Factors.rb

# This program is a demo of how to find factors for numbers

# Print message to ask the user for a number
# "puts" is shorthand for "put string".
# It's how we output text to the user.
puts "What number shall we find factors for? "

# We want to get input from the user and then store that input
# in a "variable" so that we can access it again later.
# We've decided to name our variable "number_to_factor".
# 'gets' is the name of the function that takes input from the user.
# By default 'gets' returns a variable that's called a "string" which means it's just text.
# But we actually need our variable to be a number for calculations, so adding 
# '.to_i' instructs the program to convert the input to an Integer.
# Integer is a fancy term for a "whole" number.  That is a number with no decimals.
# If the user entered anything other than an Integer this program would crash.  
# A more advanced program would double check to make sure the input was an Integer 
# so that it wouldn't crash unexpectedly.
number_to_factor = gets.to_i

# By putting number_to_factor inside a #{} we output it in our sentence below.
puts "Factors for #{number_to_factor} are:"

# now we will check every number from 1 up to number_to_factor to see if its a factor
1.upto(number_to_factor) { |number_to_test|
  # everything inside this "block" between the opening { above and the closing } below will
  # execute over and over.
  # The variable inside the ||, which we've named "number_to_test", will change each time.
  # the first time "number_to_test" will be 1, then 2, etc. all the way up to "number_to_factor"
  
  # the "%" operator tells you what the remainder would be if you divided two numbers.
  # so 10 % 3 would be 1
  # or 17 % 5 would be 2
  # or 27 % 11 would be 5
  
  # so we know if the "%" of two numbers is 0, then the one divides into the other perfectly
  # and we have a factor
  if (number_to_factor % number_to_test == 0)
    # "== 0" is how we test if the result is 0.
    # The code from here until "end" won't execute unless our "if" statement is true.
    # So we know this will only happen when number_to_test is a true factor.
    puts number_to_test
  end
}

# That's it!  
# This program works perfectly and will always give you the right answer.
# But it is extremely inefficient.

# Trying to make things more efficient is often an important element of Software Engineering,
# because you may sometimes have to deal with very large numbers or piles of data that even a 
# computer could take days or months to fully analyze.

# Remember that the 1.upto(number_to_factor) loop above will check EVERY SINGLE NUMBER
# between 1 and the number the user input.  So if the user enters 
# 10 billion, the loop will run 10 billion times!
# Can you think of how to make it faster?  
# Does the challenge of making this faster sound interesting to you?
# If so, I definitely recommend you consider software engineering.

# I won't reveal any answer here, but email me if you want to discuss it.
# [email protected]