[WIP] Add unit for removing elements from Arrays

Author: paulgoetze

4_Arrays/4_Removing_elements/solution.rb

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+baggage = ['shoes', 'underwear', 'socks', 'kettle', 'glasses', 'shirt', 'wallet']
+
+baggage-shift
+baggage.delete('kettle')
+baggage.pop

4_Arrays/4_Removing_elements/solution_spec.rb

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+require 'rspec'
+require 'code_breaker'
+
+describe 'Your code' do
+  [['solution::code']]
+
+  VARIABLE = :baggage
+  ITEMS    = ['glasses', 'shirt', 'trousers', 'socks', 'shoes']
+
+  it 'defines a variable with name "baggage"' do
+    expect(local_variables.include?(VARIABLE)).to be true
+  end
+
+  if local_variables.include?(VARIABLE)
+    it 'defines a variables "baggage" that has all the items' do
+      expect(eval(VARIABLE.to_s)).to eq ITEMS
+    end
+
+    if eval(VARIABLE.to_s) == ITEMS
+      before(:all) do
+        @statements = CodeBreaker.parse( %q{ [['solution::code']] })
+      end
+
+      it 'uses the unshift method to add "glasses"' do
+        unshift = [{ lvar: VARIABLE }, :unshift, String]
+        expect(@statements.include?(unshift)).to be true
+      end
+
+      it 'uses the push or << method to add "shoes"' do
+        push = [{ lvar: VARIABLE }, :push, String]
+        arrows = [{ lvar: VARIABLE }, :<<, String]
+
+        includes_code = @statements.include?(push) || @statements.include?(arrows)
+        expect(includes_code).to be true
+      end
+
+      it 'uses the insert method to add "trousers" and "socks"' do
+        insert = [{ lvar: VARIABLE }, :insert, Fixnum, String, String]
+        expect(@statements.include?(insert)).to be true
+      end
+    end
+  end
+end

4_Arrays/4_Removing_elements/task.md

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+# Removing elements from Arrays
+
+---
+
+*You will learn:*
+- how to remove elements from the end of an Array
+- how to remove elements from the beginning of an Array
+- how to remove elements from an Array after a specific index
+- how to remove a specific element
+
+---
+
+Once you have created an Array, there might be the case that you don’t want to keep
+all of the Array’s elements. This is where some methods for removing elements come
+in handy.
+
+This unit will look into different ways of removing elements from an Array.
+
+## Removing elements from the end of an Array
+
+For removing one ore more elements from the end of an Array you can use the `pop`
+method. The `pop` method modifies the orginal Array and returns the removes elements.
+You can pass the number of elements to remove as argument:
+
+`  numbers = [1, 2, 3, 4]`
+`  numbers.pop(2)`        *# => [3, 4]*
+`  numbers`               *# => [1, 2]*
+
+If you don’t pass the number of elements to remove it will delete 1 by default:
+
+`  numbers = [5, 6, 7, 8]`
+`  numbers.pop`             *# => 8*
+`  numbers`                 *# => [5, 6, 7]*
+
+Note that, if you remove multiple elements, `pop` will return an Array with the removed
+elements. If you just remove the last element it will return this removed element.
+
+If you try to pop an element from an empty Array, there’s no element left to remove
+and it will return *nil*:
+
+`  [].pop`                  *# => nil*
+
+## Removing elements from the beginning of an Array
+
+For removing elements from the beginning of an Array you can use the `shift` method.
+It takes an optional argument that specifies the number of elements to remove:
+
+`  numbers = [0, 1, 2, 3]`
+`  numbers.shift(3)`        *# => [2, 3]*
+`  numbers`                 *# => [0, 1]*
+
+If you don’t pass an argument it will only remove and return the very first element:
+
+`  numbers = [2, 4, 6]`
+`  numbers.shift`           *# => 2*
+`  numbers`                 *# => [4, 6]*
+
+Similar to `pop`, `shift` called on an empty Array will return *nil*:
+
+`  [].shift`                *# => nil*
+
+## Removing elements after a specific index
+
+There might be situations where you want to select all elements of an Array after
+a certain position, e.g. all elements after the the third element.
+You can do exactly this with the `drop` method:
+
+`  fruits = ['apple', 'pear', 'lemon', 'melon']`
+`  fruits.drop(2)`          *# => ["lemon", "melon"]*
+`  fruits`                  *# => ["apple", "pear", "lemon", "melon"]*
+
+Apart from `pop` and `shift`, `drop` does not modify the original Array but creates
+a new Array that comprises the dropped elements.
+
+## Removing specific elements
+
+In case you know the elements in an Array, you can specificly remove these.
+There are two similar methods for doing this: `delete` and `-`.
+
+The difference is, that `delete` modifies the original Array, while `-` creates a new
+Array and does not change the original Array.
+
+`delete` take one element to remove from the Array as argument and returns the
+removed elements:
+
+`  fruits = ['apple', 'lemon', 'lemon']`
+
+`  fruits.delete('lemon')`  *# => "lemon"*
+`  fruits`                  *# => ["apple"]*
+
+As you can see, `delete` removes duplicated elements from the Array, here all the
+*'lemon'* elements.
+
+The `-` method takes an Array as argument. It also removes duplicated elements:
+
+`  fruits = ['apple', 'lemon', 'lemon', 'melon']`
+
+`  less_fruits = fruits - ['lemon']` *# => ["apple", "melon"]*
+`  fruits`                           *# => ["apple", "lemon", "lemon", "melon"]*
+
+The `-` does not modify the original Array but creates a new one. The original
+Array that is stored in *fruits* remains the same.
+
+Before going into the task, let’s summarize all the learned methods for removing
+elements from an Array:
+
+`  pop`     *removes elements at end            modifies the original Array*
+`  shift`   *removes elements at beginning      modifies the original Array*
+`  drop`    *remove elements after given index  creates a new Array*
+`  delete`  *removes the given element          modifies the original Array*
+`  -`       *removes the given elements         creates a new Array*
+
+---
+
+Let’s use our baggage again and unpack a couple of things.
+Define a variable `baggage` and assign an array with all the things you want to
+take with you: *shoes*, *underwear*, *socks*, *kettle*, *glasses*, *shirt*, and *wallet*.
+
+You just realized, that you actually want to wear your shoes (which are the first
+thing in your baggage), so remove them from baggage.
+
+Your friend tells you that you won’t need the kettle, so delete it from your baggage, too.
+
+Last, you think that it’s maybe a better idea to carry your wallet with you.
+Thus, also pop the wallet from your baggage.
+
+Now you’re good to go. Bon voyage!
+
+---