Variables in Ruby do differ a little bit from python. There are five types of variables. Do not worry about how to use 2 - 5 in the list below, but just now they exist. We will just focus on local variables at this moment.

The five type of variables are:

1. local variable (must be loser case or start with a _)

2. global variable (starts with a $)

3. instance variable (starts with a @)

4. class variable (start with @@)

5. constant (written in upper case)

Local variable example:

luckynumber = 30

multiplier = 100

total = luckynumber * multiplier

puts total

This creates three variables, luckynumber with a value of 30, multiplier with a value of 100 and total with a value of 3000 (30 * 100). The last line with puts writes the output standard out.

Creating and using arrays differs in Ruby.  In Ruby we need to create a variable that will be our array and initialize this as a new array using the Array class. 

arr = Array.new()

arr2 = Array.new(20)

arr3 = ["A", "B", "C", "D"]

The first line of code creates a new array, but does not define the size. The second line of code passes 20 as a parameter to the new method that defines the size of the array. The third line creates an array and assigns elements to the array. 

To access an element within the array is just like Python. 

arr3 = ["A", "B", "C", "D"]

puts arr[1]

The above code would output B.

The Ruby comparison operators are the same as Python.

1.  Equal to: ==

2. Not equal to: !=

3. Greater than: >

4. Less than: <

5. Greater than or equal: >=

6. Less than or equal: <=

Keep in mind that these can be used to compare numbers, strings and other data types.

Just like Python you can define a string as a variable without needed to specify a data type. To learn more about the Ruby String class view the Ruby Documentation. 

first = "Pete"

last = "Gross"

full = first + " " + last

puts full  # Output: Pete Gross

Conditionals in Ruby have a small difference in syntax than Python. This is a great way to be able to quickly determine if the script is Ruby or Python. We will use the same example from Python, but update it to Ruby.

# Example of a Ruby conditional statement

x = 10

if x > 5

    puts "x is greater than 5"

else

    puts "x is less than or equal to 5"

end

Can you spot the difference in syntax? The first one is that we do not need to use the : character at the end of the if or else line. Also, Ruby clearly states the conditional has ended with the end keyword. 

Ruby also has a minor sytanx difference with an if-elsif-else statement. Ruby uses elsif while Python uses elif. 

# Example of a Ruby conditional statement with if-elsif-else.

x = 10

if x > 10

    puts "x is greater than 10"

elsif x < 10

    puts "x is less than 10"

else:

    puts "x is equal to 10"

Ruby for and while loops are exactly like Python, except for minor sytanx differences. Hopefully, you are realizing that once you understand how to use one scripting language it is easy to pick up another one.

Ruby for loop example:

for a in 1..5 do

 puts a

end

This for loop iterates over the range 1 to 5 (including 5) and just counts up from 1 to 5.

Ruby while loop example:

x = 5

while x >= 0

  puts x

  x=x-1

end

This while loop counts down from 5 to 0.

Error handling in Ruby differs greatly from Python and what you have learned in ICS 111. It is not simple, but we will still look at the syntax with a brief explanation.

begin

    #Where an exception might occur

rescue =>

    #This handles the exception/error

else

    #This runs when there is no error

ensure

    #This is always run

end

Let us see an example using divide by zero error again.

begin

   numerator = 281

   denominator = 0

   puts numerator/denominator

rescue ZeroDivisionError => e

   puts "You can't divide by zero, change your denominator"

   puts e

end

Ruby also provides the ability to make system calls. The first way is to use the system method. If the command was succesfully completed it will return true.  Using the system method the output will be displayed to the console.  The second way is to use backticks (` `) or %x{} syntax. This way of doing it allows the user to capture the output of the command and store it in a string value. The third way is to use exec() method. This does not provide a return value unless there is an error. The exec() method also replaces the current process with a new process. This means that anything after the exec() statement will not be run.

Example 1:

system("ls -l")

Example 2:

result = `ls -l`

Example 3:

result = %x(ls -l)

puts result

Example 4:

exec("ls -l")

puts "This will not be shown"