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Author: Aleksandar Todorović

programming/21_ruby_101.md

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+# Ruby 101
+
+* Ruby — Programming language
+* Ruby on Rails — Web framework (think: Ruby's Django)
+
+## Syntax
+
+* Everything's an object.
+* `puts "something"` prints something. `puts("something")` is the equivalent (think: Python 2.x vs. Python 3.x)
+* Classes start with `def <NAME>(<INPUT_PARAMS>)` and end with... well, `end`.
+* Common practice in the Ruby community is to use two space indentation.
+* When needing to print a value of a variable, both `result = "Good night, #{name}"` and `result = "Good night, " + name` should do the trick.
+* There's a difference between using single quotes and double quotes. Single quotes have no processing, double quotes do (you can't use something like `#{name}` or `\n` with single quotes).
+* `a = [ ​'ant'​, ​'bee'​, ​'cat'​, ​'dog'​, ​'elk'​ ]` == `a = ​%w{ ant bee cat dog elk }​`
+
+### Names:
+
+* Local variables: `something` `kthxbye`, `_x`, `_26`
+* Instance variables: `@something` `@kthxbye`, `@x`, `@_`
+* Class variable: `@@something`, `@@x_pos`, `@@sum`
+* Global variable: `$debug`, `$plan9`, `$CUSTOMER`, `$_`
+* Class name: `Something`, `Kthxbye`, `String`, `ActiveRecord`, `MyClass`
+* Constant name: `FEET_PER_MILE`, `DEBUG`
+* `Null` -> `nil` (object that represents nothing)
+
+### Hashes (something like dicts)
+
+```ruby
+inst_section = {
+  ​'cello'​     => ​'string'​,
+  ​'clarinet'​  => ​'woodwind'​,
+  ​'drum'​      => ​'percussion'​,
+  ​'oboe'​      => ​'woodwind'​,
+  ​'trumpet'​   => ​'brass'​,
+  ​'violin'​    => ​'string'​
+}
+p inst_section[​'oboe'​]    # -> Returns "woodwind"
+p inst_section[​'cello'​]		# -> Returns "string"
+p inst_section[​'bassoon'​]	# -> Returns nil
+```
+
+`:name` symbol is used commonly to replace the usage of single quotes. So, the above could also be used as such:
+
+```ruby
+inst_section = { 
+  :cello     => ​'string'​,
+  :clarinet  => ​'woodwind'​,
+  :drum      => ​'percussion'​,
+  :oboe      => ​'woodwind'​,
+  :trumpet   => ​'brass'​,
+  :violin    => ​'string'​
+}
+inst_section[:oboe]    ​# -> Returns "woodwind"
+inst_section[:cello]   ​# => Returns "string"​
+inst_section[​'cello'​]  ​# => Returns nil​, because `:cello` is not equal to `'cello'`
+```
+
+This is also a valid syntax:
+
+```ruby
+inst_section = {
+  cello:    ​'string'​,
+  clarinet: ​'woodwind'​,
+  drum:     ​'percussion'​,
+  oboe:     ​'woodwind'​,
+  trumpet:  ​'brass'​,
+  violin:   ​'string'​
+}
+puts ​"An oboe is a ​#{inst_section[:oboe]}​ instrument"
+# Outputs: An oboe is a woodwind instrument
+```
+
+### Control Structures
+
+Instead of the braces for the `if` and `while` loops, Ruby uses the keyword `end` to mark the end of the loop.
+
+`if` example:
+
+```ruby
+today = Time.now
+​if​ today.saturday?
+  puts ​"Do chores around the house"​
+​elsif​ today.sunday?
+  puts ​"Relax"​
+​else​
+  puts ​"Go to work"​
+​end​
+```
+
+`while` example:
+
+```ruby
+num_pallets = 0
+weight      = 0
+​while​ weight < 100 ​and​ num_pallets <= 5
+  pallet  = next_pallet()
+  weight += pallet.weight
+  num_pallets += 1
+​end​
+```
+
+Ruby supports something called statement modifiers, so:
+
+```ruby
+​if​ radiation > 3000
+  puts ​"Danger, Will Robinson"​
+​end​
+```
+
+...is equal to:
+
+```ruby
+puts ​"Danger, Will Robinson"​ ​if​ radiation > 3000
+```
+
+...and:
+
+```ruby
+square = 4
+​while​ square < 1000
+ square = square*square
+​end​
+```
+
+...is equal to:
+
+```ruby
+square = 4
+square = square*square ​while​ square < 1000
+```
+
+### Manipulating strings
+
+Ruby supports regex out of the box with the "match" parameter, so this is legit:
+
+```ruby
+line = gets
+​if​ line =~ /Perl|Python/
+  puts ​"Scripting language mentioned: ​#{line}​"​
+​end​
+```
+
+...and basically searches for `Perl` *or* `Python` to satisfy the criteria.
+
+* `newline   = line.sub(/Perl/, ​'Ruby'​)` will replace the first occurrence of `Perl` with `Ruby`.
+* `newerline = newline.gsub(/Python/, ​'Ruby'​)` will replace _all_ occurrences of the word `Python` with `Ruby`.
+* `newset = line.gsub(/Perl|Python/, ​'Ruby'​)` will replace _all_ occurences of the word `Perl` _and_ the word `Python` with `Ruby`.