Add unit about concatenating Strings

Author: paulgoetze

3_Strings/2_Concatenating_Strings/solution.rb

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+# e.g.
+('Ruby' << 32) + 'r' + 'o' * 5 + 'cks' << 33
+
+# or
+'Ruby'.concat(32) + 'r' + 'o' * 5 + 'cks'.concat(33)

3_Strings/2_Concatenating_Strings/solution_spec.rb

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+require 'rspec'
+
+describe 'Your code' do
+
+end

3_Strings/2_Concatenating_Strings/task.md

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+# Concatenating Strings
+
+---
+
+*You will learn:*
+- how to concatenate Strings
+- how to append a String to another String
+- about the ordinal number representation of Strings
+
+---
+
+If you look into Ruby's documentation for the the *String* class you will find several
+methods to concatenate Strings: *(ruby-doc core: String)*.
+
+Adding a String to another String is an often used operation when working with Strings.
+In Ruby there are two possibilities to combine Strings:
+
+`1) You can connect two Strings and create a third String out of them`
+`2) You can directly modify an existing String`
+
+Let's have a closer look into both the options:
+
+## Combining Strings into a new String
+
+A String comes with 2 methods to do this, namely `+` and `\*`.
+You might think "Wait, these are Number operations!". Right, but the *String* class
+does also define them.
+
+The `+` method just adds a second String to the end of the first one:
+
+`  'white' + 'space'` *# => "whitespace"*
+
+If you want to have whitespace between these words you have explicitly add it to
+the end of the first or the beginning of the second String.
+
+The `\*` method takes an Integer *n* and runs the `+`-concatenation *n* times:
+
+`  'la' \* 3`         *# => "lalala"* (here n = 3)
+
+If your first String was stored in a variable, then a concatenation with the `+` and `\*`
+methods doesn't change the value of this variable:
+
+`  name = 'Ada'`
+`  fullname = name + ' Lovelace'` *# => "Ada Lovelace"
+`  puts fullname`                 *# => "Ada Lovelace"
+`  puts name`                     *# => "Ada"
+
+The variable `name` still has the value *"Ada"*, `fullname` holds the concatenated String.
+
+## Directly modifying a String
+
+There are two methods for directly adding a String to another String: `<<` and `concat`.
+
+The `<<` method works like `+` with the difference however that it adds the second String
+to the first one. Let's take the example from above again:
+
+`  name = 'Ada'`
+`  name << ' Lovelace'`  *# => "Ada Lovelace"
+`  puts name`            *# => "Ada Lovelace"
+
+The value of *name* is now the concatenation.
+
+The method `concat` works the same but just has a more recognizable name:
+
+`  name = 'William'`
+`  name.concat(' King')` *# => "William King"
+`  puts name`            *# => "William King"
+
+Both `<<` and `concat` also take an integer number as parameter.
+Hm, how can we add an Integer value to a String? The answer is that each one-character
+String has an Integer representation (ordinal number). You can check which ordinal number
+a character has by using the `ord` method:
+
+`  'A'.ord`              *# => 65*
+`  '?'.ord`              *# => 63*
+
+This means if you write
+
+`  'Pardon'.concat(63)`  *# => "Pardon?"*
+
+then the *63* is first converted to its String representation and then added to the first String.
+
+If you want to add a number – as it is – to a String, you have to convert it to a String before.
+You can use the `to_s` method to achive this:
+
+`  'User' << 1.to_s`     *# => "User1"*
+
+Now that we learned about all these methods, let's concatenate some Strings!
+
+---
+
+Create the String *"Ruby rooooocks!"* by using the methods `+`, `\*` and `<<` or `concat`!
+
+Create the whitespace and the exclamation mark by concatenating ordinal numbers to the String.
+Use the `\*` method to add the multiple *ooooo*s to the String.
+
+---