practice_teach.md

#Objective

###What are Blocks, Procs and Lambda?

• Explain the differences between them
• Ability to perform an action on each element of array using at least two methods

###Quick Review on Loops and Arrays

• Loops
• Arrays

students = ['harry ng', 'max cantor', 'david han', 'adam bray']
students.capitalize # nope!

students.each do |student| 
  p student.capitalize
end

students.each { |student| p student.capitalize }

###Blocks

• Extremely common in Ruby

###Procs and Lambda

• Come up quite a bit as we're trying to
  • shorten,
  • modularize, and
  • DRY up our code
    • DRY - Don't Repeat Yourself

#Let's try

###Procs

• Short name for Procedure

p = Proc.new { |x| puts x * 2 }
[1,2,3].each(&p)

proc = Proc.new { puts "Hello World" }
proc.call

###Lambda

• Lambda Calculus and Anoymous Function

lam = lambda { puts "Hello World" }
lam.call

• Commonly used in Javascript as well

sqsum(x,y) = x*x + y*y  #<-- normal function
(x,y) -> x*x + y*y #<-- anonymous function

##Differences between Procs and Lambdas

proc = Proc.new { puts "Hello World" }
lam = lambda { puts "Hello World" }

proc.class
lam.class

proc #<Proc:0x007fcb34b6c450@(pry):1>
lam #<Proc:0x007fcb341bbfc8@(pry):7 (lambda)>

###Lambdas check the number of arguments, while procs do not

lam = lambda { |x| puts x }    # creates a lambda that takes 1 argument
lam.call(2)                    # prints out 2
lam.call                       # ArgumentError: wrong number of arguments (0 for 1)
lam.call(1,2,3)                # ArgumentError: wrong number of arguments (3 for 1)

• In contrast, proc don't care

proc = Proc.new { |x| puts x } # creates a proc that takes 1 argument
proc.call(2)                   # prints out 2
proc.call                      # returns nil
proc.call(1,2,3)               # prints out 1 and forgets about the extra arguments

###Lambdas and procs treat the ‘return’ keyword differently

def lambda_test
  lam = lambda { return }
  lam.call
  puts "Hello world"
end

# calling lambda_test prints 'Hello World'
lambda_test


def proc_test
  proc = Proc.new { return }
  proc.call
  puts "Hello world"
end

# calling proc_test prints nothing
proc_test

##Quiz

• Create a Proc to capitalize both first and last name of student
• Use a block to loop through an array of students' name

students = ['harry ng', 'max cantor', 'david han', 'adam bray']

# Result
Harry Ng
Max Cantor
David Han
Adam Bray

name_proc = Proc.new { |x| puts x.split(" ").map(&:capitalize).join(" ") }

students.each()&name_proc)

##Things to note

• Recognize how Blocks, Procs, Lambda look like?
• Procs and Lambda is similar, with 2 differences
• Reference to the class note and example if you cannot remember