scope.md

JumpStart Live (JSL)

Day 3

Scope

Overview

Scope in programming defines when something (usually a variable) is visible to something else. In regards to scope of variables, the scope defines what knows about and can access that variable.

Variable types and their scope

You will not need to know about most of these until a few weeks into Ada, but it's important to have a general idea about the different types of variables and their scope. You should understand local variables and their scope by the end of JumpStart Live.

Variable type	Syntax	Scope
Class variable	@@name	Visible to the class, and sub classes
Instance variable	@name	Visible to a specific object, and the instance methods of that object
Global variable	$name	Visible to everything in the file
Local variable	name	Depends!

Local variables and scope

• When you define a local variable inside of a block, it is not visible outside of that block

Examples

• In Ruby, local variables created outside of a block, are visible to everything inside of that file. What does this print?

puts "Name: "
name = gets.chomp
puts "Age: "
num = gets.chomp.to_i

while num > 0
    num -= 1
    name << " birthday!"
end

puts name
# input "susan" and 10
# susan birthday! birthday! birthday! birthday! birthday! birthday! birthday! birthday! birthday! birthday!

In Ruby, if a variable is redefined inside a block, that is a new variable with scope only inside that block.

list = ["Goofy", "Minnie", "Daisy"]
name = "Donald"

list.each do |name|
  name << " birthday! "
end

puts name
# Donald 
# name was redefined inside the block as an iterator variable

list = ["Goofy", "Minnie", "Daisy"]
greeting = ""

list.each do |name|
	name << " birthday! "
	greeting = name
end

puts greeting

Output:

Daisy birthday! 
# greeting was not redefined??

• In ruby, local variables created inside if statements, are visible outside of those if statements

   x = gets.chomp.to_i
   if x > 0
     y = 1
   else
     y = -1
   end
   
   puts y # No error, if statements are not considered blocks in Ruby

• In ruby, local variables created inside of a block are not visible outside of those blocks

   (1..3).each do |num|
     last = num
   end
   
   puts last 	# NameError: undefined local variable or method x

Resources

• Ada Scope Video (6:24)