs the most profitable pair of days on which to first buy the stock and then sell the stock. Remember, you can't sell stock before you buy it!

Towers of Hanoi

Write a Towers of Hanoi game.

Keep three arrays, which represent the piles of discs. Pick a representation of the discs to store in the arrays, maybe just a number representing their size. Don't worry too much about making the user interface pretty.

In a loop, prompt the user (using gets). Ask what pile to select a disc from and where to put it.

After each move, check to see if they have succeeded in moving all the discs to the final pile. If so, they win!

Note: Don't worry about testing the UI. Testing console I/O is tricky (i.e., don't bother checking gets or puts). Focus on:

• #move
• #won?

When you're finished, get a code review from an Instructor!

Resources

• Ruby Doc on Array
• Ruby Array Article