2013-01-22-book-problems-solution.md

Ruby Self-Study

Day 1

Find

• The Ruby API

• The free online version of Programming Ruby: The Pragmatic Programmer's Guide

• A method that substitutes part of a string

  >> foo = 'The quick brown fox jumps over the lazy dog.'
  >> foo['The'] = 'A'             # Replace first occurrence of "The" in string foo
  >> foo.sub('quick', 'fast')     # Returns copy of foo with "quick" replaced with "fast"
  >> foo.sub!(/quick/, 'speedy')  # Replaces substring "quick" with "speedy" in place (we use a regex pattern just for the sake of demonstration)
  >> foo.gsub!(/a/i, 'the')       # Case-insensitive, replace all occurrences of "a" with "the" in-place

• Ruby's regular expressions

• Ruby's ranges

Do

Print the string "Hello, World"

puts "Hello, world."

For the string “Hello, Ruby,” find the index of the word “Ruby.”

'Hello, Ruby' =~ /Ruby/     # Using regular expression matching
'Hello, Ruby'.index('Ruby') # Using the index method

Print your name 10 times

puts "Kevin Gao\n" * 10
10.times { puts "Yang Su" }

Print the string "This is sentence number 1," where the number 1 changes from 1 to 10.

(1..10).each do |num|                     # Using a range
  puts "This is sentence number #{num}"
end

1.upto(10) do |num|                       # Using an enumerator
  puts "This is sentence number #{num}"
end

Run a Ruby program from a file

ruby 01-randnumgame.rb

Bonus problem: write a program that picks a random number. Let a player guess the number, telling the player whether the guess is too low or too high.

def getRandAnswer(max = 10)
  rand(max) + 1
end

STDOUT.sync

puts "Welcome to the random number game!"
puts "\tTo quit, answer 'q' at any time"

guess = nil
max = 10
answer = getRandAnswer(10)

while true
  puts "Guess a number from 1 to #{max}:"
  guess = gets.chomp
  break if guess == 'q'

  gNum = guess.to_i
  if gNum == answer
    puts "Correct!"
    answer = getRandAnswer(10)
  elsif gNum < answer
    puts "Too low"
  else
    puts "Too high"
  end
end

Day-2: Self-Study

Find

• Find out how to access files with and without code blocks. What is the benefit of the code block?

  # With code blocks
  File.open('foo.bar', 'w') { |f| f << 'bazzbuzz' }
  
  # Without code blocks
  file = File.open('foo.bar', 'w')
  file << 'bazzbuzz'
  file.close

  The benefit of using a code block is that it wraps resource handling policies around the block, rather than the coder using the API having to deal with it. Also, it's just pretty.

• How would you translate a hash to an array? Hash.values. Can you translate arrays to hashes? Hash[*array].

• Can you iterate through a hash? Yes.

  {:array=>[1, 2, 3], :string=>"Hello"}.each {|k, v| puts "k: #{k}, v: #{v}"}
  # => k: array, v: [1, 2, 3]
  # => k: string, v: Hello

• You can use Ruby arrays as stacks. What other common data structures do arrays support? Queues, Sets

Do

Print the contents of an array of sixteen numbers, four numbers at a time, using just each. Now, do the same with each_slice in Enumerable.

a = (1..16).to_a
b = []
a.each do |x|
  b.push x
  if b.length == 4
    puts b.join(',')
    b.clear
  end
end

a = (1..16).to_a
a.each_slice(4) { |xs| puts xs.join(',') }

The solution to the Tree problem will be released after the assignment is due

Write a simple grep that will print the lines of a file having any occurrences of a phrase anywhere in that line. You will need to do a simple regular expression match and read lines from a file. (This is surprisingly simple in Ruby.) If you want, include line numbers.

def grep(filename, phrase)
  File.open(filename, 'r').each_with_index do |line, lineNumber|
    puts "#{lineNumber}: #{line}" if line =~ /#{phrase}/
  end
end

Day 3

The solution to the CSV problem will be released after the assignment is due