Add Strings chapter and Interpolation unit

Author: paulgoetze

3_Strings/1_Interpolation/solution.rb

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+# e.g.
+firstname = 'Peter'
+lastname = 'Pan'
+fullname = "#{lastname}, #{firstname}"

3_Strings/1_Interpolation/solution_spec.rb

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+require 'rspec'
+require 'rspec/expectations'
+require 'code_breaker'
+
+RSpec::Matchers.define :run_string_interpolation do |expected|
+  match do |actual|
+    interpolation = interpolation_part_of(actual)
+    return false if interpolation.nil?
+    interpolation[:lvasgn][1][:dstr] == expected
+  end
+
+  failure_message do |actual|
+    actual_line = actual.split("\n").select do |line|
+      line =~ /fullname\s?=.+/
+    end
+
+    parsed_line = CodeBreaker.parse(actual_line.join)
+    interpolation = parsed_line[:lvasgn].last
+
+    unless interpolation.is_a?(Hash) && interpolation[:dstr]
+      return "You didn't do any string interpolation."
+    end
+
+    [['solution::code']]
+
+    %Q{Your interpolated fullname shoud be "#{lastname}, #{firstname}"
+    ------- but is "#{fullname}".}
+  end
+
+  def interpolation_part_of(actual)
+    parsed = CodeBreaker.parse(actual)
+
+    interpolation = parsed.select do |part|
+      next unless part.is_a?(Hash)
+      next unless part.has_key?(:lvasgn)
+      next unless part[:lvasgn][0] == :fullname
+      next unless part[:lvasgn][1].is_a?(Hash)
+
+      part[:lvasgn][1].has_key?(:dstr)
+    end
+
+    interpolation.first
+  end
+end
+describe "Your code" do
+  [['solution::code']]
+
+  VARIABLES = [:firstname, :lastname, :fullname].freeze
+
+  VARIABLES.each do |name|
+    it "defines a variable with name \"#{name}\"" do
+      expect(local_variables.include?(name)).to be true
+    end
+
+    if name != :fullname && local_variables.include?(name)
+      it "assigns a String to the variable \"#{name}\"" do
+        expect(eval(name.to_s)).to be_a String
+      end
+    end
+  end
+
+  if (local_variables & VARIABLES).sort == VARIABLES.sort
+    it "creates an interpolated String" do
+      statement = [[{ lvar: :lastname }], String, [{ lvar: :firstname }]]
+      code_lines = %q{ [['solution::code']] }
+
+      expect(code_lines).to run_string_interpolation(statement)
+    end
+  end
+end

3_Strings/1_Interpolation/task.md

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+# Interpolation
+
+---
+
+*You will learn:*
+- how to create interpolated Strings
+
+---
+
+You already learned that you can define a String with either single quotes or
+double quotes. The resulting String is the same, but if you use the double quotes
+for creating a String you can interpolate variables within this string.
+
+The interpolated statement has to be wrapped in *#{}* – a hash followed by curly brackets.
+Let's look at an example:
+
+`  amount = 5`
+`  message = "I learned` *#{amount}* `units today."`
+
+The variable `message` now has the value *"I learned 5 units today."*.
+If you would use single quotes then the String would not evaluate the *#{}* part:
+
+`  message = 'I learned #{amount} units today.'`
+
+The variable `message` has the value *"I learned #{amount} values today."* –
+the value of `amount` will not be interpolated into the String.
+
+A common pattern is:
+
+\* You should use single quotes `''` for Strings that are not interpolated
+\* You should only use double quotes `""` for Strings that should be interpolated
+
+---
+
+Define a variable `firstname` and `lastname` with your firstname and lastname as values.
+Then create a variable `fullname` and assign an interpolated String by using
+the `lastname` and `firstname` variables.
+
+The resulting String should look like *"YourLastname, YourFirstname"*.
+
+ ---