Added selected book problem solutions

Author: yangsu

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

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+# Ruby Self-Study
+
+## Day 1
+
+### Find
+
+  * [The Ruby API][Ruby API]
+  * The free online version of [_Programming Ruby: The Pragmatic Programmer's Guide_][Programming Ruby]
+  * [A method][gsub] that substitutes part of a string
+
+      ```ruby
+      >> 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][Regex]
+  * [Ruby's ranges][Range]
+
+### Do
+
+Print the string "Hello, World"
+```ruby
+puts "Hello, world."
+```
+
+For the string “Hello, Ruby,” find the index of the word “Ruby.”
+```ruby
+'Hello, Ruby' =~ /Ruby/     # Using regular expression matching
+'Hello, Ruby'.index('Ruby') # Using the index method
+```
+
+Print your name 10 times
+```ruby
+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.
+```ruby
+(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
+```bash
+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.
+
+```ruby
+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?
+
+    ```ruby
+    # 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_.
+    ```ruby
+    {: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.
+```ruby
+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.
+```ruby
+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**
+
+<!-- links -->
+
+[Programming Ruby]: http://www.ruby-doc.org/docs/ProgrammingRuby/
+[Range]: http://www.ruby-doc.org/core-1.9.3/Range.html
+[Regex]: http://www.ruby-doc.org/core-1.9.3/Regexp.html
+[Ruby API]: http://www.ruby-doc.org
+[gsub]: http://www.ruby-doc.org/core-1.9.3/String.html#gsub-method